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Add MLX_DISABLE_NAX option to skip Metal-4 nax kernels at build time #3593

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Add MLX_DISABLE_NAX option to skip Metal-4 nax kernels at build time #3593

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3 changes: 3 additions & 0 deletions CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,9 @@ option(MLX_BUILD_METAL "Build metal backend" ON)
option(MLX_BUILD_CPU "Build cpu backend" ON)
option(MLX_BUILD_CUDA "Build cuda backend" OFF)
option(MLX_METAL_DEBUG "Enhance metal debug workflow" OFF)
option(MLX_DISABLE_NAX
"Skip the Metal-4 nax tensor GEMM/qmm/attention kernels even when the build environment otherwise qualifies (MLX_METAL_VERSION>=400 + SDK>=26.2). Lets downstream projects opt out of the nax codepath on hosts where the installed Metal toolchain miscompiles those kernels — e.g. macOS 26 metalfe-32023.x silently produces wrong matmul outputs for M>~8 rows. The existing else() branch of the metal-version/SDK gate already defines MLX_METAL_NO_NAX, so C++ dispatchers fall back automatically. Default OFF preserves current behaviour."
OFF)
option(MLX_ENABLE_X64_MAC "Enable building for x64 macOS" OFF)
option(MLX_BUILD_GGUF "Include support for GGUF format" ON)
option(MLX_BUILD_SAFETENSORS "Include support for safetensors format" ON)
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6 changes: 5 additions & 1 deletion mlx/backend/metal/kernels/CMakeLists.txt
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Expand Up @@ -156,7 +156,7 @@ if(NOT MLX_METAL_JIT)
build_kernel(steel/attn/kernels/steel_attention ${STEEL_ATTN_HEADERS})

if((MLX_METAL_VERSION GREATER_EQUAL 400) AND (MACOS_SDK_VERSION GREATER_EQUAL
26.2))
26.2) AND (NOT MLX_DISABLE_NAX))

build_kernel(steel/gemm/kernels/steel_gemm_fused_nax ${STEEL_NAX_HEADERS})
build_kernel(steel/gemm/kernels/steel_gemm_gather_nax ${STEEL_NAX_HEADERS})
Expand All @@ -172,6 +172,10 @@ if(NOT MLX_METAL_JIT)
${STEEL_NAX_ATTN_HEADERS})

else()
# Reached when (a) the toolchain doesn't qualify (METAL_VERSION<400 or
# SDK<26.2), or (b) MLX_DISABLE_NAX=ON. Either way the nax kernels
# aren't in the metallib; MLX_METAL_NO_NAX tells C++ dispatchers to
# take the non-nax codepath.
target_compile_definitions(mlx PRIVATE MLX_METAL_NO_NAX)
endif()

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