Fix -Warray-bounds build error in MLAS on clang 17+

[Copilot]

Description

The -Warray-bounds suppression pragma in sqnbitgemm_kernel_avx2_int8_blklen32.h was gated on defined(HAS_ARRAY_BOUNDS), which is set in onnxruntime_config.h. MLAS never includes that header, so the guard was dead code and the pragma never fired.

Changed the guard to #ifdef __clang__:

// Before: HAS_ARRAY_BOUNDS never defined in MLAS TU
#if defined(__clang__) && defined(HAS_ARRAY_BOUNDS)

// After
#ifdef __clang__

Note: __has_warning("-Warray-bounds") was considered but the C preprocessor does not short-circuit &&, so GCC fails to parse it even behind defined(__clang__).

Motivation and Context

Build fails on Intel Mac with Apple Clang 17.0.0 (-Werror,-Warray-bounds). Clang raises a false-positive array-bounds warning on acc[4..7] inside an if constexpr (NCols4 == 8) branch that is dead when NCols4 == 4.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Build] error: array index 4 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds]</issue_title>
<issue_description>### Describe the issue

Unable to build from main branch (0768f42 as of time writing this issue) on Intel Mac

/usr/bin/c++ --version
Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: x86_64-apple-darwin24.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Urgency

No response

Target platform

MacOS

Build script

./build.sh --config RelWithDebInfo --build_shared_lib --parallel --cmake_extra_defines CMAKE_OSX_ARCHITECTURES=x86_64

Error / output

[ 18%] Building CXX object CMakeFiles/onnxruntime_mlas.dir/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2.cpp.o
In file included from /onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2.cpp:26:
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:49: error: array index 4 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds]
1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6], acc[7]);
| ^ ~
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13: note: array 'acc' declared here
1531 | __m256 acc[NCols4];
| ^
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:57: error: array index 5 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds]
1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6], acc[7]);
| ^ ~
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13: note: array 'acc' declared here
1531 | __m256 acc[NCols4];
| ^
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:65: error: array index 6 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds]
1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6], acc[7]);
| ^ ~
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13: note: array 'acc' declared here
1531 | __m256 acc[NCols4];
| ^
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1677:73: error: array index 7 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds]
1677 | __m128 acc_1 = FoldAccumulators(acc[4], acc[5], acc[6], acc[7]);
| ^ ~
/onnxruntime/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1531:13: note: array 'acc' declared here
1531 | __m256 acc[NCols4];
| ^
4 errors generated.

Visual Studio Version

No response

GCC / Compiler Version

Apple clang version 17.0.0 (clang-1700.0.13.5)</issue_description>

<agent_instructions>Please investigate the build error. If code need fix, create a pull requests. Otherwise, suggest ways to avoid the build errors.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Build] error: array index 4 is past the end of the array (that has type '__m256[4]') [-Werror,-Warray-bounds] #27497

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[tianleiwu]

@copilot, please fix build errors in CI pipeline like in gcc 14:

/onnxruntime_src/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1673:40: error: missing binary operator before token "("
 1673 | #if defined(__clang__) && __has_warning("-Warray-bounds")
      |                                        ^
/onnxruntime_src/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1678:40: error: missing binary operator before token "("
 1678 | #if defined(__clang__) && __has_warning("-Warray-bounds")
      |                          

[Copilot]

@copilot, please fix build errors in CI pipeline like in gcc 14:

/onnxruntime_src/onnxruntime/core/mlas/lib/sqnbitgemm_kernel_avx2_int8_blklen32.h:1673:40: error: missing binary ...

Fixed in 2e038f4. __has_warning is a clang-only built-in and the C preprocessor doesn't short-circuit &&, so GCC fails to parse the expression. Simplified the guards to #ifdef __clang__ — since -Warray-bounds is supported by all clang versions, the __has_warning check was unnecessary.

[tianleiwu]

/azp run Linux QNN CI Pipeline, Win_TRT_Minimal_CUDA_Test_CI, Windows ARM64 QNN CI Pipeline, Windows GPU Doc Gen CI Pipeline

[azure-pipelines]

Azure Pipelines successfully started running 4 pipeline(s).