https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107647
Bug ID: 107647
Summary: GCC 12.2.0 may produce FMAs even with
-ffp-contract=off
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=off (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 = 0; i < n; i += 4)
{
float temp0 = da_r * x[i] - x[i+1];
float temp1 = da_r * x[i+2] - x[i+3];
x[i+1] = da_r * x[i+1] + x[i];
x[i+3] = da_r * x[i+3] + x[i+2];
x[i] = temp0;
x[i+2] = temp1;
}
}
$ gcc -S -march=haswell -O2 -ffp-contract=off cscal.c
$ grep fma cscal.s
vfmaddsub231ps %xmm0, %xmm2, %xmm1
I would expect there to be no FMA instructions in there.
