[Driver, CodeGen] pass through and apply -fassociative-math

There are 2 parts to getting the -fassociative-math command-line flag translated to LLVM FMF:

1. In the driver/frontend, we accept the flag and its 'no' inverse and deal with the 
   interactions with other flags like -ffast-math -fno-signed-zeros -fno-trapping-math. 
   This was mostly already done - we just need to translate the flag as a codegen option. 
   The test file is complicated because there are many potential combinations of flags here.
   Note that we are matching gcc's behavior that requires 'nsz' and no-trapping-math.

2. In codegen, we map the codegen option to FMF in the IR builder. This is simple code and 
   corresponding test.

For the motivating example from PR27372:

float foo(float a, float x) { return ((a + x) - x); }

$ ./clang -O2 27372.c -S -o - -ffast-math  -fno-associative-math -emit-llvm  | egrep 'fadd|fsub'
  %add = fadd nnan ninf nsz arcp contract float %0, %1
  %sub = fsub nnan ninf nsz arcp contract float %add, %2

So 'reassoc' is off as expected (and so is the new 'afn' but that's a different patch). 
This case now works as expected end-to-end although the underlying logic is still wrong:

$ ./clang  -O2 27372.c -S -o - -ffast-math  -fno-associative-math | grep xmm
	addss	%xmm1, %xmm0
	subss	%xmm1, %xmm0

We're not done because the case where 'reassoc' is set is ignored by optimizer passes. Example:

$ ./clang  -O2 27372.c -S -o - -fassociative-math -fno-signed-zeros -fno-trapping-math -emit-llvm  | grep fadd
  %add = fadd reassoc float %0, %1

$ ./clang -O2  27372.c -S -o - -fassociative-math -fno-signed-zeros -fno-trapping-math | grep xmm
	addss	%xmm1, %xmm0
	subss	%xmm1, %xmm0

Differential Revision: https://reviews.llvm.org/D39812


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320920 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 9fabcdf05a..2e8a737de4 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -639,6 +639,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
                         Args.hasArg(OPT_cl_no_signed_zeros) ||
                         Args.hasArg(OPT_cl_unsafe_math_optimizations) ||
                         Args.hasArg(OPT_cl_fast_relaxed_math));
+  Opts.Reassociate = Args.hasArg(OPT_mreassociate);
   Opts.FlushDenorm = Args.hasArg(OPT_cl_denorms_are_zero);
   Opts.CorrectlyRoundedDivSqrt =
       Args.hasArg(OPT_cl_fp32_correctly_rounded_divide_sqrt);