[all-commits] [llvm/llvm-project] 07fa6d: [InstCombine] Avoid folding `select(umin(X, Y), X)...

[Konstantin Bogdanov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 07fa6d1d90c714fa269529c3e5004a063d814c4a
      https://github.com/llvm/llvm-project/commit/07fa6d1d90c714fa269529c3e5004a063d814c4a
  Author: Konstantin Bogdanov <thevar1able at users.noreply.github.com>
  Date:   2025-06-14 (Sat, 14 Jun 2025)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    M llvm/test/Transforms/InstCombine/select.ll
    M llvm/test/Transforms/PhaseOrdering/X86/vector-reductions.ll

  Log Message:
  -----------
  [InstCombine] Avoid folding `select(umin(X, Y), X)` with min/max values in false arm (#143020)

Fixes https://github.com/llvm/llvm-project/issues/139050.

This patch adds a check to avoid folding min/max reduction into select, which may block loop vectorization.

The issue is that the following snippet:
```
declare i8 @llvm.umin.i8(i8, i8)

define i8 @masked_min_fold_bug(i8 %acc, i8 %val, i8 %mask) {
; CHECK-LABEL: @masked_min_fold_bug(
; CHECK:       %cond = icmp eq i8 %mask, 0
; CHECK:       %masked_val = select i1 %cond, i8 %val, i8 255
; CHECK:       call i8 @llvm.umin.i8(i8 %acc, i8 %masked_val)
;
  %cond = icmp eq i8 %mask, 0
  %masked_val = select i1 %cond, i8 %val, i8 255
  %res = call i8 @llvm.umin.i8(i8 %acc, i8 %masked_val)
  ret i8 %res
}
```

is being optimized to the following code, which can not be vectorized
later.
```
declare i8 @llvm.umin.i8(i8, i8) #0

define i8 @masked_min_fold_bug(i8 %acc, i8 %val, i8 %mask) {
  %cond = icmp eq i8 %mask, 0
  %1 = call i8 @llvm.umin.i8(i8 %acc, i8 %val)
  %res = select i1 %cond, i8 %1, i8 %acc
  ret i8 %res
}

attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
```

Expected:
```
declare i8 @llvm.umin.i8(i8, i8) #0

define i8 @masked_min_fold_bug(i8 %acc, i8 %val, i8 %mask) {
  %cond = icmp eq i8 %mask, 0
  %masked_val = select i1 %cond, i8 %val, i8 -1
  %res = call i8 @llvm.umin.i8(i8 %acc, i8 %masked_val)
  ret i8 %res
}

attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
```

https://godbolt.org/z/cYMheKE5r



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