https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99930
Bug ID: 99930
Summary: Failure to optimize floating point -abs(x) in
nontrivial code at -O2/3
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: core13 at gmx dot net
Target Milestone: ---
Expected compiler output for -abs(x) is an orps setting the sign bit.
It works as expected with trivial code at -O1/2/3 optimization levels:
float q(float p)
{
return -std::abs(p);
}
orps xmm0, XMMWORD PTR .LC1[rip]
ret
With more complex code the compiler uses orps at -O1 but andps + xorps at
-O2/3:
bool t(float n[2], float m)
{
for (int i = 0; i < 2; i++)
if (m > -std::abs(n[i]))
return true;
return false;
}
-O1
movss xmm1, DWORD PTR [rdi]
orps xmm1, XMMWORD PTR .LC1[rip]
comiss xmm0, xmm1
ja .L3
movss xmm1, DWORD PTR [rdi+4]
orps xmm1, XMMWORD PTR .LC1[rip]
comiss xmm0, xmm1
seta al
ret
-O2/3
movss xmm1, DWORD PTR [rdi]
movss xmm3, DWORD PTR .LC0[rip]
movss xmm2, DWORD PTR .LC1[rip]
andps xmm1, xmm3
xorps xmm1, xmm2
comiss xmm0, xmm1
ja .L3
movss xmm1, DWORD PTR [rdi+4]
andps xmm1, xmm3
xorps xmm1, xmm2
comiss xmm0, xmm1
seta al
ret
https://godbolt.org/z/5ch5ceEj7
