https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91645
--- Comment #5 from Nikita Lisitsa <lisyarus at gmail dot com> ---
Oh, thank you a lot! Indeed, this version compiles to just mulss & sqrtss
float test (float x)
{
float y = x*x;
if (std::isless(y, 0.f))
__builtin_unreachable();
return std::sqrt(y);
}
Yet, I still don't quite understand what is happening here. Is it because the
standard '<' operator is still subject to FE_* ?
Concerning pragmas, the code
#pragma GCC optimize ("no-math-errno")
float test (float x)
{
return std::sqrt(x*x);
}
produces the following assembly
std::sqrt(float):
pxor xmm2, xmm2
movaps xmm1, xmm0
ucomiss xmm2, xmm0
sqrtss xmm1, xmm1
ja .L8
movaps xmm0, xmm1
ret
.L8:
sub rsp, 24
movss DWORD PTR [rsp+12], xmm1
call sqrtf
movss xmm1, DWORD PTR [rsp+12]
add rsp, 24
movaps xmm0, xmm1
ret
test(float):
mulss xmm0, xmm0
jmp std::sqrt(float)
So, the only notable difference is that now 'std::sqrt(float)' is not inlined,
but is tail-called instead. Thus, the pragma seems not to work?
