[libc-commits] [clang] [libc] [Clang] Add width handling for <gpuintrin.h> shuffle helper (PR #125896)

[Artem Belevich via libc-commits]

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@@ -149,22 +149,23 @@ _DEFAULT_FN_ATTRS static __inline__ void __gpu_sync_lane(uint64_t __lane_mask) {
 
 // Shuffles the the lanes inside the warp according to the given index.
 _DEFAULT_FN_ATTRS static __inline__ uint32_t
-__gpu_shuffle_idx_u32(uint64_t __lane_mask, uint32_t __idx, uint32_t __x) {
+__gpu_shuffle_idx_u32(uint64_t __lane_mask, uint32_t __idx, uint32_t __x,
+                      uint32_t __width) {
   uint32_t __mask = (uint32_t)__lane_mask;
-  return __nvvm_shfl_sync_idx_i32(__mask, __x, __idx, __gpu_num_lanes() - 1u);
+  return __nvvm_shfl_sync_idx_i32(__mask, __x, __idx,
+                                  ((__gpu_num_lanes() - __width) << 8u) | 0x1f);
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Artem-B wrote:

Hmm.. Looks like CUDA SDK implements shfl_sync_idx in their own headers the same way:
![image](https://github.com/user-attachments/assets/244eabbb-a5d4-4c26-b4ce-39ad034f118b)

OK, I'm officially confused now, but given that it's been implemented this way for about a decade now, I'm fine keeping it as is, until there's concrete evidence that it's broken.

In practice it probably means that we can't (and don't) really use non-default values for width on NVIDIA GPUs.


https://github.com/llvm/llvm-project/pull/125896