https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94837
Bug ID: 94837
Summary: Failure to optimize out spurious movbe into bswap
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
float swapFloat(float x)
{
union
{
float f;
uint32_t u32;
} swapper;
swapper.f = x;
swapper.u32 = __builtin_bswap32(swapper.u32);
return swapper.f;
}
For this function, on x86-64 with `-O3 -mmovbe`, LLVM outputs this :
swapFloat(float): # @swapFloat(float)
movd eax, xmm0
bswap eax
movd xmm0, eax
ret
GCC instead outputs this :
swapFloat(float):
movd DWORD PTR [rsp-4], xmm0
movbe eax, DWORD PTR [rsp-4]
movd xmm0, eax
ret
It seems highly likely to me that a spill to memory is much slower than a
direct `bswap`.
