llvm-project

07fa6d1d - [InstCombine] Avoid folding `select(umin(X, Y), X)` with min/max values in false arm (#143020)

[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