michele.scandale added inline comments.
================
Comment at: clang/lib/CodeGen/CGExprScalar.cpp:227
+ FMF.setAllowContract(FPFeatures.allowFPContractAcrossStatement() ||
+ FPFeatures.allowFPContractWithinStatement());
}
----------------
I'm not convinced it correct to set `contract` when
`allowFPContractWithinStatement` return true. Can someone clarify this?
If I compile the following example with `-ffp-contract=on`:
```
float test1(float a, float b, float c) {
float x = a * b;
return x + c;
}
float test2(float a, float b, float c) {
return a * b + c;
}
```
Before this change the generated code was:
```
define float @test1(float %a, float %b, float %c) {
%0 = fmul float %a, %b
%1 = fadd float %0, %c
ret float %1
}
define float @test2(float %a, float %b, float %c) {
%0 = call float @llvm.fmuladd.f32(float %a, float%b, float %c)
ret float %0
}
```
And my understanding is that the in-statement contraction is implemented by
emitting the `llvm.fmuladd` call that a backend might decide to implement as
`fmul + fadd` or as `fma`.
With this change the generated code is:
```
define float @test1(float %a, float %b, float %c) {
%0 = fmul contract float %a, %b
%1 = fadd contract float %0, %c
ret float %1
}
define float @test2(float %a, float %b, float %c) {
%0 = call contract float @llvm.fmuladd.f32(float %a, float%b, float %c)
ret float %0
}
```
and it seems to me that in `test1` (where multiple statements where explicitly
used) the optimizer is now allowed to perform the contraction, violating the
original program semantic where only "in-statement" contraction was allowed.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D72841/new/
https://reviews.llvm.org/D72841
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