fix: prevent integer overflow in Psi offsets

[arcusbuilds]

Fixes #8088.

Compute Psi offsets using std::size_t to prevent integer overflow and silent incorrect results in large wave-function arrays. Adds two regression tests covering both storage layouts.

All 10 CPU tests pass with release optimization and UBSan.

Verification details

Both new tests fail against the original implementation and pass with the fix. Tested in Ubuntu WSL with GNU 15.2.0:

g++ -std=c++17 -O3 -DNDEBUG -fsanitize=undefined -fno-sanitize-recover=all -ffunction-sections -fdata-sections -Isource source/source_psi/test/psi_test.cpp source/source_psi/psi.cpp source/source_base/module_device/memory_op.cpp source/source_base/memory_recorder.cpp source/source_base/global_variable.cpp source/source_io/module_parameter/parameter.cpp -Wl,--gc-sections -lgtest_main -lgtest -pthread -o build-8088/psi_tests_release_ubsan
OMP_NUM_THREADS=1 build-8088/psi_tests_release_ubsan
g++ -std=c++11 -Isource -c source/source_psi/psi.cpp -o build-8088/psi-cxx11.o
python tools/03_code_analysis/agent_governance_check.py --base HEAD~1 --head HEAD --event-path build-8088/pr-event.json --format text
git diff --check

Results: 10/10 tests passed, no UBSan diagnostics; C++11 compilation passed; governance and whitespace checks passed. Installed GoogleTest requires C++17.

CMake configuration could not find BLAS, so verification used the real production sources linked directly. CUDA/ROCm builds and the reported Fe calculation were not run. The tests reserve 12 GiB of virtual address space and touch only a few elements.

No documentation update required: no INPUT parameters or public interfaces changed. Core impact is limited to Psi pointer arithmetic. No governance exceptions requested.

[Copilot]

🟢 Approval recommended

The offset conversions are complete, C++11-compatible, and covered by focused regression tests.

0 open findings

What changed in this PR

Fixes integer overflow in large Psi array offsets.

Changes:

• Promotes pointer-offset arithmetic to std::size_t.
• Adds regression tests for both storage layouts.
• Verified changed files use LF line endings; tests were not rerun during review.

File	Description
source/​source_psi/​psi.cpp	Uses size-safe offset calculations.
source/​source_psi/​test/​psi_test.cpp	Tests offsets exceeding INT_MAX.

🧠 Review effort: Balanced

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[mohanchen]

LGTM. Thanks for your contributions.