cudaqx::tensor fails to allocate large PCMs

[kvmto]

Summary

The cudaq::qec::generate_random_pcm() function fails with error "xbuffer_storage not resizable" when attempting to create Parity Check Matrices (PCMs) with more than ~50,000 error mechanisms. This appears to be a limitation in the underlying xtensor library used by cudaqx::tensor.

Environment

• Component: cudaqx::tensor (xtensor implementation)
• Affected Functions: cudaq::qec::generate_random_pcm() in libs/qec/lib/pcm_utils.cpp
• Error: std::runtime_error: xbuffer_storage not resizable
• nvq++ Version: (https://github.com/NVIDIA/cuda-quantum d7f1551f6c9544b6123a4daf11c391fade6ea590)

Reproduction

Test Code

#include "cudaq/qec/pcm_utils.h"
#include <iostream>
#include <random>

int main() {
  std::cout << "Testing PCM generation for different sizes...\n\n";
  
  std::vector<size_t> test_sizes = {1000, 10000, 50000, 100000};
  
  for (size_t num_mechs : test_sizes) {
    std::cout << "Attempting " << num_mechs << " mechanisms... " << std::flush;
    
    try {
      size_t n_rounds = std::max(size_t(1), num_mechs / 100);
      size_t n_errs_per_round = num_mechs / n_rounds;
      size_t n_syndromes_per_round = std::max(size_t(1), n_errs_per_round / 2);
      int weight = 3;
      
      std::mt19937_64 rng(42);
      auto pcm = cudaq::qec::generate_random_pcm(
          n_rounds, n_errs_per_round, n_syndromes_per_round, weight, std::move(rng));
      
      std::cout << " Success! Shape: " << pcm.shape()[0] << "×" << pcm.shape()[1] 
                << " (rows×cols)\n";
    } catch (const std::exception& e) {
      std::cout << " Failed! Error: " << e.what() << "\n";
    }
  }
  
  return 0;
}

Execution Results

$ ./test-pcm-allocation
Testing PCM generation for different sizes...

Attempting 1000 mechanisms...  Success! Shape: 500×1000 (rows×cols)
Attempting 10000 mechanisms...  Success! Shape: 5000×10000 (rows×cols)
Attempting 50000 mechanisms...  Success! Shape: 25000×50000 (rows×cols)
Attempting 100000 mechanisms...  Failed! Error: xbuffer_storage not resizable