forked from OSchip/llvm-project
b6a79f9916
We can't propagate FMF partially without breaking DAG-level CSE. We either need to
relax CSE to account for mismatched FMF as a temporary work-around or fully propagate
FMF throughout the DAG.
Surprisingly, there are no existing regression tests for this, but here's an example:
define float @fmf(float %a, float %b) {
%mul1 = fmul fast float %a, %b
%nega = fsub fast float 0.0, %a
%mul2 = fmul fast float %nega, %b
%abx2 = fsub fast float %mul1, %mul2
ret float %abx2
}
$ llc -o - badflags.ll -march=x86-64 -mattr=fma -enable-unsafe-fp-math -enable-fmf-dag=0
...
vmulss %xmm1, %xmm0, %xmm0
vaddss %xmm0, %xmm0, %xmm0
retq
$ llc -o - badflags.ll -march=x86-64 -mattr=fma -enable-unsafe-fp-math -enable-fmf-dag=1
...
vmulss %xmm1, %xmm0, %xmm2
vfmadd213ss %xmm2, %xmm1, %xmm0 <--- failed to recognize that (a * b) was already calculated
retq
llvm-svn: 244053
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README.txt
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you're writing a package for LLVM, see docs/Packaging.rst for our suggestions.