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[Clang] Fix GPU intrinsic helpers incorrectly sign extending #129560

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Merged
merged 1 commit into from
Mar 3, 2025
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[Clang] Fix GPU intrinsic helpers incorrectly sign extending #129560

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2 changes: 1 addition & 1 deletion clang/lib/Headers/amdgpuintrin.h
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Original file line number Diff line number Diff line change
Expand Up @@ -121,7 +121,7 @@ __gpu_read_first_lane_u64(uint64_t __lane_mask, uint64_t __x) {
uint32_t __hi = (uint32_t)(__x >> 32ull);
uint32_t __lo = (uint32_t)(__x & 0xFFFFFFFF);
return ((uint64_t)__builtin_amdgcn_readfirstlane(__hi) << 32ull) |
((uint64_t)__builtin_amdgcn_readfirstlane(__lo));
((uint64_t)__builtin_amdgcn_readfirstlane(__lo) & 0xFFFFFFFF);
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@arsenm arsenm Mar 3, 2025

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why not just cast to uint32_t?

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@arsenm arsenm Mar 3, 2025

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also the builtin really should have been unsigned to begin with

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@jhuber6 jhuber6 Mar 3, 2025

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These are being combined, so they both should be 64-bit. I could cast to unsigned, then cast to unsigned 64-bit, but I think explicitly masking is clearer here.

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@arsenm arsenm Mar 3, 2025

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this is also clamping the thing you just clamped 2 lines above

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@jhuber6 jhuber6 Mar 3, 2025

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The clamp above likely isn't necessary, as a cast to unsigned will probably truncate, that's probably a no-op after optimizations. But these are different, the input is unsigned but the output is signed, so we need to clamp it again.

}

// Returns a bitmask of threads in the current lane for which \p x is true.
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3 changes: 2 additions & 1 deletion clang/lib/Headers/nvptxintrin.h
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Expand Up @@ -131,7 +131,8 @@ __gpu_read_first_lane_u64(uint64_t __lane_mask, uint64_t __x) {
__gpu_num_lanes() - 1)
<< 32ull) |
((uint64_t)__nvvm_shfl_sync_idx_i32(__mask, __lo, __id,
__gpu_num_lanes() - 1));
__gpu_num_lanes() - 1) &
0xFFFFFFFF);
}

// Returns a bitmask of threads in the current lane for which \p x is true.
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