diff --git a/Makefile b/Makefile
index 4f95d85f..2ddaeb4e 100644
--- a/Makefile
+++ b/Makefile
@@ -270,6 +270,7 @@ check:
 	  mkdir -p $(COVER_DB) ;                                            \
 	  echo "*** Run once, force parallel ***" ;                         \
 	  LCOV_FORCE_PARALLEL=1 $(MAKE) -s -C tests check LCOV_HOME=`pwd` ; \
+	  LCOV_FORCE_PARALLEL=1 $(MAKE) -s -C example LCOV_HOME=`pwd` ;     \
 	  echo "*** Run again, no force ***" ;                              \
 	fi
 	@$(MAKE) -s -C tests check LCOV_HOME=`pwd`
@@ -293,7 +294,7 @@ else
 	@echo "Checking changes in source files for coding style issues (MODE=diff):"
 endif
 	@RC=0 ;                                                  \
-	CHECKFILES=`find . -path ./.git -prune -o \( \( -type f -exec grep -q '^#!.*perl' {} \; \) -o -name '*.pm' \) -not \( -name '*.tdy' -o -name '*.orig' -o -name '*~' \) -print `; \
+	CHECKFILES=`find bin lib scripts tests -path ./.git -prune -o \( \( -type f -exec grep -q '^#!.*perl' {} \; \) -o -name '*.pm' \) -not \( -name '*.tdy' -o -name '*.orig' -o -name '*~' \) -print `; \
 	for FILE in $$CHECKFILES ; do                            \
 	  $(CHECKSTYLE) "$$FILE";                                \
 	  if [ 0 != $$? ] ; then                                 \
diff --git a/README b/README
index db82db4d..c30f7b94 100644
--- a/README
+++ b/README
@@ -1,6 +1,6 @@
 -------------------------------------------------
 - README file for the LTP GCOV extension (LCOV) -
-- Last changes: 2025-10-01
+- Last changes: 2026-02-10
 -------------------------------------------------
 
 Description
@@ -430,6 +430,9 @@ LCOV features and capabilities fall into 7 major categories:
         vii) compress the 'function detail' table to improve readability
              by shortening long C++ template and function names.
              Sample script:  simplify.pm
+        viii) use history from previous execution to improve runtime
+             performance of current execution by improving load balancing.
+             Sample script:  history.pm
 
      The callback may be any desired script or executable - but there
      may be performance advantages if it is written as a Perl module.
@@ -445,7 +448,7 @@ LCOV features and capabilities fall into 7 major categories:
      Related options:
        --annotate-script, --criteria-script, --version-script
        --resolve-script, --select-script, --context-script
-       --simplify-script
+       --simplify-script --history-script
 
   f) Performance
 
@@ -466,7 +469,7 @@ LCOV features and capabilities fall into 7 major categories:
 
      See the genhtml/lcov/geninfo man pages for details
 
-     Related options: --parallel, --memory, --profile
+     Related options: --parallel, --memory, --profile, --history-script
 
   g) Language/tool support
 
diff --git a/bin/genhtml b/bin/genhtml
index f9684987..38ae1f56 100755
--- a/bin/genhtml
+++ b/bin/genhtml
@@ -2231,18 +2231,18 @@ sub type
 
 sub lineNo
 {
-    my ($self, $which, $lineNo) = @_;
+    my ($self, $which, $lineNo, $filename) = @_;
     my $loc;
     if ($which eq "current") {
         $loc = LINENO_CURRENT;
     } else {
-        die("unknown key $which - should be 'base' or 'current'")
+        die("unknown key '$which' - should be 'base' or 'current'")
             unless $which eq "base";
         $loc = LINENO_BASE;
     }
     lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
-                              "inconsistent $which line location $loc: " .
-                                  $self->[$loc] . " -> $lineNo")
+                        "\"$filename\": inconsistent '$which' line location: " .
+                            $self->[$loc] . " -> $lineNo")
         if (defined($lineNo) &&
             defined($self->[$loc]) &&
             $self->[$loc] != $lineNo);
@@ -2320,7 +2320,7 @@ sub _mergeBranchData
             if (!$branchData->hasBlock($branchId)) {
                 # don't know how to get here...but someone on the internet
                 # managed to do it - so we need to handle the error
-                my $which = $loc == DATA_BASELINE ? 'baseline' : 'current';
+                my $which = $loc == DATA_BASELINE ? 'base' : 'current';
                 lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                                         $filename . ':' . $current->line() .
                                             ": '$which' line " .
@@ -2341,7 +2341,7 @@ sub _mergeBranchData
             # LCOV_EXCL_START
             if ($nc != $nd) {
                 #  similarly: this should not happen - but it might
-                my $which = $loc == DATA_BASELINE ? 'baseline' : 'current';
+                my $which = $loc == DATA_BASELINE ? 'base' : 'current';
                 lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                     $filename . ':' . $current->line() .
                         ": '$which' line " . $self->lineNo($which) .
@@ -2351,7 +2351,7 @@ sub _mergeBranchData
             # LCOV_EXCL_STOP
             for (my $i = ($nc > $nd ? $nd : $nc) - 1; $i >= 0; --$i) {
                 my $br = $d->[$i];
-                $c->[$i]->merge($br);
+                $c->[$i]->merge($br, $filename, $current->line());
             }
             # now append any new values from 'branchData':
             #  (go here only if |D| > |C| and we ignore the mismatch error
@@ -2498,6 +2498,16 @@ sub differential_function
     return $function->[DATA_DIFFERENTIAL];
 }
 
+# forward-declare some types
+package DiffMap;
+
+use constant {
+              EQUAL  => 0,
+              INSERT => 1,
+              DELETE => 2,
+
+};
+
 # structure holding coverage data for a particular file:
 #  - associated with a line line number:
 #     - line coverage
@@ -2683,52 +2693,29 @@ sub _findLineData
     my $lineDataMap = $self->lineMap();
     my $linedata;
 
-    if ($type ne "delete") {
+    if ($type != DiffMap::DELETE) {
         if (!defined($lineDataMap->{$current_lineNo})) {
             $linedata = LineData->new($type);
             $lineDataMap->{$current_lineNo} = $linedata;
-            $linedata->lineNo('current', $current_lineNo);
+            $linedata->lineNo('current', $current_lineNo, $filename);
         } else {
             $linedata = $lineDataMap->{$current_lineNo};
         }
-        $linedata->lineNo('base', $base_lineNo);
+        $linedata->lineNo('base', $base_lineNo, $filename);
     } else {
         # nothing walks the keylist so a prefix is sufficient to distinguish
         # records that should be summarized but not displayed
         my $dline = "<<<" . $base_lineNo;
         if (!exists($lineDataMap->{$dline})) {
             $linedata = LineData->new($type);
-            $linedata->lineNo('base', $base_lineNo);
+            $linedata->lineNo('base', $base_lineNo, $filename);
             $lineDataMap->{$dline} = $linedata;
             # look up and/or down to find the first baseline line
             # which is not deleted - and store that as the corresponding
             # 'current' line.
             # this way, we can know the extents of the deleted region
-            my $c;
-            for (my $i = $base_lineNo - 1; $i > 0; --$i) {
-                if ('delete' ne $diffMap->type($filename, $diffMap->OLD, $i)) {
-                    $c = $diffMap->lookup($filename, $diffMap->OLD, $i);
-                    last;
-                }
-            }
-            if (!defined($c)) {
-                # there were no 'current' lines above me - so I must be
-                # at the first line in the file.  It must not be deleted
-                $c = 1;
-                die("$filename:1: incorrectly marked 'delete'")
-                    if (
-                      'delete' eq $diffMap->type($filename, $diffMap->OLD, $c));
-            }
-            die("$filename:  no current block for deleted line $base_lineNo")
-                unless defined($c);
-            $linedata->lineNo('current', -$c);
-            # keep track of where deleted lines were - so we can
-            # mark them in the source view
-            if (exists($self->[DELETED_LINE_LEADER]->{$c})) {
-                push(@{$self->[DELETED_LINE_LEADER]->{$c}}, $linedata);
-            } else {
-                $self->[DELETED_LINE_LEADER]->{$c} = [$linedata];
-            }
+            my $c = $diffMap->find_deleted_line_leader($filename, $base_lineNo);
+            $linedata->lineNo('current', -$c, $filename);
         } else {
             $linedata = $lineDataMap->{$dline};
         }
@@ -2742,31 +2729,34 @@ sub _categorizeLineCov
     my ($self, $filename, $base_data, $current_data, $diffMap, $verbose) = @_;
     my $lineDataMap = $self->lineMap();
 
-    if ($verbose) {
-        print("categorize lines $filename\n");
-    }
+    print("categorize lines $filename\n")
+        if ($verbose);
+
     # $lineCovBase, $lineCovCurrent are CountData objects
     my $lineCovBase    = $base_data->sum() if defined($base_data);
     my $lineCovCurrent = $current_data->sum();
 
-    # walk the branch coverpoints to check for data consistency:
-    #  - we expect a line coverpoint in every location which has branches
-    #  - if not found, the generate message and/or create a fake coverpoint
-    # LLVM seems to like to generate inconsistent data.
-    my $branchCurrent = $current_data->sumbr();
-    foreach my $line ($branchCurrent->keylist()) {
-        # just ignore bogus data - we already warned when we read the data
-        next if ($line <= 0);
-
-        my $type = $diffMap->type($filename, $diffMap->NEW, $line);
-        # LCOV_EXCL_START
-        if ($type eq 'delete') {
-            lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
-                "'current' line $filename:$line should not be marked 'delete'");
-            delete($branchCurrent->{$line});
-            next;
+    if ($lcovutil::check_data_consistency) {
+        # walk the branch coverpoints to check for data consistency:
+        #  - we expect a line coverpoint in every location which has branches
+        #  - if not found, the generate message and/or create a fake coverpoint
+        # LLVM seems to like to generate inconsistent data.
+        my $branchCurrent = $current_data->sumbr();
+        foreach my $line ($branchCurrent->keylist()) {
+            # just ignore bogus data - we already warned when we read the data
+            next if ($line <= 0);
+
+            my $type = $diffMap->type($filename, $diffMap->NEW, $line);
+            # LCOV_EXCL_START
+            if ($type == DiffMap::DELETE) {
+                lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
+                    "'current' line $filename:$line should not be marked 'delete'"
+                );
+                delete($branchCurrent->{$line});
+                next;
+            }
+            # LCOV_EXCL_STOP
         }
-        # LCOV_EXCL_STOP
     }
 
     # it is sufficient to just walk the 'global' (merged) line
@@ -2778,7 +2768,7 @@ sub _categorizeLineCov
         # just ignore bogus data - we already warned when we read the data
         next if ($line <= 0);
         my $type = $diffMap->type($filename, $diffMap->NEW, $line);
-        if ($type eq 'delete') {
+        if ($type == DiffMap::DELETE) {
             # can happen in some inconsistent case, when there are certain
             # out-of-range references in a file which contained diffs - and we
             # ignored the error check
@@ -2795,7 +2785,7 @@ sub _categorizeLineCov
             $linedata = $lineDataMap->{$line};
         }
         my $val = $lineCovCurrent->value($line);
-        $linedata->lineNo("current", $line);
+        $linedata->lineNo("current", $line, $filename);
         $linedata->curr_count($val);
         $linedata->tla($val == 0 ? 'UNC' : 'GNC')
             if (!defined($lineCovBase));
@@ -2831,7 +2821,7 @@ sub _categorizeLineCov
     foreach my $line (sort keys %$lineDataMap) {
         my $linedata = $lineDataMap->{$line};
         my $tla;
-        if ($linedata->type() eq "insert") {
+        if ($linedata->type() == DiffMap::INSERT) {
             if (!$linedata->in_curr()) {
                 # can get here if the 'diff' file is wrong with respect to
                 # baseline vs. current coverage data - e.g., showing that
@@ -2843,7 +2833,7 @@ sub _categorizeLineCov
             }
             $tla = ($linedata->curr_count() > 0) ? "GNC" : "UNC";
             print("  insert $line $tla\n") if ($verbose);
-        } elsif ($linedata->type() eq "delete") {
+        } elsif ($linedata->type() == DiffMap::DELETE) {
             if (!$linedata->in_base()) {
                 # similarly:  can get here if the diff vs baseline/current data
                 # is inconsistent.
@@ -2856,7 +2846,7 @@ sub _categorizeLineCov
             print("  delete $line $tla\n") if ($verbose);
         } else {
             die("FileCoverageInfo:: deleted segment line=$line file=$filename")
-                unless $linedata->type() eq "equal";
+                unless $linedata->type() == DiffMap::EQUAL;
 
             if ($linedata->in_base() && $linedata->in_curr()) {
                 $tla =
@@ -2924,13 +2914,15 @@ sub _categorizeBranchCov
 
         $branchCovLines{$line} = 1;
         # we expect that the line number matches...
-        $data->lineNo("current", $line);
+        $data->lineNo("current", $line, $filename);
         # append this branch data for the line
         my $currBranchData = $branchCurrent->value($line);
         $data->current_branch($currBranchData, $filename);
         if (!defined($branchBaseline)) {
             my $categorized = BranchEntry->new($line);
             $data->differential_branch($categorized);
+            die("no branch data for deleted $filename:$line")
+                unless defined($currBranchData);
             _cloneBranchEntry($categorized, $currBranchData, 'UNC', 'GNC');
         }
     }    # foreach line in 'current' branch data
@@ -2943,7 +2935,7 @@ sub _categorizeBranchCov
         my $data      = $self->_findLineData($diffMap, $filename, $base_line);
         my $curr_line = $data->lineNo('current');
         my $type      = $data->type();
-        if ($type ne 'delete') {
+        if ($type != DiffMap::DELETE) {
             $branchCovLines{$curr_line} = 1;
         } else {
             # the line has been deleted...just record the data
@@ -2966,24 +2958,39 @@ sub _categorizeBranchCov
         my @currBlocks = defined($curr) ? $curr->blocks() : ();
         my @baseBlocks = defined($base) ? $base->blocks() : ();
 
-        if ($type eq 'insert') {
+        if ($type == DiffMap::INSERT) {
             # can get here if diff data vs baseline/current is not consistent
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "baseline branch data should not be defined for inserted line $filename:$line"
             ) if defined($base);
-
-            _cloneBranchEntry($categorized, $curr, 'UNC', 'GNC');
-        } elsif ($type eq 'delete') {
+            if (defined($curr)) {
+                _cloneBranchEntry($categorized, $curr, 'UNC', 'GNC');
+            } else {
+                # LCOV_EXCL_START
+                lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
+                    "current branch data should not be defined for inserted line $filename:$line"
+                );
+                # LCOV_EXCL_STOP
+            }
+        } elsif ($type == DiffMap::DELETE) {
             # similarly: get here if diff data vs baseline/current is not consistent
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "current branch data should not be defined for deleted line $filename:$line"
             ) if defined($curr);
 
-            _cloneBranchEntry($categorized, $base, 'DUB', 'DCB');
+            if (defined($base)) {
+                _cloneBranchEntry($categorized, $base, 'DUB', 'DCB');
+            } else {
+                # LCOV_EXCL_START
+                lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
+                    "baseline branch data should not be defined for deleted line $filename:$line"
+                );
+                # LCOV_EXCL_STOP
+            }
         } else {
             lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                      "unexpected branch coverage type $type at $filename:$line")
-                if $type ne 'equal';
+                if $type != DiffMap::EQUAL;
 
             # branch might or might not be in both baseline and current
             foreach my $branchId (@baseBlocks) {
@@ -3096,7 +3103,7 @@ sub _categorizeMcdcCov
 
         $mcdcCovLines{$line} = 1;
         # we expect that the line number matches...
-        $data->lineNo("current", $line);
+        $data->lineNo("current", $line, $filename);
         # append this MC/DC data for the line (this is an MCDC_Block)
         my $currMcdcData = $mcdcCurrent->value($line);
         $data->current_mcdc($currMcdcData, $filename);
@@ -3116,7 +3123,7 @@ sub _categorizeMcdcCov
         my $data      = $self->_findLineData($diffMap, $filename, $base_line);
         my $curr_line = $data->lineNo('current');
         my $type      = $data->type();
-        if ($type ne 'delete') {
+        if ($type != DiffMap::DELETE) {
             $mcdcCovLines{$curr_line} = 1;
         } else {
             # the line has been deleted...just record the data
@@ -3139,14 +3146,14 @@ sub _categorizeMcdcCov
         my $currGroups = defined($curr) ? $curr->groups() : ();
         my $baseGroups = defined($base) ? $base->groups() : ();
 
-        if ($type eq 'insert') {
+        if ($type == DiffMap::INSERT) {
             # can get here if diff data vs baseline/current is not consistent
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "baseline MC/DC data should not be defined for inserted line $filename:$line"
             ) if defined($base);
 
             _cloneMcdcEntry($categorized, $curr, 'UNC', 'GNC');
-        } elsif ($type eq 'delete') {
+        } elsif ($type == DiffMap::DELETE) {
             # similarly: get here if diff data vs baseline/current is not consistent
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "current MC/DC data should not be defined for deleted line $filename:$line"
@@ -3156,7 +3163,7 @@ sub _categorizeMcdcCov
         } else {
             lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                       "unexpected MC/DC coverage type $type at $filename:$line")
-                if $type ne 'equal';
+                if $type != DiffMap::EQUAL;
             # group might or might not be in both baseline and current
             while (my ($groupSize, $bgroup) = each(%$baseGroups)) {
 
@@ -3236,7 +3243,7 @@ sub _categorizeFunctionCov
         $funcCovLines{$line} = 1;
         lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                  "'current' line $filename:$line should not be marked 'delete'")
-            if $type eq 'delete';
+            if $type == DiffMap::DELETE;
         my $data;
         if (!exists($lineDataMap->{$line})) {
             $data = LineData->new($type);
@@ -3249,7 +3256,7 @@ sub _categorizeFunctionCov
                 unless $data->type() eq $type;
         }
         # we expect that the line number matches...
-        $data->lineNo("current", $line);
+        $data->lineNo("current", $line, $filename);
         # function data for the line
         $data->current_function($func);
         if (!defined($funcBase)) {
@@ -3275,7 +3282,7 @@ sub _categorizeFunctionCov
 
         my $type      = $data->type();
         my $curr_line = $data->lineNo('current');
-        if ($type ne 'delete') {
+        if ($type != DiffMap::DELETE) {
             $funcCovLines{$curr_line} = 1;
         } else {
             # the line has been deleted...just record the data
@@ -3310,21 +3317,23 @@ sub _categorizeFunctionCov
             #   wasn't recognized as a function before (e.g., unused template)
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "$filename:$line: unexpected undef baseline function data for deleted $name"
-            ) if $type eq 'delete';
-            _cloneFunctionEntry($categorized, $curr,
-                                $type eq 'insert' ? 'UNC' : 'UIC',
-                                $type eq 'insert' ? 'GNC' : 'GIC');
+            ) if $type == DiffMap::DELETE;
+            _cloneFunctionEntry($categorized,
+                                $curr,
+                                $type == DiffMap::INSERT ? 'UNC' : 'UIC',
+                                $type == DiffMap::INSERT ? 'GNC' : 'GIC');
         } elsif (!defined($curr)) {
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                 "$filename:$line: unexpected undef current function data for inserted $name"
-            ) if $type eq 'insert';
-            _cloneFunctionEntry($categorized, $base,
-                                $type eq 'delete' ? 'DUB' : 'EUB',
-                                $type eq 'delete' ? 'DCB' : 'ECB');
+            ) if $type == DiffMap::INSERT;
+            _cloneFunctionEntry($categorized,
+                                $base,
+                                $type == DiffMap::DELETE ? 'DUB' : 'EUB',
+                                $type == DiffMap::DELETE ? 'DCB' : 'ECB');
         } else {
             lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
                    "unexpected function coverage type $type at $filename:$line")
-                if $type ne 'equal';
+                if $type != DiffMap::EQUAL;
             # if we know end lines for this function, then check if there
             # have been any changes in the function body.  If any changes,
             # then mark GNC or UNC
@@ -3345,7 +3354,7 @@ sub _categorizeFunctionCov
                     #   - line is new and is source code, OR
                     #   - line is unchanged and was code before and isn't code
                     #      now, or line wasn't code before and is now
-                    if ($type eq 'insert' &&
+                    if ($type == DiffMap::INSERT &&
                         defined($lineCovCurrent->value($line)))   # line is code
                     {
                         $changed = 1;
@@ -3373,7 +3382,8 @@ sub _categorizeFunctionCov
                         # claim a change if line is deleted and was code
                         #  before.
                         # note that we already checked unchanged lines, above
-                        if ($type ne 'equal' &&  # line is in old but not in new
+                        if ($type !=
+                            DiffMap::EQUAL &&    # line is in old but not in new
                             defined($lineCovBase->value($bline))
                             )    # line is code
                         {
@@ -3477,7 +3487,7 @@ use constant {
               TYPE => 2,
 
               _START => 0,
-              _END   => 1
+              _END   => 1,
 };
 
 sub new
@@ -3490,7 +3500,7 @@ sub new
                 [{}, {}],    # def location
                              # element 0: old filename -> line number where this
                              #             entry starts
-                             # element 1: new filename -> line numbern
+                             # element 1: new filename -> line number
                 {}           # unchanged
     ];
     bless $self, $class;
@@ -3588,7 +3598,7 @@ sub recreateBaseline
     my @lines;
     my $didWarn = 0;
     foreach my $chunk (@$diffs) {
-        if ($chunk->[TYPE] eq 'equal') {
+        if ($chunk->[TYPE] == EQUAL) {
             my ($from, $to) = @{$chunk->[NEW]};
             my $f = $from - 1;
             my $t = $to - 1;
@@ -3607,7 +3617,7 @@ sub recreateBaseline
                     push(@lines, $currentSrcLines->[$f++]);
                 }
             }
-        } elsif ($chunk->[TYPE] eq 'delete') {
+        } elsif ($chunk->[TYPE] == DiffMap::DELETE) {
             my $r = $chunk->[OLD];
             for (my $i = $r->[_START]; $i <= $r->[_END]; ++$i) {
                 die("missing baseline line $i")
@@ -3622,8 +3632,128 @@ sub recreateBaseline
     return \@lines;
 }
 
+sub _findChunk
+{
+    my ($lineMap, $vers, $line, $filename) = @_;
+    my $notThis = $vers == OLD ? INSERT : DELETE;
+    # LCOV_EXCL_START
+    my $linearSearch = sub {
+        my $idx = 0;
+        while ($idx <= $#$lineMap) {
+            my $chunk = $lineMap->[$idx];
+            my $v     = $chunk->[$vers];
+            last
+                if ($v->[_START] <= $line &&
+                    $line <= $v->[_END] &&
+                    $chunk->[TYPE] != $notThis);
+            ++$idx;
+        }
+        return $idx;
+    };
+    # LCOV_EXCL_STOP
+
+    # might be marginally faster to use linear search up to some small
+    #   list size threshold...maybe 5 or 10?
+    my $max = $#$lineMap;
+    my $min = 0;
+    my $mid;
+    while (1) {
+        $mid = int(($max + $min) / 2);
+        my $chunk = $lineMap->[$mid];
+        my $d     = $chunk->[$vers];
+        if ($line < $d->[_START]) {
+            $max = $mid;
+        } elsif ($line > $d->[_END]) {
+            $min = $mid;
+        } else {
+            # this entry is in range - match is either this one
+            #  or the one on one side or the other.
+            # There are at most 2 entries for any particular line -
+            #  and there are two only if this is the insert or delete point
+            if ($chunk->[TYPE] != $notThis) {
+                die("bad lookup")
+                    unless (!$lcovutil::debug ||
+                            $mid == &$linearSearch());
+                return $chunk;
+            }
+            # is it one side or the other?
+            die("unexpectedly at beginning") if $mid == 0;
+            my $prev = $lineMap->[$mid - 1];
+            my $pd   = $prev->[$vers];
+            if ($pd->[_END] >= $line) {
+                die("unexpected prev element")
+                    unless ($pd->[_START] <= $line &&
+                            $prev->[TYPE] != $notThis);
+                die("bad lookup 2")
+                    unless (!$lcovutil::debug ||
+                            $mid - 1 == &$linearSearch());
+                return $prev;
+            }
+            if ($mid == $#$lineMap) {
+                # we are at last element
+                die("wrong place")
+                    unless (!$lcovutil::debug ||
+                            &$linearSearch() == $#$lineMap + 1);
+                return undef;
+            }
+            my $next = $lineMap->[$mid + 1];
+            my $nd   = $next->[$vers];
+            die("expected 'next' to match but did not")
+                unless ($nd->[_START] <= $line &&
+                        $nd->[_END] >= $line &&
+                        $next->[TYPE] != $notThis);
+            die("bad lookup 3")
+                unless (!$lcovutil::debug ||
+                        $mid - 1 == &$linearSearch());
+            return $next;
+        }
+        my $diff = $max - $min;
+        if ($diff <= 1) {
+            $mid = $min;
+            $mid = $max
+                if $line > $lineMap->[$min]->[$vers]->[_END];
+            last;
+        }
+    }
+    my $chunk = $lineMap->[$mid];
+    my $d     = $chunk->[$vers];
+    if ($d->[_START] <= $line &&
+        $d->[_END] >= $line) {
+        if ($chunk->[TYPE] != $notThis) {
+            die("bad lookup 4")
+                unless (!$lcovutil::debug ||
+                        $mid == &$linearSearch());
+            return $chunk;
+        }
+        if ($mid <= $#$lineMap) {
+            # we aren't at the last element in the list
+            my $next = $lineMap->[$mid + 1];
+            my $nd   = $next->[$vers];
+            if ($nd->[_START] <= $line &&
+                $nd->[_END] >= $line &&
+                $next->[TYPE] != $notThis) {
+
+                die("wrong lookup")
+                    unless (!$lcovutil::debug ||
+                            $mid + 1 == &$linearSearch());
+                return $next;
+            }
+        }
+    }
+    die("didn't find element for $filename:$line: $#$lineMap")
+        unless (!$lcovutil::debug ||
+                $#$lineMap + 1 == &$linearSearch());
+    return undef;
+}
+
 sub lookup
 {
+    # given line in OLD or NEW version, return corresponding line in the
+    # other.
+    #  $vers == OLD if $line is in OLD (ie., we are looking for NEW
+    #     and
+    #  $vers == NEW if $line is in NEW and we are looking for line number in OLD
+
     my ($self, $file, $vers, $line) = @_;
 
     $file = $self->findName($file);
@@ -3633,20 +3763,22 @@ sub lookup
         return $line;
     }
 
-    my @candidates =
-        grep { $_->[$vers]->[_START] < $line } @{$self->[LINEMAP]->{$file}};
-    # candidates is empty if $line==1 - which is unusual, as there is typically
-    # a comment, copyright notice, #include, or whatever on the first line
-    return $line unless @candidates;
-
-    my $chunk = pop @candidates;
-
-    my $alt = ($vers == OLD) ? NEW : OLD;
-
-    if ($line > $chunk->[$vers]->[_END]) {
-        return ($chunk->[$alt]->[_END] + ($line - $chunk->[$vers]->[_END]));
+    # search from end of list to find last chunk which starts before this line
+    my $lineMap = $self->[LINEMAP]->{$file};
+    my $chunk   = _findChunk($lineMap, $vers, $line, $file);
+    my $alt     = ($vers == OLD) ? NEW : OLD;
+    my $c;
+    if (defined($chunk)) {
+        my $delta = $line - $chunk->[$vers]->[_START];
+        $c = $chunk->[$alt]->[_START] + $delta;
+        $c += $chunk->[TYPE] == DiffMap::DELETE;
+    } else {
+        # this element is past the last chunk
+        my $v     = $lineMap->[-1];
+        my $delta = $line - $v->[$vers]->[_END];
+        $c = $v->[$alt]->[_END] + $delta;
     }
-    return ($chunk->[$alt]->[_START] + ($line - $chunk->[$vers]->[_START]));
+    return $c;
 }
 
 sub type
@@ -3661,39 +3793,56 @@ sub type
             (@main::base_filenames ||
                 $main::diff_filename)
         ) {
-            return "equal";    # categories will be "GIC", "UIC"
+            return DiffMap::EQUAL;    # categories will be "GIC", "UIC"
         } else {
-            return "insert";    # categories will be "GNC", "UNC"
+            return DiffMap::INSERT;    # categories will be "GNC", "UNC"
         }
     }
 
     if (!defined($self->[FILEMAP]->{$file})) {
         #mapping with no filemap when baseline file was deleted
-        return "delete";
+        return DiffMap::DELETE;
     }
+    my $lineMap = $self->[LINEMAP]->{$file};
+    my $chunk   = _findChunk($lineMap, $vers, $line, $file);
+    # '$line' is past the end of the last chunk in the diff -
+    #   all the lines which weren't mentioned must be equal
+    return EQUAL unless defined($chunk);
 
-    # ->{start} equal $line only if beginning of range or omitted in ->{type}
-    my @candidates =
-        grep { $_->[$vers]->[_START] <= $line } @{$self->[LINEMAP]->{$file}};
-    my $chunk = pop @candidates;
-    my $prev  = pop @candidates;
-    while (defined($prev) &&
-           $line >= $prev->[$vers]->[_START] &&
-           $line <= $prev->[$vers]->[_END]) {
-        $chunk = $prev;
-        $prev  = pop @candidates;
-    }
-    if (!defined($chunk)) {
-        warn "DiffMap::type(): got undef chunk at $file, $vers, $line\n";
-        return "undef chunk";
-    }
-    if (!defined($chunk->[TYPE])) {
-        warn "DiffMap::type(): got undef type at $file, $vers, $line\n";
-        return "undef type";
-    }
+    die("DiffMap::type(): got undef type at $file, $vers, $line")
+        unless defined($chunk->[TYPE]);
     return $chunk->[TYPE];
 }
 
+sub find_deleted_line_leader
+{
+    my ($self, $filename, $base_lineNo) = @_;
+
+    # find the 'delete' chunk which contains this line -
+    #   - the line number of the leader in the baseline is the line
+    #     immediately before the first line in the chunk (in the 'OLD'
+    #     section
+    #   - the line number of the leader in the current file is the
+    #     first line in the 'NEW' section of this chunk.
+    #       - we expect the 'NEW' begin and end lines are the same
+    #       - we also expect the leader to be the 'end' line of the
+    #         'NEW' section in the previous chunk.
+
+    my $file    = $self->findName($filename);
+    my $lineMap = $self->[LINEMAP]->{$file};
+    # there must be a lineMap, if we have a deleted line
+    die("lineMap must exist $filename:$base_lineNo") unless defined($lineMap);
+
+    my $chunk = _findChunk($lineMap, OLD, $base_lineNo, $filename);
+    # should not fall off end of 'diff' - there must be a 'delete' chunk
+    die("missing 'delete' chunk in diff $filename:$base_lineNo")
+        unless defined($chunk);
+    die("unexpected chunk $filename:$base_lineNo")
+        unless ($chunk->[TYPE] == DELETE &&
+                $chunk->[NEW]->[_START] == $chunk->[NEW]->[_END]);
+    return $chunk->[NEW]->[_START];
+}
+
 sub compute_deleted_lines
 {
     my ($self, $filename) = @_;
@@ -3702,7 +3851,7 @@ sub compute_deleted_lines
     return undef unless defined($lineMap);
     my %hash;
     foreach my $chunk (@$lineMap) {
-        next unless $chunk->[TYPE] eq 'delete';
+        next unless $chunk->[TYPE] == DELETE;
 
         $hash{$chunk->[NEW]->[_START]} = $chunk->[OLD];
     }
@@ -3988,7 +4137,8 @@ sub _read_udiff
     my $time =
         '[1-9]{1}[0-9]{3}\-[0-9]{2}\-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2}(\.[0-9]*)?( .[0-9]+)?';
     # Parse diff file line by line
-    my $verbose = 0;
+    my $verbose         = 0;
+    my $skipCurrentFile = 0;
     while (<$diffHdl>) {
         chomp($_);
         s/\r//g;
@@ -4010,7 +4160,7 @@ sub _read_udiff
             # === <filename>
             /^=== (.+)$/ && do    # note: filename may contain whitespace
             {
-                if ($filename) {
+                if ($filename && !$skipCurrentFile) {
                     die("not case insensitive")
                         unless !$lcovutil::case_insensitive ||
                         ($filename eq lc($filename));
@@ -4020,6 +4170,10 @@ sub _read_udiff
                 my $file = $1;
                 $file = lcovutil::strip_directories($file, $main::strip);
                 my $key = ReadCurrentSource::resolve_path($file, 1);
+                # skip diff data for excluded files - but skip quietly
+                $skipCurrentFile = TraceFile::skipCurrentFile($key, 'diff');
+                last if $skipCurrentFile;
+
                 $key = lc($key) if $lcovutil::case_insensitive;
                 if (exists($self->[UNCHANGED]->{$key})) {
                     # unchanged entry flag value should be 1
@@ -4047,6 +4201,8 @@ sub _read_udiff
                 last;
             };
 
+            next if $skipCurrentFile;
+
             # Filename of old file:
             # --- <filename>
             /^--- (.+)$/ && do {
@@ -4103,7 +4259,7 @@ sub _read_udiff
                         $.;    # record original name too.. $file_new
                 }
                 # new file - chunk starts here
-                $chunk = _newChunk('equal', 1, 1);
+                $chunk = _newChunk(EQUAL, 1, 1);
                 last;
             };
             # Start of diff block:
@@ -4111,13 +4267,13 @@ sub _read_udiff
             /^\@\@\s+-(\d+),(\d+)\s+\+(\d+),(\d+)\s+\@\@.*$/ && do {
                 if ($1 > ($chunk->[OLD]->[_END])) {
                     # old start skips "equal" lines
-                    if ($chunk->[TYPE] ne "equal") {
+                    if ($chunk->[TYPE] != EQUAL) {
                         if ($filename) {
                             push(@{$self->[LINEMAP]->{$filename}}, $chunk);
                             _printChunk($chunk) if ($verbose);
                         }
                         $chunk =
-                            _newChunk('equal',
+                            _newChunk(EQUAL,
                                       $chunk->[OLD]->[_END] + 1,
                                       $chunk->[NEW]->[_END] + 1);
                     }
@@ -4136,13 +4292,13 @@ sub _read_udiff
                 if ($old_block == 0 && $new_block == 0) {
                     last;
                 }
-                if ($chunk->[TYPE] ne "equal") {
+                if ($chunk->[TYPE] != EQUAL) {
                     if ($filename) {
                         push(@{$self->[LINEMAP]->{$filename}}, $chunk);
                         _printChunk($chunk) if ($verbose);
                     }
                     # next chunk starts right after current one
-                    $chunk = _newChunk('equal',
+                    $chunk = _newChunk(EQUAL,
                                        $chunk->[OLD]->[_END] + 1,
                                        $chunk->[NEW]->[_END] + 1);
                 } else {
@@ -4169,12 +4325,12 @@ sub _read_udiff
                 }
                 $lines->{$baseline_lineno} = $1;
 
-                if ($chunk->[TYPE] ne "delete") {
+                if ($chunk->[TYPE] != DELETE) {
                     if ($filename) {
                         push(@{$self->[LINEMAP]->{$filename}}, $chunk);
                         _printChunk($chunk) if ($verbose);
                     }
-                    $chunk = _newChunk('delete', $baseline_lineno,
+                    $chunk = _newChunk(DELETE, $baseline_lineno,
                                        $chunk->[NEW]->[_END]);
                 } else {
                     $chunk->[OLD]->[_END] = $baseline_lineno;
@@ -4187,12 +4343,12 @@ sub _read_udiff
                 if ($old_block == 0 && $new_block == 0) {
                     last;
                 }
-                if ($chunk->[TYPE] ne "insert") {
+                if ($chunk->[TYPE] != INSERT) {
                     if ($filename) {
                         push(@{$self->[LINEMAP]->{$filename}}, $chunk);
                         _printChunk($chunk) if ($verbose);
                     }
-                    $chunk = _newChunk('insert',
+                    $chunk = _newChunk(INSERT,
                                        $chunk->[OLD]->[_END],
                                        $chunk->[NEW]->[_END] + 1);
                 } else {
@@ -4205,12 +4361,12 @@ sub _read_udiff
                 if ($old_block == 0 && $new_block == 0) {
                     last;
                 }
-                if ($chunk->[TYPE] ne "equal") {
+                if ($chunk->[TYPE] != EQUAL) {
                     if ($filename) {
                         push(@{$self->[LINEMAP]->{$filename}}, $chunk);
                         _printChunk($chunk) if ($verbose);
                     }
-                    $chunk = _newChunk('equal',
+                    $chunk = _newChunk(EQUAL,
                                        $chunk->[OLD]->[_END] + 1,
                                        $chunk->[NEW]->[_END] + 1);
                 } else {
@@ -4944,6 +5100,9 @@ sub _countLineTlaData
     if ($SummaryInfo::tlaLocation{$tla} & 0x1) {
         # skip "DUB' and 'DCB' categories - which are not in current
         #   and thus have no line associated
+        # @todo if desired, we _could_ enhance the 'annotate' callback
+        #       to look up the baseline SHA/CL and get age/author information
+        #       from that.  Seems unnecessary, though.
 
         #lcovutil::info("$l: $tla" . $lineData->in_curr() . "\n");
 
@@ -5049,7 +5208,8 @@ sub _countFunctionTlaData
     # LCOV_EXCL_START
     if (!defined($mergedTla)) {
         lcovutil::ignorable_error($lcovutil::ERROR_INTERNAL,
-                                  "undef TLA for function '" . $func->name() .
+                                  "undef TLA for function '" .
+                                      $func->name() .
                                       "' hit " . $h->[0] . " at line " .
                                       $line . " (" . $lineData->tla() .
                                       ' ' . $lineData->curr_count() . ")");
@@ -5742,9 +5902,10 @@ sub _synthesize
             $suffix = ' ... synthesizing fake content for last ' . $plural;
         }
         lcovutil::ignorable_error($lcovutil::ERROR_RANGE,
-                              $self->path() . ' contains only ' . $currentLast .
-                                  ' lines but coverage data refers to line ' .
-                                  $last_line . $suffix);
+                                $self->path() .
+                                    ' contains only ' . $currentLast .
+                                    ' lines but coverage data refers to line ' .
+                                    $last_line . $suffix);
         $why = 'not long enough';
     }
     # Simulate gcov behavior
@@ -5979,10 +6140,68 @@ sub new
                       {}
     ];
 
-    # sort the worklist so segment will tend to contain files from the same directory
-    foreach my $f (sort $current_data->files()) {
+    # if the worklist is sorted alphabetically, then each segment tends to
+    #   contain files from the same directory.
+    # However, large files (which may take a long time to process) often
+    #   cluster inside the same directory - so a random (hash) order often
+    #   performs better.
+    # Better is to look at profile data from the prior execution of
+    #   this job - say, from last night's regressions - and to distribute
+    #   the files we think will be long-running (...because they ran a long
+    #   the last time) into different segments so they can run in parallel.
+    # We also want to schedule those long-running files so they are executed
+    #   first.  That way, we don't end up starting some long job late in the
+    #   game - and then have to wait a long time for a single task to finish
+    #   (as opposed to it running in parallel with a lot of other stuff).
+    my @predicted;
+    my @unknown;    # no history
+    foreach my $f ($current_data->files()) {
         my $traceInfo = $current_data->data($f);
         my $filename  = ReadCurrentSource::resolve_path($traceInfo->filename());
+
+        my $predicted;
+        if (   defined($lcovutil::profileHistoryCallback)
+            &&
+            defined($predicted =
+                        $lcovutil::profileHistoryCallback->history($filename))
+        ) {
+            push(@predicted, [$f, $filename, $predicted]);
+        } else {
+            # put the ones we don't know about at the start of the list
+            #  - work on them first, trusting that we will have parallelism
+            #    capacity for the ones we know about
+            push(@unknown, [$f, $filename]);
+        }
+    }
+    lcovutil::info(
+          scalar(@unknown) . ' (of ' . (scalar(@predicted) + scalar(@unknown)) .
+              ") files do not appear in \'--history\' profile.\n")
+        if (defined($lcovutil::profileHistoryCallback) &&
+            @unknown);
+
+    # sort the 'predicted' ones based on expected execution time
+    #  tie break alphabetically
+    @predicted =
+        sort({ $b->[2] <=> $a->[2] or $a->[1] cmp $b->[1] } @predicted);
+    # interleave the long-running files so they get distributed between segments
+    my @fileOrder;
+    my $offset = 0;
+    my $idx    = 0;
+    for (my $i = 0; $i <= $#predicted; ++$i) {
+        push(@fileOrder, $predicted[$idx]);
+        $idx += $lcovutil::maxParallelism;
+        if ($idx > $#predicted) {
+            ++$offset;
+            $idx = $offset;
+        }
+    }
+    die("wrong calculation") unless $#predicted == $#fileOrder;
+    push(@fileOrder, @unknown);    # unknown ones to the back
+
+    foreach my $d (@fileOrder) {
+        my ($f, $filename) = @$d;
+        #my $traceInfo = $current_data->data($f);
+        #my $filename  = ReadCurrentSource::resolve_path($traceInfo->filename());
         my ($vol, $parentDir, $file) = File::Spec->splitpath($filename);
         if (!File::Spec->file_name_is_absolute($filename)) {
             if ($parentDir) {
@@ -7998,7 +8217,8 @@ sub write_function_page($$$$$$$$$$$$$$$)
             File::Spec->catfile($rel_dir, "$base_name.func-b.$html_ext");
     }
     html_create(*HTML_HANDLE, $filename);
-    my $pagetitle = "LCOV - $title - " .
+    my $pagetitle =
+        "LCOV - $title - " .
         File::Spec->catfile($trunc_dir, $base_name) . " - functions";
     write_html_prolog(*HTML_HANDLE, $base_dir, $pagetitle);
     write_header(*HTML_HANDLE,
@@ -8031,8 +8251,7 @@ sub process_file($$$$$)
              .
              (($main::diff_filename && $diff_data->containsFile($filename)) ?
                   ' (source code changed)' :
-                  '') .
-             "\n");
+                  ''));
 
     my $base_name = basename($filename);
     my $base_dir  = get_relative_base_path($rel_dir);
@@ -8077,9 +8296,10 @@ sub process_file($$$$$)
                               $diff_data,
                               defined($main::verboseScopeRegexp) &&
                                   $filename =~ m/$main::verboseScopeRegexp/);
-    my $then = Time::HiRes::gettimeofday();
-    $lcovutil::profileData{categorize}{$filename} = $then - $now;
-
+    my $then    = Time::HiRes::gettimeofday();
+    my $elapsed = $then - $now;
+    $lcovutil::profileData{categorize}{$filename} = $elapsed;
+    info(sprintf(" (%0.3fs)\n", $elapsed));
     my $r = "  lines=$lines_found hit=$lines_hit";
     $r .= " functions=$fn_found hit=$fn_hit"
         if $lcovutil::func_coverage && $fn_found != 0;
@@ -8230,6 +8450,7 @@ sub process_file($$$$$)
                 }
             }
             if (%$differentialFunctionMap) {
+                my $funcStart = Time::HiRes::gettimeofday();
                 foreach my $sort_type (@funcview_sortlist) {
                     write_function_page([$differentialFunctionMap,
                                          \%lineCov, \%branchCov, \%mcdcCov,
@@ -8249,6 +8470,10 @@ sub process_file($$$$$)
                                         $sort_type,
                                         $fileSummary);
                 }
+                my $funcEnd = Time::HiRes::gettimeofday();
+                my $elapsed = $funcEnd - $funcStart;
+                $lcovutil::profileData{function_html}{$filename} = $elapsed;
+                #info(sprintf("    function HTML %0.3fs\n", $elapsed));
             }
         }
 
@@ -8285,7 +8510,10 @@ sub process_file($$$$$)
         }
     }
     my $to = Time::HiRes::gettimeofday();
-    $lcovutil::profileData{html}{$filename} = $to - $from;
+    $elapsed = $to - $from;
+    $lcovutil::profileData{html}{$filename} = $elapsed;
+    #info(sprintf("    HTML %0.3fs\n", $elapsed));
+
     return ($testdata, $testfncdata, $testbrdata, $testcase_mcdc);
 }
 
@@ -10831,7 +11059,8 @@ sub get_branch_html($$)
                     my $line = $differentialBranch->line();
                     my $next = $fileDetail->nextBranchTlaGroup($tla, $line);
                     $href =
-                        "<a href=\"#" . (defined($next) ? "L$next" : 'top') .
+                        "<a href=\"#" .
+                        (defined($next) ? "L$next" : 'top') .
                         "\" title=\"TITLE\" class=\"branchTla\">$char</a>";
                     $tlaLinks{$tla} = $href;
                 }
@@ -10929,7 +11158,8 @@ sub get_mcdc_html($$)
             my ($char, $title);
 
             my $expr_name = $expr->expression();
-            $title = ${prefix} . ($sense ? 'True' : 'False') .
+            $title =
+                ${prefix} . ($sense ? 'True' : 'False') .
                 ' sense of expression "' . $expr->expression() . '" ';
             if ($expr->parent()->num_groups() > 1) {
                 $title .= 'in group "' . $expr->groupSize() . '" ';
@@ -10968,7 +11198,8 @@ sub get_mcdc_html($$)
                     my $line = $differentialMcdc->line();
                     my $next = $fileDetail->nextMcdcTlaGroup($tla, $line);
                     $href =
-                        "<a href=\"#" . (defined($next) ? "L$next" : 'top') .
+                        "<a href=\"#" .
+                        (defined($next) ? "L$next" : 'top') .
                         "\" title=\"TITLE\" class=\"mcdcTla\">SENSE</a>";
                     # cache the href as 'next' lookup is moderately expensive
                     $tlaLinks{$tla} = $href;
@@ -12085,7 +12316,8 @@ sub write_header(*$$$$$$$)
     } elsif ($type == $HDR_TESTDESC) {
         # Test description header
         $base_dir = "";
-        $view     = "<a href=\"$base_dir" . "index.$html_ext\">" .
+        $view =
+            "<a href=\"$base_dir" . "index.$html_ext\">" .
             "$overview_title</a> - test case descriptions";
     }
 
@@ -12097,12 +12329,14 @@ sub write_header(*$$$$$$$)
         if ($frames && ($type == $HDR_SOURCE || $type == $HDR_FUNC)) {
             # Need to break frameset when clicking this link
             $test .=
-                " ( <span style=\"font-size:80%;\">" . "<a href=\"$base_dir" .
+                " ( <span style=\"font-size:80%;\">" .
+                "<a href=\"$base_dir" .
                 "descriptions.$html_ext\" target=\"_parent\">" .
                 "view descriptions</a></span> )";
         } else {
             $test .=
-                " ( <span style=\"font-size:80%;\">" . "<a href=\"$base_dir" .
+                " ( <span style=\"font-size:80%;\">" .
+                "<a href=\"$base_dir" .
                 "descriptions.$html_ext\">" . "view descriptions</a></span> )";
         }
     }
@@ -12578,8 +12812,8 @@ sub get_sort_code($$$)
     }
     my $help = " title=\"Click to sort table by $alt\"";
     $alt = "Sort by $alt";
-    return " <span $help class=\"tableHeadSort\">" .
-        $link_start . '<img src="' . $base . $png . '" width=10 height=14 ' .
+    return " <span $help class=\"tableHeadSort\">" . $link_start .
+        '<img src="' . $base . $png . '" width=10 height=14 ' .
         'alt="' . $alt . '"' . $help . ' border=0>' . $link_end . '</span>';
 }
 
@@ -12615,7 +12849,8 @@ sub get_line_code($$$$$$$)
     } elsif ($type == $HEAD_DETAIL_HIDDEN) {
         # Text + link to detail view
         my $help = "title=\"Click to go to per-testcase coverage details\"";
-        my $detail_link = 'index' . $key . $bin_type .
+        my $detail_link =
+            'index' . $key . $bin_type .
             '-detail' . $fileview_sortname[$sort_type] . '.' . $html_ext;
         $result .=
             " ( <a $help " .
@@ -13315,7 +13550,8 @@ sub write_file_table(*$$$$$$)
             my $href = $testname;
             # Insert link to description of available
             if ($test_description{$testname}) {
-                $href = "<a href=\"$base_dir" . "descriptions.$html_ext#T" .
+                $href =
+                    "<a href=\"$base_dir" . "descriptions.$html_ext#T" .
                     escape_id($testname) . "\">" . "$testname</a>";
             }
             write_file_table_detail_entry(*HTML_HANDLE, $base_dir,
@@ -13421,7 +13657,8 @@ sub write_source($$$$$$$$)
         }
         # using 8 characters for line number field, with different
         # foreground/background - to make distinctive
-        $empty .= '  <span class="lineNumWithDelete">' .
+        $empty .=
+            '  <span class="lineNumWithDelete">' .
             (' ' x (8 - 3)) . '...</span>';
         $empty .= ' ' x $main::br_field_width if $showBranches;
         $empty .= ' ' x $main::mcdc_field_width if $showMcdc;
diff --git a/bin/geninfo b/bin/geninfo
index e6521397..2d60569a 100755
--- a/bin/geninfo
+++ b/bin/geninfo
@@ -223,6 +223,7 @@ our $defaultChunkSize;
 our $defaultInterval;
 our %childRetryCounts;
 our @large_files;
+our $callFromLcov;
 
 our $cwd = getcwd();
 chomp($cwd);
@@ -243,6 +244,7 @@ $ENV{"LC_ALL"} = "C";
 
 # retrieve settings from RC file - use these if not overridden on command line
 my %geninfo_opts = ("test-name|t=s"       => \$test_name,
+                    'call-from-lcov'      => \$callFromLcov,
                     "output-filename|o=s" => \$output_filename,
                     "base-directory|b=s"  => \$base_directory,
                     "follow|f"            => \$lcovutil::opt_follow,
@@ -906,15 +908,15 @@ sub add_worklist_entry
 {
     my ($self, $filename, $directory) = @_;
     if (exists($self->[1]->{$filename})) {
-        lcovutil::ignorable_error(
-                                 $lcovutil::ERROR_USAGE,
-                                 "duplicate file $filename in both " .
-                                     $self->[1]->{$filename} . " and $directory"
-                                     .
-                                     (lcovutil::is_ignored(
+        lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
+                                  "duplicate file $filename in both " .
+                                      $self->[1]->{$filename} .
+                                      " and $directory"
+                                      .
+                                      (lcovutil::is_ignored(
                                                        $lcovutil::ERROR_USAGE) ?
-                                          ' (skip latter)' :
-                                          ''));
+                                           ' (skip latter)' :
+                                           ''));
         return;
     }
     $self->[1]->{$filename} = $directory;
@@ -1333,8 +1335,45 @@ sub gen_info(@)
     }
     my @worklist;
     my $serialChunk = [1, []];
-    my $chunk       = [0, []];    # [isSerial, [fileList]]
-    FILE: foreach my $j (@$filelist) {
+    my $nChunks =
+        int(floor((scalar(@$filelist) + $chunkSize - 1) / $chunkSize));
+    for (my $i = 0; $i < $nChunks; ++$i) {
+        push(@worklist, [0, []]);
+    }
+    my $work = $filelist;    # default to the original order
+    if ($lcovutil::profileHistoryCallback &&
+        ($chunkSize > 1 || exists($ENV{LCOV_FORCE_PARALLEL}))) {
+        my (@predicted, @unknown);
+        foreach my $j (@$filelist) {
+            my ($dir, $gcda, $gcno) = @$j;
+            my $key       = defined($gcda) ? $gcda : $gcno;
+            my $k         = substr($key, length($dir) + 1);
+            my $predicted = $lcovutil::profileHistoryCallback->history($k);
+            if (defined($predicted)) {
+                push(@predicted, [$j, $predicted, $key]);
+            } else {
+                # put the ones we don't know about at the start of the list
+                #  - work on them first, trusting that we will have parallelism
+                #    capacity for the ones we know about
+                push(@unknown, $j);
+            }
+        }
+        @predicted =
+            sort({ $b->[1] <=> $a->[1] or $a->[2] cmp $b->[2] } @predicted);
+
+        lcovutil::info(scalar(@unknown) .
+                           ' (of ' . (scalar(@predicted) + scalar(@unknown)) .
+                           ") files do not appear in \'--history\' profile.\n")
+            if (@unknown);
+        if (@predicted) {
+            my @order = map({ $_->[0] } @predicted);
+            # unknown to the back...
+            push(@order, @unknown);
+            $work = \@order;
+        }
+    }
+    my $chunkId = 0;
+    FILE: foreach my $j (@$work) {
         my ($dir, $gcda, $gcno) = @$j;
         foreach my $f ($gcda, $gcno) {
             next unless defined($f);    # might not be a GCDA file
@@ -1345,15 +1384,15 @@ sub gen_info(@)
                 next FILE;
             }
         }
+        die("unexpected chunkId $chunkId") if $chunkId >= $nChunks;
+        my $chunk = $worklist[$chunkId++];
         push(@{$chunk->[1]}, $j);
-        if (scalar(@{$chunk->[1]}) == $chunkSize) {
-            push(@worklist, $chunk);
-            $chunk = [0, []];
+        if ($chunkId == $nChunks) {
+            $chunkId = 0;
         }
     }    #foreach DATA_FILE
-    push(@worklist, $chunk) if @{$chunk->[1]};
-    # serial chunk is at the top of the stack - so serial processing
-    #  happens before we fork multiple processes
+         # serial chunk is at the top of the stack - so serial processing
+         #  happens before we fork multiple processes
     push(@worklist, $serialChunk)
         if (@{$serialChunk->[1]});
 
diff --git a/bin/get_version.sh b/bin/get_version.sh
index e515231b..3d158527 100755
--- a/bin/get_version.sh
+++ b/bin/get_version.sh
@@ -25,7 +25,7 @@ else
 fi
 
 # Fallback
-[ -z "$VERSION" ] && VERSION="2.4"
+[ -z "$VERSION" ] && VERSION="2.4.1"
 [ -z "$RELEASE" ] && RELEASE="beta"
 [ -z "$FULL" ]    && FULL="$VERSION-$RELEASE"
 
diff --git a/bin/lcov b/bin/lcov
index 4e1fef23..7d3eeaba 100755
--- a/bin/lcov
+++ b/bin/lcov
@@ -821,6 +821,7 @@ sub lcov_geninfo(@)
     @param = (File::Spec->catfile($tool_dir, 'geninfo'), @dir);
     # make things less confusing for user, by using the name they actually invoked
     push(@param, '--toolname', $lcovutil::tool_name);
+    push(@param, '--call-from-lcov');
     if ($output_filename) {
         push(@param, "--output-filename", $output_filename);
     }
@@ -914,6 +915,9 @@ sub lcov_geninfo(@)
     # memory has not been multiplied by Mb yet - so just pass the integer value
     push(@param, '--memory', $lcovutil::maxMemory)
         if defined($lcovutil::maxMemory);
+    # function coverage was enabled by default
+    push(@param, '--no-function-coverage')
+        if (!$lcovutil::func_coverage);
     push(@param, "--branch-coverage") if $lcovutil::br_coverage;
     push(@param, "--mcdc") if $lcovutil::mcdc_coverage;
     push(@param, '--fail-under-lines', $lcovutil::fail_under_lines)
@@ -935,6 +939,9 @@ sub lcov_geninfo(@)
                          ['--version-script', \@lcovutil::extractVersionScript],
                          ['--resolve-script', \@lcovutil::resolveCallback],
                          ['--substitute', \@lcovutil::file_subst_patterns],
+                         ['--history-script',
+                          \@lcovutil::profileHistoryCallback
+                         ],
                          ['--omit-lines', \@lcovutil::omit_line_patterns],
                          ['--erase-functions',
                           \@lcovutil::exclude_function_patterns
diff --git a/example/Makefile b/example/Makefile
index 51a5ca78..d88e4317 100644
--- a/example/Makefile
+++ b/example/Makefile
@@ -83,7 +83,7 @@ gauss.o: methods/gauss.c gauss.h
 output: example descriptions test_noargs test_2_to_2000 test_overflow test_differential
 	@echo
 	@echo '*'
-	@echo '* Generating HTML output'
+	@echo '* Generating HTML output from data captured from tests'
 	@echo '*'
 	@echo
 	$(GENHTML) trace_noargs.info trace_args.info trace_overflow.info \
@@ -95,7 +95,7 @@ output: example descriptions test_noargs test_2_to_2000 test_overflow test_diffe
 	@echo '* See '`pwd`/output/index.html
 	@echo '*'
 	@echo
-	@echo "Generate HTML with hierarchical report and additional navigation features"
+	@echo "Generate HTML with hierarchical report and additional navigation features (same testcase data)"
 	@echo '*'
 	$(GENHTML) trace_noargs.info trace_args.info trace_overflow.info \
 		   --output-directory hierarchical \
@@ -175,7 +175,8 @@ test_differential:
 
 	@echo "Step 2: Capture initial coverage"
 	(cd $(REPO) ; \
-	$(LCOV) --capture -o baseline.info -d . --version-script $(SCRIPTDIR)/gitversion.pm )
+	$(LCOV) --capture -o baseline.info -d . \
+		--version-script $(SCRIPTDIR)/gitversion.pm --profile )
 
 	@echo "Step 3: Modify source code"
 	@(cd $(REPO) ; \
@@ -192,27 +193,34 @@ test_differential:
 	./example )
 
 	@echo "Step 5: Capture new coverage (after source changes)"
+	@echo "   (note that this example is too tiny for history reuse"
+	@echo "   to affect runtime performance)"
 	(cd $(REPO) ; \
-	 $(LCOV) --capture -o current.info -d . --version-script $(SCRIPTDIR)/gitversion.pm )
+	$(LCOV) --capture -o current.info -d .              \
+		--version-script $(SCRIPTDIR)/gitversion.pm \
+		--profile                                   \
+		--history $(SCRIPTDIR)/history.pm,baseline.info.json )
 	@echo "Compute source diffs"
 	(cd $(REPO) ;                                          \
 	$(SCRIPTDIR)/gitdiff -b . `git rev-list -n 1 baseline` \
 	  `git rev-list -n 1 current` > udiff.txt )
 
 	@echo "Step 6: Generate differential coverage report"
-	@echo "        (caching revision control data may result in improved runtime performance)"
+	@echo "      - caching revision control data may result in improved runtime performance)"
+	@echo "      - capture 'genhtml' execution profile that we can use later"
 	(cd $(REPO) ;                                            \
 	$(GENHTML) -o differential --baseline-file baseline.info \
 	  --diff-file udiff.txt --show-owners                    \
 	  --title "Differential coverage example"                \
 	  --annotate $(SCRIPTDIR)/gitblame.pm,--cache,./my_cache \
-	  --version-script $(SCRIPTDIR)/gitversion.pm            \
+	  --version-script $(SCRIPTDIR)/gitversion.pm --profile  \
 	  -- current.info )
 
 	@echo "point your browser to `realpath $(REPO)`/differential/index.html"
 
 	@echo "Step 7: Generate subset report for code review:"
-	@echo "        (reuse revision control data cached in previous step)"
+	@echo "      - reuse execution profile and cached revision control data"
+	@echo "        captured in previous step"
 	(cd $(REPO) ;                                            \
 	$(GENHTML) -o review --baseline-file baseline.info       \
 	  --diff-file udiff.txt --show-owners                    \
@@ -221,7 +229,9 @@ test_differential:
 	  --version-script $(SCRIPTDIR)/gitversion.pm            \
 	  --select-script $(SCRIPTDIR)/select.pm                 \
 	  --select-script --tla --select-script UNC,UIC,LBC      \
-	  -- current.info )
+	  --history-script $(SCRIPTDIR)/history.pm               \
+	  --history-script ./differential/genhtml.json --profile \
+          -- current.info )
 
 	@echo "point your browser to `realpath $(REPO)`/review/index.html"
 
diff --git a/example/README b/example/README
index 6c3facbc..d0cf08cb 100644
--- a/example/README
+++ b/example/README
@@ -66,5 +66,16 @@ Code review:
       - Real use cases are likely to use more sophisticated select-script
         callbacks (e.g., to select from a range of changelists).
 
+      - The example uses caching and profile history to improve runtime
+        performance - see the man pages for a more detailed description
+        of the features.  There is no effect with a tiny example - but
+        a real project may see benefit.
+        The 'spreadsheet.py' application script can be used to convert
+        JSON profile files into more readable excel spreadsheets.  This
+        can be useful to see the effect (if any) of the caching and/or
+        history features, and can show where time is spent for your example.
+        This can be helpful, to suggest opportunities to optimize the LCOV
+        implementation.
+
 Feel free to edit the Makefile or to run the lcov utilities directly,
 to see the effect of other options that you find in the lcov man pages.
diff --git a/lib/lcovutil.pm b/lib/lcovutil.pm
index 7f55c4ff..8fe91f81 100644
--- a/lib/lcovutil.pm
+++ b/lib/lcovutil.pm
@@ -490,7 +490,12 @@ our %pngMap = ('=' => ['CBC', 'LBC']
 our @opt_rc;        # list of command line RC overrides
 
 our %profileData;
-our $profile;       # the 'enable' flag/name of output file
+our $profile;    # the 'enable' flag/name of output file
+# historical profile - optimize performance by somewhat carefully sorting
+# job list - use callback mechanism to provide more configurable support
+# for complex build environments
+our @profileHistoryCallback;
+our $profileHistoryCallback;
 
 # need to defer any errors until after the options have been
 #  processed as user might have suppressed the error we were
@@ -998,6 +1003,8 @@ sub configure_callback
             #$package->import(qw(new));
             # the first value in @_ is the script name
             $$cb = $class->new(@args);
+            die("callback constructor returned 'undef'")
+                unless defined($$cb);
             if (exists($ENV{LCOV_FORCE_PARALLEL}) ||
                 (defined($lcovutil::maxParallelism) &&
                     1 != $lcovutil::maxParallelism)
@@ -1030,7 +1037,8 @@ sub configure_callback
         # not module
         $$cb = ScriptCaller->new(@args);
     }
-    push(@configured_callbacks, $cb);
+    push(@configured_callbacks, $cb)
+        if defined($$cb);
 }
 
 sub cleanup_callbacks
@@ -1258,8 +1266,9 @@ our %argCommon = ("tempdir=s"         => \$tempdirname,
                   "config-file=s"          => \@unsupported_config,
                   "rc=s%"                  => \%unsupported_rc,
                   "profile:s"              => \$lcovutil::profile,
-                  "exclude=s"              => \@lcovutil::exclude_file_patterns,
-                  "include=s"              => \@lcovutil::include_file_patterns,
+                  'history-script=s'  => \@lcovutil::profileHistoryCallback,
+                  "exclude=s"         => \@lcovutil::exclude_file_patterns,
+                  "include=s"         => \@lcovutil::include_file_patterns,
                   "erase-functions=s" => \@lcovutil::exclude_function_patterns,
                   "omit-lines=s"      => \@lcovutil::omit_line_patterns,
                   "substitute=s"      => \@lcovutil::file_subst_patterns,
@@ -1325,24 +1334,13 @@ my @include_stack;
 sub read_config($$)
 {
     my ($filename, $opts) = @_;
-    my $key;
-    my $value;
-    local *HANDLE;
-
     my $set_value = 0;
     info(1, "read_config: $filename\n");
-    my $path = abs_path($filename);
-    if (!defined($path)) {
-        lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
-                                  "config file '$filename' does not exist");
-        # as below - this line is unreachable because we can't ignore
-        # the message due to order of processing - see below.
-        return 0;    # LCOV_UNREACHABLE_LINE
-    } elsif (exists($included_config_files{$path})) {
+    my $f;
+    eval { $f = InOutFile->in($filename); };
+    if ($@) {
         lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
-                                  'config file inclusion loop detected: "' .
-                                      join('" -> "', @include_stack) .
-                                      '" -> "' . $filename . '"');
+                              "cannot read configuration file '$filename': $!");
         # this line is unreachable as we can't ignore the 'usage' error
         #   because it is generated when we parse the config-file options
         #   but the '--ignore-errors' option isn't parsed until later, after
@@ -1350,18 +1348,21 @@ sub read_config($$)
         # This could be fixed by doing some early processing on the command
         #   line (similar to how config file options are handled) - but that
         #   seems like overkill.  Just force the user to fix the issues.
-        return 0;    # LCOV_UNREACHABLE_LINE
+        return 0;    # didn't set anything LCOV_UNREACHABLE_LINE
     }
-
-    if (!open(HANDLE, "<", $filename)) {
+    my $path = abs_path($filename);
+    die("abs_path returned undef for $filename") unless defined($path);
+    if (exists($included_config_files{$path})) {
         lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
-                              "cannot read configuration file '$filename': $!");
-        # similarly, this line is also unreachable for the same reasons as
-        #   described above.
-        return 0;    # didn't set anything LCOV_UNREACHABLE_LINE
+                                  'config file inclusion loop detected: "' .
+                                      join('" -> "', @include_stack) .
+                                      '" -> "' . $filename . '"');
+        return 0;    # LCOV_UNREACHABLE_LINE
     }
     $included_config_files{$path} = 1;
     push(@include_stack, $filename);
+
+    local *HANDLE = $f->hdl();
     VAR: while (<HANDLE>) {
         chomp;
         # Skip comments
@@ -1371,7 +1372,7 @@ sub read_config($$)
         # Remove trailing blanks
         s/\s+$//;
         next unless length;
-        ($key, $value) = split(/\s*=\s*/, $_, 2);
+        my ($key, $value) = split(/\s*=\s*/, $_, 2);
         # is this an environment variable?
         while (defined($value) &&
                $value =~ /\$ENV\{([^}]+)\}/) {
@@ -1413,7 +1414,6 @@ sub read_config($$)
                 ]);
         }
     }
-    close(HANDLE) or die("unable to close $filename: $!\n");
     delete $included_config_files{$path};
     pop(@include_stack);
     return $set_value;
@@ -1591,12 +1591,22 @@ sub parseOptions
     $lcovutil::verify_checksum = 0
         if (defined($rc_no_checksum));
 
+    # Determine which errors the user wants us to ignore
+    parse_ignore_errors(@opt_ignore_errors);
+
+    # if lcov --capture:  no further initialization required - is handled
+    #   in geninfo call
+    return 1 if $lcov_capture;
+
     foreach my $cb ([\$versionCallback, \@lcovutil::extractVersionScript],
                     [\$resolveCallback, \@lcovutil::resolveCallback],
                     [\$CoverageCriteria::criteriaCallback,
                      \@CoverageCriteria::coverageCriteriaScript
                     ],
                     [\$contextCallback, \@lcovutil::contextCallback],
+                    [\$profileHistoryCallback,
+                     \@lcovutil::profileHistoryCallback
+                    ],
     ) {
         lcovutil::configure_callback($cb->[0], @{$cb->[1]})
             if (@{$cb->[1]});
@@ -1623,35 +1633,30 @@ sub parseOptions
     $lcovutil::profile = ''
         if ($contextCallback && !defined($lcovutil::profile));
 
-    if (!$lcov_capture) {
-        if ($lcovutil::compute_file_version &&
-            !defined($versionCallback)) {
-            lcovutil::ignorable_warning($lcovutil::ERROR_USAGE,
-                "'compute_file_version=1' option has no effect without either '--version-script' or 'version_script=...'."
-            );
-        }
-        lcovutil::munge_file_patterns();
-        lcovutil::init_parallel_params();
-        # Determine which errors the user wants us to ignore
-        parse_ignore_errors(@opt_ignore_errors);
-        parse_expected_message_counts(@opt_expected_message_counts);
-        # Determine what coverpoints the user wants to filter
-        push(@opt_filter, 'exception') if $lcovutil::exclude_exception_branch;
-        parse_cov_filters(@opt_filter);
-
-        # Ensure that the c++filt tool is available when using --demangle-cpp
-        lcovutil::do_mangle_check();
-
-        foreach my $entry (@deferred_rc_errors) {
-            my ($isErr, $type, $msg) = @$entry;
-            if ($isErr) {
-                lcovutil::ignorable_error($type, $msg);
-            } else {
-                lcovutil::ignorable_warning($type, $msg);
-            }
-        }
+    if ($lcovutil::compute_file_version &&
+        !defined($versionCallback)) {
+        lcovutil::ignorable_warning($lcovutil::ERROR_USAGE,
+            "'compute_file_version=1' option has no effect without either '--version-script' or 'version_script=...'."
+        );
     }
+    lcovutil::munge_file_patterns();
+    lcovutil::init_parallel_params();
+    parse_expected_message_counts(@opt_expected_message_counts);
+    # Determine what coverpoints the user wants to filter
+    push(@opt_filter, 'exception') if $lcovutil::exclude_exception_branch;
+    parse_cov_filters(@opt_filter);
+
+    # Ensure that the c++filt tool is available when using --demangle-cpp
+    lcovutil::do_mangle_check();
 
+    foreach my $entry (@deferred_rc_errors) {
+        my ($isErr, $type, $msg) = @$entry;
+        if ($isErr) {
+            lcovutil::ignorable_error($type, $msg);
+        } else {
+            lcovutil::ignorable_warning($type, $msg);
+        }
+    }
     return 1;
 }
 
@@ -1785,14 +1790,24 @@ sub munge_file_patterns
 sub warn_pattern_list
 {
     my ($type, $patterns) = @_;
+    my $unused = 0;
     foreach my $pat (@$patterns) {
         my $count = $pat->[-1];
         if (0 == $count) {
             my $str = $pat->[-2];
             lcovutil::ignorable_error($ERROR_UNUSED,
                                       "'$type' pattern '$str' is unused.");
+            ++$unused;
         }
     }
+    if ($unused) {
+        lcovutil::ignorable_error($ERROR_UNUSED,
+                                  "$unused of " .
+                                      scalar(@$patterns) .
+                                      " '$type' pattern" .
+                                      (scalar(@$patterns) == 1 ? '' : 's') .
+                                      ' were never applied.');
+    }
 }
 
 sub warn_file_patterns
@@ -1927,7 +1942,8 @@ sub summarize_messages
     foreach my $type ('error', 'warning', 'ignore') {
         next unless $total{$type};
         $found = 1;
-        my $leader = $header . '  ' . $total{$type} . " $type message" .
+        my $leader =
+            $header . '  ' . $total{$type} . " $type message" .
             ($total{$type} > 1 ? 's' : '') . ":\n";
         my $h = $message_types{$type};
         foreach my $k (sort keys %$h) {
@@ -2586,7 +2602,8 @@ sub summarize_cov_filters
         next if 0 == $histogram->[-2];
         my $points = '';
         if ($histogram->[-2] != $histogram->[-1]) {
-            $points = '    ' . $histogram->[-1] . ' coverpoint' .
+            $points =
+                '    ' . $histogram->[-1] . ' coverpoint' .
                 ($histogram->[-1] > 1 ? 's' : '') . "\n";
         }
         info(-1,
@@ -3403,7 +3420,6 @@ sub annotate
 sub check_criteria
 {
     my ($self, $name, $type, $data) = @_;
-
     my $iter =
         $self->pipe('criteria', $name, $type, JsonSupport::encode($data));
     return (0) unless $iter;    # constructor will have given error message
@@ -3420,7 +3436,6 @@ sub check_criteria
 sub select
 {
     my ($self, $lineData, $annotateData, $filename, $lineNo) = @_;
-
     my @params = ('select',
                   defined($lineData) ?
                       JsonSupport::encode($lineData->to_list()) : '',
@@ -3434,19 +3449,32 @@ sub select
 sub simplify
 {
     my ($self, $func) = @_;
-
     my $name;
     my $pipe = $self->pipe('simplify', $func);
     die("broken 'simplify' callback")
         unless ($pipe &&
                 ($name = $pipe->next()));
-
     chomp($name);
     $name =~ s/\r//;
     lcovutil::info(1, "  simplify: $name\n");
     return $name;
 }
 
+sub history
+{
+    my ($self, $item) = @_;
+
+    my $time;
+    my $pipe = $self->pipe('history', $item);
+    die("broken 'history' callback")
+        unless ($pipe &&
+                ($time = $pipe->next()));
+    chomp($time);
+    $time =~ s/\r//;
+    lcovutil::info(1, "  history: $item = $time\n");
+    return $time eq '' ? undef : $time;
+}
+
 package JsonSupport;
 
 our $rc_json_module = 'auto';
@@ -4143,9 +4171,9 @@ sub merge
     # return 1 if something changed, 0 if nothing new covered or discovered
     my ($self, $that, $filename, $line) = @_;
     # should have called 'iscompatible' first
-    die('attempt to merge incompatible expressions for id' .
-        $self->id() . ', ' . $that->id() .
-        ": '" . $self->exprString() . "' -> '" . $that->exprString() . "'")
+    die("$filename:$line: attempt to merge incompatible expressions for id" .
+        $self->id() . ', ' . $that->id() . ": '" .
+        $self->exprString() . "' -> '" . $that->exprString() . "'")
         if ($self->exprString() ne $that->exprString());
 
     if ($self->is_exception() != $that->is_exception()) {
@@ -4656,7 +4684,8 @@ sub merge
 {
     my ($self, $that) = @_;
     lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
-                              $self->name() . " has different location than " .
+                              $self->name() .
+                                  " has different location than " .
                                   $that->name() . " during merge")
         if ($self->line() != $self->line());
     while (my ($name, $count) = each(%{$that->[ALIASES]})) {
@@ -4831,7 +4860,8 @@ sub define_function
         ) {
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
                                       "mismatched end line for $fnName at " .
-                                          $self->filename() . ":$start_line: "
+                                          $self->filename() .
+                                          ":$start_line: "
                                           .
                                           (defined($data->end_line()) ?
                                                $data->end_line() : 'undef') .
@@ -4941,8 +4971,8 @@ sub union
         my $thisData;
         if (!exists($myData->{$key})) {
             $thisData =
-                $self->define_function($thatData->name(),
-                                      $thatData->line(), $thatData->end_line());
+                $self->define_function($thatData->name(), $thatData->line(),
+                                       $thatData->end_line());
             $changed = 1;    # something new...
         } else {
             $thisData = $myData->{$key};
@@ -6300,7 +6330,8 @@ sub parseLines
                 lcovutil::ignorable_error($lcovutil::ERROR_MISMATCH,
                            "$filename: overlapping exclude directives. Found " .
                                $d->[4] .
-                               " at line $line - but no matching " . $d->[5] .
+                               " at line $line - but no matching " .
+                               $d->[5] .
                                ' for ' . $d->[4] . ' at line ' . $$ref->[0])
                     if $$ref;
                 $$ref = [$line, $reason, $d->[4], $d->[5]];
@@ -6309,7 +6340,8 @@ sub parseLines
                 lcovutil::ignorable_error($lcovutil::ERROR_MISMATCH,
                               "$filename: found " . $d->[5] .
                                   " directive at line $line without matching " .
-                                  ($$ref ? $$ref->[2] : $d->[4]) . ' directive')
+                                  ($$ref ? $$ref->[2] : $d->[4]) .
+                                  ' directive')
                     unless $$ref &&
                     $$ref->[2] eq $d->[4] &&
                     $$ref->[3] eq $d->[5];
@@ -6531,16 +6563,17 @@ sub isExcluded
                     lcovutil::message_count($lcovutil::ERROR_RANGE) == 0
             ]);
         lcovutil::store_deferred_message(
-              $lcovutil::ERROR_RANGE,
-              1, $key,
-              "unknown line '$lineNo' in " . $self->filename()
-                  .
-                  (
-                  defined($data->[EXCLUDE]) ?
-                      (" there are only " .
-                       scalar(@{$data->[EXCLUDE]}) . " lines in the file.") :
-                      "") .
-                  $suffix) if lcovutil::warn_once($lcovutil::ERROR_RANGE, $key);
+                                $lcovutil::ERROR_RANGE,
+                                1, $key,
+                                "unknown line '$lineNo' in " . $self->filename()
+                                    .
+                                    (defined($data->[EXCLUDE]) ?
+                                         (" there are only " .
+                                          scalar(@{$data->[EXCLUDE]}) .
+                                          " lines in the file.") :
+                                         "") .
+                                    $suffix
+        ) if lcovutil::warn_once($lcovutil::ERROR_RANGE, $key);
         return 0;    # even though out of range - this is not excluded by filter
     }
     my $reason;
@@ -6991,8 +7024,13 @@ sub print_summary
 
 sub skipCurrentFile
 {
-    my $filename = shift;
+    my ($filename, $fileTypeName) = @_;
 
+    if (defined($fileTypeName)) {
+        $fileTypeName .= ' ';
+    } else {
+        $fileTypeName = '';
+    }
     my $filt = $lcovutil::cov_filter[$lcovutil::FILTER_MISSING_FILE];
     if ($filt) {
         my $missing = !-r $filename;
@@ -7005,7 +7043,8 @@ sub skipCurrentFile
 
         if ($missing) {
             lcovutil::info(
-                   "Excluding \"$filename\": does not exist/is not readable\n");
+                "Excluding $fileTypeName\"$filename\": does not exist/is not readable\n"
+            );
             ++$filt->[-2];
             ++$filt->[-1];
             return 1;
@@ -7016,7 +7055,8 @@ sub skipCurrentFile
     foreach my $p (@lcovutil::exclude_file_patterns) {
         my $pattern = $p->[0];
         if ($filename =~ $pattern) {
-            lcovutil::info(1, "exclude $filename: matches '" . $p->[1] . "\n");
+            lcovutil::info(1,
+                  "exclude $fileTypeName$filename: matches '" . $p->[1] . "\n");
             ++$p->[-1];
             return 1;    # all done - explicitly excluded
         }
@@ -7026,13 +7066,15 @@ sub skipCurrentFile
             my $pattern = $p->[0];
             if ($filename =~ $pattern) {
                 lcovutil::info(1,
-                              "include: $filename: matches '" . $p->[1] . "\n");
+                               "include: $fileTypeName$filename: matches '" .
+                                   $p->[1] . "\n");
                 ++$p->[-1];
                 return 0;    # explicitly included
             }
         }
-        lcovutil::info(1, "exclude $filename: no include matches\n");
-        return 1;            # not explicitly included - so exclude
+        lcovutil::info(1,
+                       "exclude $fileTypeName$filename: no include matches\n");
+        return 1;    # not explicitly included - so exclude
     }
     return 0;
 }
@@ -7236,9 +7278,8 @@ sub _eraseFunctions
                         lcovutil::info(
                                   1 + (0 == $isMasterList),
                                   "exclude FN $name line range $source_file:[" .
-                                      $fcn->line() .
-                                      ":$end_line] due to '" . $p->[-2] . "'\n"
-                        );
+                                      $fcn->line() . ":$end_line] due to '" .
+                                      $p->[-2] . "'\n");
                     }
                     _eraseFunction($fcn, $alias, $end_line,
                                    $source_file, $functionMap, $lineData,
@@ -7379,9 +7420,10 @@ sub _deriveFunctionEndLines
                 lcovutil::explain_once('derive_end_line',
                       "  See lcovrc man entry for 'derive_function_end_line'.");
             lcovutil::ignorable_error($lcovutil::ERROR_INCONSISTENT_DATA,
-                                '"' . $func->filename() . '":' . $func->line() .
-                                    ': failed to set end line for function ' .
-                                    $func->name() . '.' . $suffix);
+                                  '"' .
+                                      $func->filename() . '":' . $func->line() .
+                                      ': failed to set end line for function ' .
+                                      $func->name() . '.' . $suffix);
             next FUNC;
         }
 
@@ -8659,8 +8701,8 @@ sub _read_info
     }
 
     # Check for .gz extension
-    my $inFile  = InOutFile->in($tracefile, $lcovutil::demangle_cpp_cmd);
-    my $infoHdl = $inFile->hdl();
+    my $inFile = InOutFile->in($tracefile, $lcovutil::demangle_cpp_cmd);
+    local *INFO = $inFile->hdl();
 
     $testname = "";
     my $fileData;
@@ -8678,7 +8720,7 @@ sub _read_info
     my %excludedFunction;
     my $skipCurrentFile = 0;
     my %fnIdxMap;
-    while (<$infoHdl>) {
+    while (<INFO>) {
         chomp($_);
         my $line = $_;
         $line =~ s/\s+$//;    # whitespace
diff --git a/man/genhtml.1 b/man/genhtml.1
index 0990f9b8..179dec58 100644
--- a/man/genhtml.1
+++ b/man/genhtml.1
@@ -99,7 +99,10 @@ genhtml \- Generate HTML view from LCOV coverage data files
 .IR config\-file ]
 .br
 .RB [ \-\-profile
-.IR [profile\-file ]
+.IR [profile\-file ]]
+.br
+.RB [ \-\-history-script
+.IR callback ]
 .br
 .RB [ \-\-rc
 .IR keyword = value ]
@@ -439,6 +442,8 @@ following options:
 .br
 .B \-\-version\-script
 .br
+.B \-\-history\-script
+.br
 .RE
 
 While each script performs a separate function there are some common aspects
@@ -626,6 +631,37 @@ and does not modify information in the coveage DB.
 
 .PP
 
+.IP history\-script
+$cpu_seconds = $callback_obj->history($element_name)
+.br
+.br
+
+where
+.I $cpu_seconds
+is the predicted time taken to process
+.I $element_name
+or
+.I undef
+if there is no prediction/the element is unknown.
+
+.br
+See the sample implementation
+.RS
+\*[scriptdir]/history.pm,
+which uses the
+.I "\-\-profile"
+data generated by a previous
+.B genhtml
+execution to predict the time required this time.
+.br
+The prediction may improve load balancing - and thus improve overall
+runtime performance (
+.I i.e.,
+because we won't be waiting for some "long pole" task to complete while
+all other threads are idle.
+
+.PP
+
 .RE
 
 .IP 2. 3
@@ -2287,6 +2323,13 @@ and
 .I fork_fail_timeout
 entries in man
 .B lcovrc(5).
+.br
+A previously generated execution profile may help to enable better utilization
+and faster parallel execution.  See the
+.I "\-\-profile"
+and
+.I "\-\-history\-script"
+sections of this man page.
 
 
 .RE
@@ -3415,9 +3458,35 @@ If the optional
 is not specified, then the profile data is written to a file named
 .I "genhtml.json"
 in the output directory.
+.br
+Profile data is useful if you are trying to optimize the
+.B lcov
+implementation (see
+.B $LCOV_ROOT/share/lcov/support-scripts/spreadsheet.py
+), and can also enable faster 'genhtml --parallel' execution (see the
+"\-\-history\-script" section of this man page).
+
 
 .RE
 
+.BI "\-\-history-script" script
+.br
+.RS
+Tell the tool to use performance data from a prior job to predict resource usage by the current job.
+This may allow better segmentation to enable more balanced workloads between parallel threads - thus improving wall clock execution time.
+
+A common source for the
+.I "previous\-profile\-data\-file"
+is the profile data generated by a prior regression suite execution.
+See
+.B $LCOV_ROOT/share/lcov/example
+and
+.B $LCOV_ROOT/share/lcov/support-scripts/history.pm
+in the installed release (or
+.B .../example
+in the source repository).
+.RE
+
 
 .B \-\-rc
 .IR keyword = value
diff --git a/man/geninfo.1 b/man/geninfo.1
index d5a9892f..70fd9ed5 100644
--- a/man/geninfo.1
+++ b/man/geninfo.1
@@ -88,6 +88,9 @@ geninfo \- Generate tracefiles from GCOV coverage data files
 .RB [ \-\-profile
 .IR [ profile\-file ] ]
 .br
+.RB [ \-\-history\-script
+.IR callback ]
+.br
 .RB [ \-\-derive\-func\-data ]
 .RB [ \-\-compat
 .IR  mode =on|off|auto]
@@ -652,9 +655,45 @@ basename as the
 .I \-\-output\-filename, with suffix
 .I ".json"
 appended.
+.br
+Profile data is useful if you are trying to optimize the
+.B geninfo
+implementation (see
+.B $LCOV_ROOT/share/lcov/support-scripts/spreadsheet.py
+), and can also enable faster 'geninfo --parallel' execution (see the
+"\-\-history\-script" section of this man page).
+
+.RE
+
+
+.B \-\-history\-script
+.I callback
+.br
+.RS
+Tell the tool to use performance data from a prior job to predict resource usage by the current job.
+This may allow better segmentation to enable more balanced workloads between parallel threads - thus improving wall clock execution time.
 
+A common source for the performance history is the
+.I "previous\-profile\-data\-file"
+generated by the
+.I "\-\-profile"
+argument.
+
+See a sample callback implementation in
+.B $LCOV_ROOT/share/lcov/support\-scripts/history.pm
+and its use in
+.B $LCOV_ROOT/share/lcov/example
+in the installed release (or
+.B .../example
+and
+.B .../scripts/history.pm
+in the source repository).
 .RE
 
+See man
+.B genhtml(1)
+for more details.
+
 
 .B \-\-derive\-func\-data
 .br
@@ -1389,7 +1428,6 @@ option described below may be necessary to enable parallelism to succeed
 in the presence of data files which consume excessive memory in
 .B gcov.
 
-
 Also see the
 .I memory, memory_percentage, max_fork_fails, fork_fail_timeout, geninfo_chunk_size
 and
@@ -1397,6 +1435,13 @@ and
 entries in man
 .B lcovrc(5)
 for a description of some options which may aid in parameter tuning and performance optimization.
+.br
+A previously generated execution profile may help to enable better utilization
+and faster parallel execution.  See the
+.I "\-\-profile"
+and
+.I "\-\-history\-script"
+sections of this man page.
 
 .RE
 .BI "\-\-large\-file "
diff --git a/man/lcov.1 b/man/lcov.1
index 6838d0a5..2f637f3b 100644
--- a/man/lcov.1
+++ b/man/lcov.1
@@ -96,6 +96,9 @@ format is described in man
 .RB [ \-\-version\-script
 .IR script_file ]
 .br
+.RB [ \-\-history\-script
+.IR callback ]
+.br
 .RB [ \-\-comment
 .IR comment_string ]
 .br
@@ -103,7 +106,7 @@ format is described in man
 .IR regexp ]
 .br
 .RE
-.RE
+RE
 
 Generate tracefile (from compiler-generated data) with all counter values set to zero:
 .br
@@ -260,7 +263,6 @@ such that coverpoints found only in the set on the left will be retained and all
 .RE
 
 
-
 Summarize tracefile content:
 .br
 
@@ -704,6 +706,29 @@ basename as the
 .I \-\-output\-filename, with suffix
 .I ".json"
 appended.
+Profile data is useful if you are trying to optimize the
+.B lcov
+implementation (see
+.B $LCOV_ROOT/share/lcov/support\-scripts/spreadsheet.py
+), and can also enable faster 
+.I --parallel
+execution (see the
+.I "\-\-history\-script"
+ section of this man page).
+
+.B \-\-history-script
+.I callback
+.br
+.RS
+Use
+.I callback
+to predict current runtime cost using observed cost from prior execution.
+
+See man
+.B genhtml(1)
+for more information.
+.RE
+
 
 .RE
 
@@ -1723,6 +1748,12 @@ and
 .I fork_fail_timeout
 entries in man
 .B lcovrc(5).
+A previously generated execution profile may help to enable better utilization
+and faster parallel execution.  See the
+.I "\-\-profile"
+and
+.I "\-\-history"
+sections of this man page.
 
 .RE
 .BI "\-\-memory "
diff --git a/rpm/lcov.spec b/rpm/lcov.spec
index 591cb163..b7d3c71b 100644
--- a/rpm/lcov.spec
+++ b/rpm/lcov.spec
@@ -1,6 +1,6 @@
 Summary: A graphical code coverage front-end
 Name: lcov
-Version: 2.4
+Version: 2.4.1
 Release: 1
 License: GPLv2+
 Group: Development/Tools
diff --git a/scripts/gitblame.pm b/scripts/gitblame.pm
index a26b9d70..d4f2aa19 100644
--- a/scripts/gitblame.pm
+++ b/scripts/gitblame.pm
@@ -130,7 +130,8 @@ sub annotate
             (-f $pathname || -l $pathname) &&
             -r $pathname) {
         $context = ':' . $context if $context;
-        die($self->[SCRIPT] . $context . ' expected readable file, found \'' .
+        die($self->[SCRIPT] .
+            $context . ' expected readable file, found \'' .
             (defined($pathname) ? $pathname : '<undef>') . "'");
     }
 
diff --git a/scripts/history.pm b/scripts/history.pm
new file mode 100644
index 00000000..8de53d03
--- /dev/null
+++ b/scripts/history.pm
@@ -0,0 +1,196 @@
+#!/usr/bin/env perl
+
+#   Copyright (c) MediaTek USA Inc., 2025-2026
+#
+#   This program is free software;  you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation; either version 2 of the License, or (at
+#   your option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY;  without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+#   General Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License
+#   along with this program;  if not, see
+#   <http://www.gnu.org/licenses/>.
+#
+# history
+#
+#   This script is used as a "--history-script history" callback for
+#   genhtml/lcov/geninfo.
+#   The intent is to reuse profile history from a prior tool execution
+#   to enable better load balancing and thus better performance in a
+#   subsequent execution.
+#   The basic observation is that the next regression job is very similar to
+#   the previous one - so we expect similar resource utilization.
+#
+#   The 'history-script' is called as:
+#
+#        history->new(previousProfileFile)
+#
+#   where 'previousProfileFile' is the path to the '--profile' data generated
+#   in your previous regression job.
+#
+#   The configured callback is executed as
+#
+#       $callback->history(name)
+#
+#   where 'name' is typically the filename seen by genhtml or lcov.
+#   The return value is expected to be the execution time associated
+#   with 'name'.
+#
+#   This example script can be used with 'genhtml' and 'lcov --capture'
+#   and should work as expected if:
+#     - your command line is the same as the previous execution
+#     - file 'previousProfileFile' was generated by the previous execution, and
+#     - source and data files (.gdca, etc) are in the same relative locations
+#       as the previous execution
+#
+#   It may be useful to dig into the format of the '--profile' data
+#   to implement your own callbacks.
+#
+#   Note that history is not going to help if:
+#     - Individual tasks all have roughly the same resource utilization -
+#       there is no 'long pole' which needs to be scheduled first.
+#     - Random testing/tests are not reused: either test names never
+#       repeat, or the same name is not the same test.
+#       (You will need to implement your own prediction callback in that
+#        case...history won't help.)
+#
+
+use strict;
+use lcovutil;
+use Time::HiRes;
+
+package history;
+
+our @ISA       = qw(Exporter);
+our @EXPORT_OK = qw(new);
+
+sub appendElements
+{
+    my ($predicted, $in, $sub) = @_;
+
+    while (my ($name, $time) = each(%$in)) {
+        $name = $sub->($name) if defined($sub);
+        if (!exists($predicted->{$name})) {
+            $predicted->{$name} = [$time, 1];
+        } else {
+            my $p = $predicted->{$name};
+            $p->[0] += $time;
+            $p->[1] += 1;
+        }
+    }
+}
+
+sub new
+{
+    my $class      = shift;
+    my $script     = shift;
+    my $standalone = $script eq $0;
+
+    my $start = Time::HiRes::gettimeofday();
+    # these keys should be present in the data file
+    #  (else user has given us the wrong one)
+    my @verifyKeysList;
+    if ($lcovutil::tool_name eq 'genhtml') {
+        push(@verifyKeysList, 'file');
+    } elsif ($lcovutil::tool_name eq 'geninfo' ||
+             ($lcovutil::tool_name eq 'lcov' &&
+              $main::callFromLcov)
+    ) {
+        push(@verifyKeysList, 'file', 'find');
+    }
+    die("--history-script:  JSON file argument required")
+        unless (@_);
+    # is the profile history a valid JSON profile file
+    my @data;
+    foreach my $glob (@_) {
+        my @glob = glob($glob);
+        lcovutil::ignorable_error($lcovutil::ERROR_EMPTY,
+                           "'--history-script '$glob' does not match any files")
+            unless (@glob);
+        foreach my $history (@glob) {
+            eval {
+                my $data = JsonSupport::load($history);
+                for my $k (@verifyKeysList) {
+                    die("profile key '$k' not found")
+                        unless (exists($data->{$k}));
+                }
+                push(@data, $data);
+            };
+            if ($@) {
+                $@ =~ s/.+?ERROR: //;
+                lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
+                    "'--history-script $script $history' is not a valid $lcovutil::tool_name profile file: $@."
+                );
+                next;
+            }
+        }
+    }
+    unless (@data) {
+        lcovutil::ignorable_error($lcovutil::ERROR_USAGE,
+                                  "'--history-script $script " .
+                                      join(',', @_) .
+                                      "': profile history not found.");
+        return undef;
+    }
+    my %predicted;
+    if ($lcovutil::tool_name eq 'genhtml') {
+        # just keep the 'files' hash
+        if (1 < scalar(@data)) {
+            # compute average
+            foreach my $d (@data) {
+                appendElements(\%predicted, $d);
+            }
+        } else {
+            return bless $data[0]->{$verifyKeysList[0]}, $class;
+        }
+    } elsif (scalar(@verifyKeysList)) {
+        # this is geninfo..munge the data
+        #   - there may have been multiple 'capture' calls - say, if the
+        #     testsuite is segmented across machines.
+        #   - the individual capture directories may have different content
+        #     from one regression run to the next - say, if tests are
+        #     distributed adaptively.
+        # Thus:  the only way to have a complete history is if we merge
+        # the data from all of those jobs.
+        foreach my $d (@data) {
+            # read the profile data to figure out how much each file costs
+            my $sep = '^(';
+            my $pat = '';
+            for my $dir (keys %{$d->{find}}) {
+                $pat .= $sep . $dir;
+                $sep = '|';
+            }
+            $pat .= ')/';
+            my $re  = qr($pat);
+            my $sub = sub {
+                my $n = shift;
+                $n =~ s/$re//;
+                return $n;
+            };
+            appendElements(\%predicted, $d->{file}, $sub);
+        }
+    }
+    for my $name (keys %predicted) {
+        my $p = $predicted{$name};
+        # storing the average
+        $predicted{$name} = $p->[0] / $p->[1];
+    }
+    my $now = Time::HiRes::gettimeofday();
+    $lcovutil::profileData{history} = $now - $start;
+
+    return bless \%predicted, $class;
+}
+
+sub history
+{
+    my ($self, $name) = @_;
+
+    return exists($self->{$name}) ? $self->{$name} : undef;
+}
+
+1;
diff --git a/scripts/simplify.pm b/scripts/simplify.pm
index db9d8207..b14338bc 100644
--- a/scripts/simplify.pm
+++ b/scripts/simplify.pm
@@ -92,7 +92,18 @@ EOF
 
     # verify that the patterns are valid...
     lcovutil::verify_regexp_patterns($script, \@patterns);
-    my @munged = map({ [$_, 0]; } @patterns);
+    # pre-compile the regexps (to the extent that we can)
+    my @munged;
+    foreach my $p (@patterns) {
+        my $sep = substr($p, 1, 1);
+        my @s   = split($sep, $p);
+        die("unexpected substitution pattern '$p'")
+            unless ($s[0] eq 's' &&
+                    $#s == 3 &&
+                    $s[3] eq 'g');
+
+        push(@munged, [qr/$s[1]/, $s[2], $p, 0]);
+    }
 
     return bless \@munged, $class;
 }
@@ -103,13 +114,12 @@ sub simplify
 
     foreach my $p (@$self) {
         my $orig = $name;
-        # sadly, no support for pre-compiled patterns
-        eval "\$name =~ $p->[0] ;";    # apply pattern that user provided...
-            # $@ should never match:  we already checked pattern validity during
-            #   initialization - above.  Still: belt and braces.
-        die("invalid 'simplify' regexp '$p->[0]': $@")
+        eval { $name =~ s/$p->[0]/$p->[1]/g; };
+        # $@ should never match:  we already checked pattern validity during
+        #   initialization - above.  Still: belt and braces.
+        die("invalid 'simplify' regexp '$p->[-2]': $@")
             if ($@);
-        ++$p->[1]
+        ++$p->[-1]
             if ($name ne $orig);
     }
     return $name;
@@ -119,7 +129,7 @@ sub start
 {
     my $self = shift;
     foreach my $p (@$self) {
-        $p->[1] = 0;
+        $p->[-1] = 0;
     }
 }
 
@@ -128,7 +138,7 @@ sub save
     my $self = shift;
     my @data;
     foreach my $p (@$self) {
-        push(@data, $p->[1]);
+        push(@data, $p->[-1]);
     }
     return \@data;
 }
diff --git a/scripts/spreadsheet.py b/scripts/spreadsheet.py
index ebb37813..fb6d6d43 100755
--- a/scripts/spreadsheet.py
+++ b/scripts/spreadsheet.py
@@ -24,7 +24,7 @@ def __init__(self, excelFile, files, args):
 
         # keep a list of sheets so we can insert a summary..
         geninfoSheets = []
-        summarySheet = s.add_worksheet("geninfo_summary") if 1 < len(files) else None
+        summarySheet = s.add_worksheet("capture_summary") if 1 < len(files) else None
 
         # order:  order of processing
         # file: time to process one GCDA file
@@ -41,7 +41,7 @@ def __init__(self, excelFile, files, args):
         # queue: time between child finish and start of merge in parent
         # merge: time to merge returned chunk info
         geninfoChunkKeys = ('work', 'chunk', 'queue', 'child', 'process', 'undump', 'merge')
-        geninfoSpecialKeys = ('total', 'parallel', 'filter', 'write')
+        geninfoSpecialKeys = ('total', 'parallel', 'filter', 'write', 'history')
 
         # keys related to filtering
         filterKeys = ('filt_chunk', 'filt_queue',  'filt_child', 'filt_proc', 'filt_undump', 'filt_merge', 'derive_end')
@@ -166,6 +166,7 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
             insertConditional(sheet, avgRow, devRow,
                               beginRow, firstCol, endRow, col)
 
+        activeSheet = None
         for name in files:
             try:
                 with open(name) as f:
@@ -179,6 +180,9 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
 
                 try:
                     tool = data['config']['tool']
+                    if (tool == 'lcov' and
+                        -1 != data['config']['cmdLine'].find('--call-from-lcov')):
+                        tool = 'geninfo'
                 except:
                     tool = 'unknown'
                     print("%s: unknown tool" %(name))
@@ -198,6 +202,8 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
             for i in range(1000):
                 try:
                     sheet = s.add_worksheet(sn[-31:])
+                    if activeSheet == None:
+                        activeSheet = sheet
                     break
                 except:
                     sn = sheetname + "_" + str(i)
@@ -217,7 +223,7 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
             for n in sorted(data['config'].keys()):
                 try:
                     sheet.write_string(row, 1, n)
-                    if n in ("tool", 'date', ):
+                    if n in ("tool", 'cmdLine', 'date', ):
                         sheet.write_string(row, 2, data['config'][n])
                     else:
                         sheet.write_number(row, 2, data['config'][n], intFormat)
@@ -266,7 +272,7 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
                                     effectiveParallelism += sep + xl_rowcol_to_cell(row, 2)
                                     sep = "+"
                             except:
-                                print("%s: failed to write %s for lcov[seg %d][%s]" % (
+                                print("Warning: %s: unable to write %s for lcov[seg %d][%s]" % (
                                     name, str(d[k]) if k in d else "??", seg, k))
                             row += 1
                         begin = row
@@ -280,10 +286,10 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
                                     try:
                                         sheet.write_number(row, 3, float(d2[f]), twoDecimal)
                                     except:
-                                        print("%s: failed to write %s for lcov[seg %d][%s][$s]" % (name, str(d2[f]), seg, k, f))
+                                        print("Warning: %s: unable to write %s for lcov[seg %d][%s][$s]" % (name, str(d2[f]), seg, k, f))
                                 row += 1
                             except:
-                                print("%s: failed to write %s for lcov[seg %d]" % (name, k, seg))
+                                print("Warning: %s: unable to write %s for lcov[seg %d]" % (name, k, seg))
                     effectiveParallelism += ")/%(total)s" % {
                         'total': total,
                     }
@@ -300,7 +306,7 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
                             val = data[k]
                             sheet.write_number(row, 2, float(val), twoDecimal)
                         except:
-                            print("%s: failed to write %s for lcov[%s]" % (name, str(val), k))
+                            print("Warning: %s: unable to write %s for lcov[%s]" % (name, str(val), k))
                             row += 1
                     for k in ('parse', 'append'):
                         try:
@@ -311,10 +317,10 @@ def insertStats(keys, sawData, sumRow, avgRow, devRow, beginRow, endRow, col):
                                 try:
                                     sheet.write_number(row, 3, float(d2[f]), twoDecimal)
                                 except:
-                                    print("%s: failed to write %s for lcov[%s][$s]" % (name, str(d2[f]), k, f))
+                                    print("Warning: %s: unable to write %s for lcov[%s][$s]" % (name, str(d2[f]), k, f))
                             row += 1
                         except:
-                            print("%s: failed to find key '%s'" %(name, k))
+                            print("Warning: %s: failed to find key '%s'" %(name, k))
 
                 # go on to the next file
                 continue
@@ -557,11 +563,15 @@ def cmpFile(a, b):
             elif tool == 'genhtml':
 
                 for k in ('parse_source', 'parse_diff',
-                          'parse_current', 'parse_baseline'):
+                          'parse_current', 'parse_baseline',
+                          'history'):
                     if k in data:
-                        sheet.write_string(row, 0, k)
-                        sheet.write_number(row, 1, data[k], twoDecimal)
-                        row += 1
+                        try:
+                            sheet.write_string(row, 0, k)
+                            sheet.write_number(row, 1, data[k], twoDecimal)
+                            row += 1
+                        except:
+                            pass # 'history' key might not be there
 
                 # total: time from start to end of the particular unit -
                 # child: time from start to end of child process
@@ -625,7 +635,7 @@ def printDataRow(name):
                                 else:
                                     sawData[k] = 1
                             except:
-                                print("%s: failed to write %s" %(name, data[k][name]))
+                                print("Warning: %s: unable to write %s" %(name, data[k][name]))
                         col += 1
 
                 def visitScope(f):
@@ -674,7 +684,7 @@ def visitScope(f):
                         try:
                             sheet.write_number(row, 2, float(d[n]), twoDecimal)
                         except:
-                            print("%s: failed to write %s for [%s][%s]" %(name, str(d[n]), k, n))
+                            print("Warning: %s: unable to write %s for [%s][%s]" %(name, str(d[n]), k, n))
                         row += 1;
                     continue
                 elif k in ('config', 'overall', 'total'):
@@ -684,6 +694,7 @@ def visitScope(f):
 
         if summarySheet:
             if len(geninfoSheets) < 2:
+                activeSheet.activate()
                 summarySheet.hide()
 
             # insert the average and variance data...
diff --git a/tests/gendiffcov/errs/msgtest.sh b/tests/gendiffcov/errs/msgtest.sh
index f2f8ea54..dd0d9430 100755
--- a/tests/gendiffcov/errs/msgtest.sh
+++ b/tests/gendiffcov/errs/msgtest.sh
@@ -4,7 +4,7 @@ set +x
 source ../../common.tst
 
 rm -f test.cpp *.gcno *.gcda a.out *.info *.log *.json diff.txt loop*.rc markers.err* readThis.rc testing.rc
-rm -rf select criteria annotate empty unused_src scriptErr scriptFixed epoch inconsistent highlight etc mycache cacheFail expect subset context labels sortTables simplify_* simplify missingRestore selectErr1 selectErr2 selectErr3
+rm -rf select criteria annotate empty unused_src scriptErr scriptFixed epoch inconsistent highlight etc mycache cacheFail expect subset context labels sortTables simplify_* simplify missingRestore selectErr1 selectErr2 selectErr3 mcdc
 
 clean_cover
 
@@ -23,6 +23,7 @@ GITBLAME_SCRIPT=$SCRIPT_DIR/gitblame.pm
 GITVERSION_SCRIPT=$SCRIPT_DIR/gitversion.pm
 P4VERSION_SCRIPT=$SCRIPT_DIR/P4version.pm
 SIMPLIFY_SCRIPT=$SCRIPT_DIR/simplify.pm
+HISTORY_SCRIPT=$SCRIPT_DIR/history.pm
 
 if [ 1 == "$USE_GIT" ] ; then
     # this is git
@@ -222,13 +223,7 @@ for missing in noSuchFile missingDirectory/nofile ; do
 	fi
     fi
     grep "cannot read configuration file '$missing'" err_missing.log
-    FOUND=$?
-    if [ 0 != $FOUND ] ; then
-	# look for alternate message found with some perl versions...see #450
-	grep "config file '$missing' does not exist" err_missing.log
-	FOUND=$?
-    fi
-    if [ 0 != $FOUND ] ; then
+    if [ 0 != $? ] ; then
 	echo "ERROR: missing config file '$missing' message"
 	if [ 0 == $KEEP_GOING ] ; then
             exit 1
@@ -241,7 +236,7 @@ echo "message_log = message_file.log" > testing.rc
 echo "config_file = testing.rc" > readThis.rc
 echo lcov $LCOV_OPTS --summary initial.info --config-file readThis.rc
 $COVER $LCOV_TOOL $LCOV_OPTS --summary initial.info --config-file readThis.rc
-if [ ! == ${PIPESTATUS[0]} ] ; then
+if [ 0 != $? ] ; then
     echo "ERROR: didn't read config file"
     if [ 0 == $KEEP_GOING ] ; then
         exit 1
@@ -428,7 +423,7 @@ for arg in "--annotate-script $ANNOTATE_SCRIPT --select-script $SELECT_SCRIPT,--
     fi
     # run again  without error
     echo genhtml $DIFCOV_OPTS initial.info -o scriptFixed ${arg}
-    $COVER $GENHTML_TOOL $DIFFCOV_OPTS initial.info -o scriptFixed ${arg} --ignore annotate 2>&1 | tee script_err.log
+    $COVER $GENHTML_TOOL $DIFFCOV_OPTS initial.info -o scriptFixed ${arg} --ignore annotate --profile 2>&1 | tee script_err.log
     if [ 0 != ${PIPESTATUS[0]} ] ; then
         echo "ERROR: genhtml scriptFixed failed"
         if [ 0 == $KEEP_GOING ] ; then
@@ -971,8 +966,60 @@ if [ 0 != $? ] ; then
     fi
 fi
 
+# test profile history fails
+for f in noFile initial.info ; do
+    $COVER $GENHTML_TOOL $DIFFCOV_OPTS initial.info -o history --parallel --history $HISTORY_SCRIPT,$f 2>&1 | tee history.log
+    if [ 0 == ${PIPESTATUS[0]} ] ; then
+	echo "ERROR: genhtml --history $f didn't fail"
+	if [ 0 == $KEEP_GOING ] ; then
+            exit 1
+	fi
+    fi
+    grep -E "ERROR.*usage.*--history.* is not a valid genhtml profile file" history.log
+    if [ 0 != $? ] ; then
+	echo "ERROR: didn't find expected --history message"
+	if [ 0 == $KEEP_GOING ] ; then
+            exit 1
+	fi
+    fi
+done
+
+# wrong profile type
+$COVER $GENINFO_TOOL $LCOV_OPTS . -o profileTest.info --parallel --history $HISTORY_SCRIPT,scriptFixed.info.json 2>&1 | tee geninfo_history.log
+if [ 0 == ${PIPESTATUS[0]} ] ; then
+    echo "ERROR: geninfo --history scriptFixed.info.json didn't fail"
+    if [ 0 == $KEEP_GOING ] ; then
+        exit 1
+    fi
+fi
+grep -E "ERROR.*usage.*--history.* is not a valid geninfo profile file" geninfo_history.log
+if [ 0 != $? ] ; then
+    echo "ERROR: didn't find expected geninfo --history message"
+    if [ 0 == $KEEP_GOING ] ; then
+        exit 1
+    fi
+fi
+
+# ignore the wrong history message - also need to ignore the resulting
+# 'package' error when the callback can't be installed
+$COVER $GENINFO_TOOL $LCOV_OPTS . -o profileTest.info --parallel --history $HISTORY_SCRIPT,scriptFixed.info.json --ignore usage,package 2>&1 | tee geninfo_history_ignore.log
+if [ 0 != ${PIPESTATUS[0]} ] ; then
+    echo "ERROR: geninfo --history --ignore failed"
+    if [ 0 == $KEEP_GOING ] ; then
+        exit 1
+    fi
+fi
+grep -E "WARNING.*usage.*--history.* profile history not found" geninfo_history_ignore.log
+if [ 0 != $? ] ; then
+    echo "ERROR: didn't find expected geninfo --history --ignore message"
+    if [ 0 == $KEEP_GOING ] ; then
+        exit 1
+    fi
+fi
+
+
 if [ "$ENABLE_MCDC" != 1 ] ; then
-    $COVER $GENINFO_TOOL . -o mccd --mcdc-coverage $LCOV_OPTS --msg-log mcdc_errs.log
+    $COVER $GENINFO_TOOL . -o mcdc --mcdc-coverage $LCOV_OPTS --msg-log mcdc_errs.log
     if [ 0 == $? ] ; then
         echo "ERROR: no error for unsupported MC/DC"
         if [ 0 == $KEEP_GOING ] ; then
diff --git a/tests/gendiffcov/simple/script.sh b/tests/gendiffcov/simple/script.sh
index 72e11515..65bc20d8 100755
--- a/tests/gendiffcov/simple/script.sh
+++ b/tests/gendiffcov/simple/script.sh
@@ -26,6 +26,7 @@ echo *
 
 CRITERIA=${SCRIPT_DIR}/criteria
 SELECT=${SCRIPT_DIR}/select.pm
+HISTORY=${SCRIPT_DIR}/history.pm
 
 #PARALLEL=''
 #PROFILE="''
@@ -248,7 +249,7 @@ gzip -c baseline_nobranch.info > baseline_nobranch.info.gz
 #genhtml baseline.info --output-directory ./baseline
 
 echo genhtml $DIFFCOV_OPTS baseline_orig.info --output-directory ./baseline $IGNORE --rc memory_percentage=50 --serialize ./baseline/coverage.dat
-$COVER $GENHTML_TOOL $DIFFCOV_OPTS baseline_orig.info --output-directory ./baseline --save $IGNORE --rc memory_percentage=50 --serialize ./baseline/coverage.dat
+$COVER $GENHTML_TOOL $DIFFCOV_OPTS baseline_orig.info --output-directory ./baseline --save $IGNORE --rc memory_percentage=50 --serialize ./baseline/coverage.dat --profile
 if [ 0 != $? ] ; then
     echo "ERROR: genhtml baseline failed"
     status=1
@@ -279,7 +280,7 @@ fi
 gzip -c baseline-filter.info > baseline-filter.info.gz
 #genhtml baseline.info --output-directory ./baseline
 echo genhtml $DIFFCOV_OPTS baseline-filter.info --output-directory ./baseline-filter $IGNORE --missed
-$COVER $GENHTML_TOOL $DIFFCOV_OPTS baseline-filter.info --output-directory ./baseline-filter $IGNORE --missed
+$COVER $GENHTML_TOOL $DIFFCOV_OPTS baseline-filter.info --output-directory ./baseline-filter $IGNORE --missed --profile --history $HISTORY,baseline/genhtml.json,baseline/genhtml.json
 if [ 0 != $? ] ; then
     echo "ERROR: genhtml baseline-filter failed"
     status=1
@@ -290,7 +291,7 @@ fi
 
 #genhtml baseline.info --dark --output-directory ./baseline
 echo genhtml $DIFFCOV_OPTS --dark baseline-filter.info --output-directory ./baseline-filter-dark $IGNORE
-$COVER $GENHTML_TOOL $DIFFCOV_OPTS --dark baseline-filter.info --output-directory ./baseline-filter-dark $IGNORE
+$COVER $GENHTML_TOOL $DIFFCOV_OPTS --dark baseline-filter.info --output-directory ./baseline-filter-dark $IGNORE --history $HISTORY,baseline-filter/genhtml.json
 if [ 0 != $? ] ; then
     echo "ERROR: genhtml baseline-filter-dark failed"
     status=1
diff --git a/tests/lcov/add/Makefile b/tests/lcov/add/Makefile
index d9e5359c..4733f40e 100644
--- a/tests/lcov/add/Makefile
+++ b/tests/lcov/add/Makefile
@@ -8,4 +8,4 @@ TESTS := zero.sh zero2.sh full.sh full2.sh  prune.sh track.sh
 DISABLED: part.sh part2.sh concatenated4.sh
 
 clean:
-	rm -f *.info prune prune2 prune3 track
+	rm -f *.info prune prune2 prune3 track prune3s prune3t
diff --git a/tests/lcov/add/prune.sh b/tests/lcov/add/prune.sh
index 7ad38ccd..8c22f4c6 100755
--- a/tests/lcov/add/prune.sh
+++ b/tests/lcov/add/prune.sh
@@ -4,6 +4,8 @@ set +x
 
 source ../../common.tst
 
+rm -f prune3s prune3t prune3
+
 if [[ 1 == $CLEAN_ONLY ]] ; then
     exit 0
 fi
diff --git a/tests/lcov/demangle/demangle.sh b/tests/lcov/demangle/demangle.sh
index fdf82bd2..d5e9705f 100755
--- a/tests/lcov/demangle/demangle.sh
+++ b/tests/lcov/demangle/demangle.sh
@@ -59,7 +59,7 @@ for k in FNA ; do
 done
 
 # see if we can "simplify" the function names..
-for callback in './simplify.pl' "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/;s/Cat::Cat/subst2/;s/subst2/subst3/" "${SIMPLIFY_SCRIPT},--file,simplify.cmd" ; do
+for callback in './simplify.pl' "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/g;s/Cat::Cat/subst2/g;s/subst2/subst3/g" "${SIMPLIFY_SCRIPT},--file,simplify.cmd" ; do
 
     $COVER $GENHTML_TOOL --branch $PARLLEL $PROFILE -o simplify demangle.info --flat --simplify $callback
     if [ $? != 0 ] ; then
@@ -90,23 +90,23 @@ done
 
 # test unused regexp in simplify callback
 for PAR in '' '--parallel' ; do
-    $COVER $GENHTML_TOOL --branch $PARLLEL $PROFILE -o simplify demangle.info --flat --simplify "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/;s/Cat::Cat/subst2/;s/subst2/subst3/;s/foo/bar/" $PAR 2>&1 | tee simplifyErr.log
+    $COVER $GENHTML_TOOL --branch $PARLLEL $PROFILE -o simplify demangle.info --flat --simplify "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/g;s/Cat::Cat/subst2/g;s/subst2/subst3/g;s/foo/bar/g" $PAR 2>&1 | tee simplifyErr.log
     if [ ${PIPESTATUS[0]} == 0 ] ; then
 	echo "genhtml --simplify unused regexp didn't fail"
 	exit 1
     fi
-    grep "'simplify' pattern 's/foo/bar/' is unused" simplifyErr.log
+    grep "'simplify' pattern 's/foo/bar/g' is unused" simplifyErr.log
     if [ $? != 0 ] ; then
 	echo "didn't find expected unused error"
 	exit 1
     fi
 
-    $COVER $GENHTML_TOOL --branch $PARLLEL $PROFILE -o simplify demangle.info --flat --simplify "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/;s/Cat::Cat/subst2/;s/subst2/subst3/;s/foo/bar/" $PAR --ignore unused 2>&1 | tee simplifyWarn.log
+    $COVER $GENHTML_TOOL --branch $PARLLEL $PROFILE -o simplify demangle.info --flat --simplify "${SIMPLIFY_SCRIPT},--sep,;,--re,s/Animal::Animal/subst1/g;s/Cat::Cat/subst2/g;s/subst2/subst3/g;s/foo/bar/g" $PAR --ignore unused 2>&1 | tee simplifyWarn.log
     if [ ${PIPESTATUS[0]} != 0 ] ; then
 	echo "genhtml --simplify unused regexp warn didn't pass"
 	exit 1
     fi
-    grep "'simplify' pattern 's/foo/bar/' is unused" simplifyWarn.log
+    grep "'simplify' pattern 's/foo/bar/g' is unused" simplifyWarn.log
     if [ $? != 0 ] ; then
 	echo "didn't find expected unused error"
 	exit 1
diff --git a/tests/lcov/demangle/simplify.cmd b/tests/lcov/demangle/simplify.cmd
index 8a628285..809f0834 100644
--- a/tests/lcov/demangle/simplify.cmd
+++ b/tests/lcov/demangle/simplify.cmd
@@ -1,6 +1,6 @@
 # test the 'simplify.pm --file ...' option
 
-s/Animal::Animal/subst1/
-s/Cat::Cat/subst2/
-s/subst2/subst3/
+s/Animal::Animal/subst1/g
+s/Cat::Cat/subst2/g
+s/subst2/subst3/g
 
diff --git a/tests/lcov/errs/errs.sh b/tests/lcov/errs/errs.sh
index 16e0831a..0fe0e55a 100755
--- a/tests/lcov/errs/errs.sh
+++ b/tests/lcov/errs/errs.sh
@@ -3,7 +3,7 @@ set +x
 
 source ../../common.tst
 
-rm -f  *.log *.json dumper* *.out
+rm -f  *.log *.json dumper* *.out emptyDir.info
 rm -rf emptyDir
 
 clean_cover
diff --git a/tests/lcov/extract/extract.sh b/tests/lcov/extract/extract.sh
index fe61beb5..f69b44f3 100755
--- a/tests/lcov/extract/extract.sh
+++ b/tests/lcov/extract/extract.sh
@@ -64,7 +64,7 @@ if [ "${VER[0]}" -lt 8 ] ; then
 fi
 
 if [ 1 != "$NO_INITIAL_CAPTURE" ] ; then
-    $COVER $CAPTURE . $LCOV_OPTS --initial -o initial.info $IGNORE_EMPTY $IGNORE_USAGE
+    $COVER $CAPTURE . $LCOV_OPTS --initial -o initial.info $IGNORE_EMPTY $IGNORE_USAGE --profile
     if [ 0 != $? ] ; then
         echo "Error:  unexpected error code from lcov --initial"
         if [ $KEEP_GOING == 0 ] ; then
@@ -81,7 +81,7 @@ fi
 
 if [ "$NO_INITIAL_CAPTURE" != 1 ] ; then
     # capture 'all' - which will pick up the unused file
-    $COVER $CAPTURE . $LCOV_OPTS --all -o all_initial.info $IGNORE_EMPTY $IGNORE_USAGE
+    $COVER $CAPTURE . $LCOV_OPTS --all -o all_initial.info $IGNORE_EMPTY $IGNORE_USAGE --history $SCRIPT_DIR/history.pm,initial.info.json --profile
     if [ 0 != $? ] ; then
         echo "Error:  unexpected error code from lcov --capture --all"
         if [ $KEEP_GOING == 0 ] ; then
@@ -105,7 +105,8 @@ if [ 0 != $? ] ; then
     exit 1
 fi
 
-$COVER $CAPTURE . $LCOV_OPTS -o external.info $FILTER $IGNORE
+# test an empty/trivial history callback
+$COVER $CAPTURE . $LCOV_OPTS -o external.info $FILTER $IGNORE --profile --histor ./history.sh
 if [ 0 != $? ] ; then
     echo "Error:  unexpected error code from lcov --capture"
     if [ $KEEP_GOING == 0 ] ; then
@@ -130,7 +131,7 @@ fi
 
 # callback tests
 echo $COVER $CAPTURE . $LCOV_OPTS -o callback.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,90,--branch,65,--function,100
-$COVER $CAPTURE . $LCOV_OPTS -o callback.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,90,--branch,65,--function,100 2>&1 | tee callback_fail.log
+$COVER $CAPTURE . $LCOV_OPTS -o callback.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,90,--branch,65,--function,100 --history $SCRIPT_DIR/history.pm,external.info.json 2>&1 | tee callback_fail.log
 if [ 0 == ${PIPESTATUS[0]} ] ; then
     echo "Error:  expected criteria fail from lcov --capture - but not found"
     if [ $KEEP_GOING == 0 ] ; then
@@ -144,8 +145,8 @@ if [ 0 != $? ] ; then
         exit 1
     fi
 fi
-echo $COVER $CAPTURE . $LCOV_OPTS -o callback2.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,20
-$COVER $CAPTURE . $LCOV_OPTS -o callback2.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,20
+echo $COVER $CAPTURE . $LCOV_OPTS -o callback2.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,20 --history $SCRIPT_DIR/history.pm,external.info.json,callback.info.json
+$COVER $CAPTURE . $LCOV_OPTS -o callback2.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,20 --history $SCRIPT_DIR/history.pm,external.info.json,callback.info.json
 if [ 0 != $? ] ; then
     echo "Error:  expected criteria pass from lcov --capture - but failed"
     if [ $KEEP_GOING == 0 ] ; then
@@ -153,7 +154,6 @@ if [ 0 != $? ] ; then
     fi
 fi
 
-
 echo $COVER $LCOV_TOOL $LCOV_OPTS -o aggregata.info -a callback.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,90,--branch,65,--function,100
 $COVER $LCOV_TOOL $LCOV_OPTS -o aggregata.info -a callback.info $FILTER $IGNORE --criteria $SCRIPT_DIR/threshold.pm,--line,90,--branch,65,--function,100 2>&1 | tee callback_fail2.log
 if [ 0 == ${PIPESTATUS[0]} ] ; then
diff --git a/tests/lcov/extract/history.sh b/tests/lcov/extract/history.sh
new file mode 100755
index 00000000..efce51d4
--- /dev/null
+++ b/tests/lcov/extract/history.sh
@@ -0,0 +1,2 @@
+#!/bin/sh
+echo ''
diff --git a/tests/llvm2lcov/llvm2lcov.sh b/tests/llvm2lcov/llvm2lcov.sh
index 9b4d279e..03bf79f4 100755
--- a/tests/llvm2lcov/llvm2lcov.sh
+++ b/tests/llvm2lcov/llvm2lcov.sh
@@ -19,6 +19,12 @@ fi
 
 LCOV_OPTS="--branch-coverage $PARALLEL $PROFILE"
 
+which clang
+if [ $? != 0 ] ; then
+    echo "clang unavailable - skipping test"
+    exit 0
+fi
+
 IFS='.' read -r -a LLVM_VER <<< `clang -dumpversion`
 if [ "${LLVM_VER[0]}" -ge 18 ] ; then
     ENABLE_MCDC=1