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From: "bartoldeman at users dot sourceforge.net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107647] New: GCC 12.2.0 may produce FMAs even
 with -ffp-contract=off
Date: Fri, 11 Nov 2022 16:59:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107647

            Bug ID: 107647
           Summary: GCC 12.2.0 may produce FMAs even with
                    -ffp-contract=3Doff
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bartoldeman at users dot sourceforge.net
  Target Milestone: ---

I stumped upon an example where GCC generates FMA instruction even when FMAs
are disabled using -ffp-contract=3Doff (extracted from
https://github.com/xianyi/OpenBLAS/blob/develop/kernel/x86_64/cscal.c)

$ cat cscal.c
void cscal(int n, float da_r, float *x)
{
  for (int i =3D 0; i < n; i +=3D 4)
    {
      float temp0  =3D  da_r * x[i]   - x[i+1];
      float temp1  =3D  da_r * x[i+2] - x[i+3];
      x[i+1]       =3D  da_r * x[i+1] + x[i];
      x[i+3]       =3D  da_r * x[i+3] + x[i+2];
      x[i]         =3D  temp0;
      x[i+2]       =3D  temp1;
    }
}
$ gcc -S -march=3Dhaswell -O2 -ffp-contract=3Doff cscal.c
$ grep fma cscal.s
        vfmaddsub231ps  %xmm0, %xmm2, %xmm1

I would expect there to be no FMA instructions in there.=