llvm / llvm/llvm-project

Missed optimization: fold fcmp ogt (select x > 0.0, x, 0.0), 1.0 to fcmp ogt x, 1.0

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#186,556 0 comments 0 reactions 1 assignee Claimed by @tclin914 View on GitHub
llvm:instcombine missed-optimization
Dominant language
LLVM
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Description

```llvm
define i1 @src(float %arg1, ptr %arg0, ptr %arg2) {
%v0 = getelementptr inbounds nuw i8, ptr %arg2, i64 4
%v1 = load i16, ptr %v0, align 2
%v2 = zext i16 %v1 to i64
%v3 = getelementptr inbounds nuw float, ptr %arg0, i64 %v2
%v4 = load float, ptr %v3, align 4
%v5 = fmul float %arg1, %v4
%v6 = fcmp ogt float %v5, 0.000000e+00
%v7 = select i1 %v6, float %v5, float 0.000000e+00
%v8 = fcmp ogt float %v7, 1.000000e+00
ret i1 %v8
}

define i1 @tgt(float %arg1, ptr %arg0, ptr %arg2) {
%v0 = getelementptr inbounds nuw i8, ptr %arg2, i64 4
%v1 = load i16, ptr %v0, align 2
%v2 = zext i16 %v1 to i64
%v3 = getelementptr inbounds nuw float, ptr %arg0, i64 %v2
%v4 = load float, ptr %v3, align 4
%v5 = fmul float %arg1, %v4
%v6 = fcmp ogt float %v5, 1.000000e+00
ret i1 %v6
}
```
godbolt: https://godbolt.org/z/hP74zo37e
alive2: Available when timeout limit = 50000
```llvm
define i1 @src(float %arg1, ptr %arg0, ptr %arg2) {
#0:
%v0 = gep inbounds nuw ptr %arg2, 1 x i64 4
%v1 = load i16, ptr %v0, align 2
%v2 = zext i16 %v1 to i64
%v3 = gep inbounds nuw ptr %arg0, 4 x i64 %v2
%v4 = load float, ptr %v3, align 4
%v5 = fmul float %arg1, %v4
%v6 = fcmp ogt float %v5, 0.000000
%v7 = select i1 %v6, float %v5, float 0.000000
%v8 = fcmp ogt float %v7, 1.000000
ret i1 %v8
}
=>
define i1 @tgt(float %arg1, ptr %arg0, ptr %arg2) {
#0:
%v0 = gep inbounds nuw ptr %arg2, 1 x i64 4
%v1 = load i16, ptr %v0, align 2
%v2 = zext i16 %v1 to i64
%v3 = gep inbounds nuw ptr %arg0, 4 x i64 %v2
%v4 = load float, ptr %v3, align 4
%v5 = fmul float %arg1, %v4
%v6 = fcmp ogt float %v5, 1.000000
ret i1 %v6
}
Transformation seems to be correct!
```

Pattern found in: https://github.com/dtcxzyw/llvm-opt-benchmark/blob/main/bench/openexr/optimized/makePreview.ll

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