Clang and llvm IR - Generate FMA

Clang Frontend
1

Hi,

I am trying to generate FMA for X86 target (“bdver2”) through -ffp-contract=fast. [Sanity checks: bdver2 supports fma]

While I am able to generate FMA for “C” input, I could not when LLVM IR is input. Example illustrated below:

#input - C file
float test(float a, float b, float c) {
return a*b+c;
}

The below arguments work fine -

$clang -march=bdver2 -mfma -ffp-contract=fast -S test.c

However, when I input the below llvm assembly(pasted at the end), clang (not llc) fails to generate FMA. Any idea why and how I will be able to do that through clang?

$ clang -march=bdver2 -ffp-contract=fast -mfma -S test.ll -O3
$ more test.s

test: # @test
.cfi_startproc

BB#0: # %entry

vmulss %xmm1, %xmm0, %xmm0
vaddss %xmm2, %xmm0, %xmm0
ret
.Ltmp0:
.size test, .Ltmp0-test
.cfi_endproc

; ModuleID = ‘test.c’
target datalayout = “e-p:64:64:64-S128-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f16:16:16-f32:32:32-f64:64:64-f128:128:128-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64”
target triple = “x86_64-unknown-linux-gnu”
module asm “\09.ident\09\22GCC: (GNU) 4.7.2 LLVM: 3.2svn\22”
define float @test(float %a, float %b, float %c) nounwind uwtable readnone {
entry:
%0 = fmul float %a, %b
%1 = fadd float %0, %c
ret float %1
}

2

Adding Lang Hames who implemented fpcontract in LLVM and Clang & might
know what's going on here.

4

Hi Anitha,

Sorry about the delayed response. This was caused by -ffp-contract being implemented as a language option, which aren’t parsed when compiling bitcode. I have made -ffp-contract a codegen option in r168027, which should fix your issue.

  • Lang.
5

Hi Anitha,

Sorry about the delayed response. This was caused by -ffp-contract being
implemented as a language option, which aren't parsed when compiling
bitcode. I have made -ffp-contract a codegen option in r168027, which should
fix your issue.

Thanks Lang! Just back-ported the patch to older revision(162999). It worked.

-Anitha