https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93039
Bug ID: 93039
Summary: Fails to use SSE bitwise ops for float-as-int
manipulations
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: amonakov at gcc dot gnu.org
Target Milestone: ---
(the non-regression part of PR 92905)
libm functions need to manipulate individual bits of float/double
representations with good efficiency, but on x86 gcc typically does them on
gprs even when it results in sse-gpreg-sse move chain:
float foo(float x)
{
union {float f; unsigned i;} u = {x};
u.i &= ~0x80000000;
return u.f;
}
foo:
movd eax, xmm0
and eax, 2147483647
movd xmm0, eax
ret
It's good to use bitwise ops on general registers if the source or destination
needs to be in a general registe, but for cases like the above creating a
roundtrip is not desirable.
(GCC gets this example right on aarch64; LLVM on x86 compiles this to SSE/AVX
bitwise 'and', taking the immediate from memory)
