[all-commits] [llvm/llvm-project] c836b8: [mlir][OpenACC] Atomicize contended shared array r...

[Matsu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c836b82d088424baa8a60de63136747add192732
      https://github.com/llvm/llvm-project/commit/c836b82d088424baa8a60de63136747add192732
  Author: Matsu <47756807+khaki3 at users.noreply.github.com>
  Date:   2026-07-30 (Thu, 30 Jul 2026)

  Changed paths:
    M mlir/lib/Dialect/OpenACC/Transforms/ACCCGToGPU.cpp
    M mlir/test/Dialect/OpenACC/acc-cg-to-gpu-reduction-array-shared.mlir

  Log Message:
  -----------
  [mlir][OpenACC] Atomicize contended shared array reduction updates (#212971)

Example:
```fortran
!$acc parallel loop gang reduction(+:a)
do i = 1, N
  !$acc loop worker reduction(+:a)
  do j = 1, M
    a(i) = a(i) + b(j,i)
  end do
end do
```

In this code, the worker accumulator is block-shared, so every worker
updates
the same element with a plain read-modify-write and all but one partial
is lost.

Fix: make in-place updates of a block-shared array accumulator atomic,
unless
the element index varies per thread, in which case each thread owns its
element
and the plain update is already race-free.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply at anthropic.com>



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