Add Windows OpenMP 2.0 compatibility

[tony-davis]

Summary

This PR adds compatibility for OpenMP 2.0 implementations (e.g., MSVC, older GCC) that lack functions introduced in OpenMP 3.0. BLIS currently uses omp_get_active_level() and omp_get_max_active_levels() which are not available in OpenMP 2.0, preventing compilation on Windows with MSVC and other OpenMP 2.0 environments.

Changes

frame/thread/bli_thread.c

Added OpenMP 2.0 compatibility shims that detect OpenMP version using the standard _OPENMP macro:

// OpenMP 2.0 compatibility: Provide fallbacks for OpenMP 3.0 functions
// Some compilers (e.g., MSVC, older GCC) only support OpenMP 2.0 which lacks:
//   - omp_get_active_level() (OpenMP 3.0)
//   - omp_get_max_active_levels() (OpenMP 3.0)
// The _OPENMP macro is set to version-specific values by OpenMP-compliant compilers:
//   200203 = OpenMP 2.0, 200811 = OpenMP 3.0, 201107 = OpenMP 3.1, etc.
#if defined(BLIS_ENABLE_OPENMP) && defined(_OPENMP) && _OPENMP < 200811
static inline int omp_get_active_level(void) {
    return 0;  // Always assume top-level (no nested parallelism support)
}
static inline int omp_get_max_active_levels(void) {
    return 1;  // OpenMP 2.0 doesn't support nested parallelism
}
#endif

Why _OPENMP < 200811 instead of _MSC_VER?

• ✅ Portable: Works with any OpenMP 2.x implementation, not just MSVC
• ✅ Future-proof: Won't break when MSVC adds OpenMP 3.0 support
• ✅ Accurate: Detects actual OpenMP version rather than compiler vendor
• ✅ Standard: _OPENMP is defined by all OpenMP-compliant compilers

(Credit: GitHub Copilot suggested this improvement over the original _MSC_VER check)

Testing

Environment:

• Windows 11
• Visual Studio 2022 (MSVC 19.50.35721.0)
• OpenMP 2.0 (via MSVC's /openmp flag)

Results:

• ✅ BLIS builds successfully with OpenMP threading enabled
• ✅ ILP64 support functional
• ✅ Zen4-optimized kernels compile and link correctly
• ✅ No performance overhead (shims only active for OpenMP 2.0)

Compatibility

Backward Compatibility: ✅ Fully maintained

• OpenMP 3.0+ systems: No changes (native functions used)
• OpenMP 2.0 systems: Shims provide safe fallbacks
• No OpenMP: No changes (shims not compiled)

Impact:

• Nested parallelism disabled on OpenMP 2.0 systems (rare in BLAS workloads)
• Safe defaults: single-threaded assumptions for compatibility

Checklist

• Code follows BLIS coding style
• Changes are minimal and focused
• Backward compatibility maintained
• Tested on target platform (Windows/MSVC)
• No performance regression on existing platforms
• Documentation inline via comments

[Copilot]

Pull request overview

This PR enables BLIS to build on Windows using GNU-style Clang compilers (e.g., LLVM/Clang built from source) and adds compatibility for OpenMP 2.0 implementations. The changes support AMD's ROCm testing infrastructure while maintaining full backward compatibility with existing MSVC/Clang-CL toolchains.

Changes:

• Added compiler frontend detection in CMake to distinguish GNU-style Clang from MSVC/Clang-CL on Windows
• Added OpenMP 2.0 compatibility shims for functions introduced in OpenMP 3.0

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
CMakeLists.txt	Adds detection of Clang with GNU-style frontend on Windows to use -fopenmp-simd instead of /openmp:experimental
frame/thread/bli_thread.c	Provides fallback implementations of omp_get_active_level() and omp_get_max_active_levels() for OpenMP 2.0 environments

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[kvaragan]

Thanks @tony-davis - will look into this.

[sireeshasanga]

@tony-davis ,
Thank you for raising the Pull Request. However, we can process the PRs that are raised on dev branch only.
Kindly raise the PR on dev branch for further processing. In parallel, we are reviewing the changes internally.

For more details on Contribution Guidelines and the process, please refer to https://github.com/amd/blis/blob/master/CONTRIBUTING.md.