https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97887
Bug ID: 97887
Summary: Failure to optimize neg plus div to avoid using x87
floating point stack
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
float f(float a)
{
return -a / a;
}
On x86 -O3, LLVM outputs this:
.LCPI0_0:
.long 0x80000000 # float -0
.long 0x80000000 # float -0
.long 0x80000000 # float -0
.long 0x80000000 # float -0
f(float):
movaps xmm1, xmmword ptr [rip + .LCPI0_0] # xmm1 =
[-0.0E+0,-0.0E+0,-0.0E+0,-0.0E+0]
xorps xmm1, xmm0
divss xmm1, xmm0
movaps xmm0, xmm1
ret
GCC outputs this:
f(float):
movss DWORD PTR [rsp-4], xmm0
fld DWORD PTR [rsp-4]
movaps xmm1, xmm0
fchs
fstp DWORD PTR [rsp-4]
movss xmm0, DWORD PTR [rsp-4]
divss xmm0, xmm1
ret
I'm *pretty sure* that loading the value into the x87 stack (especially mixed
with SSE instructions) is much slower than using SSE instructions for this.
