[all-commits] [llvm/llvm-project] 03461c: [mlir][gpu][spirv] Remove rotation semantics of gp...

[Hsiangkai Wang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 03461c9c6e21e43a6e1c699bfb254ddb3d575c93
      https://github.com/llvm/llvm-project/commit/03461c9c6e21e43a6e1c699bfb254ddb3d575c93
  Author: Hsiangkai Wang <hsiangkai.wang at arm.com>
  Date:   2025-06-19 (Thu, 19 Jun 2025)

  Changed paths:
    M mlir/include/mlir/Dialect/GPU/IR/GPUOps.td
    M mlir/lib/Conversion/GPUToSPIRV/GPUToSPIRV.cpp
    M mlir/test/Conversion/GPUToSPIRV/shuffle.mlir

  Log Message:
  -----------
  [mlir][gpu][spirv] Remove rotation semantics of gpu.shuffle up/down (#139105)

>From the description of gpu.shuffle operation, shuffle up/down rotates
values in the subgroup because it applies modulo on the shifted value to
calculate the result lane ID. It is inconsistent with the definition of
SPIR-V shuffle up/down and NVVM data movement definitions within
subgroup.

In NVVM, it says

"If the computed source lane index j is in range, the returned i32 value
will be the value of %a from lane j; otherwise, it will be the the value
of %a from the current thread."

It will keep the original value if the result land ID is out of range.

In SPIR-V OpGroupNonUniformShuffleUp and OpGroupNonUniformShuffleDown,
it says

"The resulting value is undefined if Delta is greater than the current
invocation’s id within the scope or if the identified invocation is not
in scope restricted tangle."

It's an undefined value if the result land ID is out of range.

Anyway, there is no circular movement in shuffle up/down from these 2
specifications. This patch removes the circular movement in gpu.shuffle
up/down and lower gpu.shuffle up/down to SPIR-V
OpGroupNonUniformShuffleUp and OpGroupNonUniformShuffleDown directly.

Reference:

https://docs.nvidia.com/cuda/archive/12.2.1/nvvm-ir-spec/index.html#data-movement

https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpGroupNonUniformShuffleUp

https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpGroupNonUniformShuffleDown



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications