https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91645
Bug ID: 91645
Summary: Missed optimization with sqrt(x*x)
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lisyarus at gmail dot com
Target Milestone: ---
Based on a discussion on stackoverflow:
https://stackoverflow.com/questions/57673825/how-to-force-gcc-to-assume-that-a-floating-point-expression-is-non-negative.
With gcc-trunk and '-std=c++17 -O3', the function
float test (float x)
{
return std::sqrt(x*x);
}
produces the following assembly:
test(float):
mulss xmm0, xmm0
pxor xmm2, xmm2
ucomiss xmm2, xmm0
movaps xmm1, 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
As far as I can tell, it calls sqrtf, unless the argument to sqrt is >= 0, to
check for negatives/NaN's and set the appropriate errno. The behavior is
reasonable, as expected.
Adding '-fno-math-errno', '-ffast-math', or '-ffinite-math-only' removes all
the clutter and compiles the same code into the neat
test(float):
mulss xmm0, xmm0
sqrtss xmm0, xmm0
ret
Now, the problem is that GCC doesn't seem to optimize away the call to sqrtf
based on some surrounding code. As an example, it would be neat to have this
(or something similar) to get compiled into the same mulss-sqrtss-ret:
float test (float x)
{
float y = x*x;
if (y >= 0.f)
return std::sqrt(y);
__builtin_unreachable();
}
If I understand it correctly, the 'y >= 0.f' excludes 'y' being NaN and 'y'
being negative (though this is excluded by 'y = x*x'), so there is no need to
check if the argument to `std::sqrt` is any bad, enabling to just do 'sqrtss'
and return.
Furthemore, adding e.g. '#pragma GCC optimize ("no-math-errno")' before the
'test' function doesn't lead to optimizing it either, though I'm not sure
whether this is expected to work and/or requires a separate bugtracker issue.
