https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107006
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|rtl-optimization |tree-optimization
Last reconfirmed| |2022-09-22
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Keywords| |missed-optimization
--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
The reason is that the loops are not unrolled (early enough or at all) for the
bswap/load detection. So to work you have to unroll the loops manually or
direct GCC to do that, for example
inline uint64_t get_le64 (const unsigned char x[64/8]) {
uint64_t y = 0;
#pragma GCC unroll 8
for (size_t i = 0; i < sizeof y; i++)
if (0) y |= (uint64_t)x[i] << ((sizeof y - 1 - i)*8);
else y |= (uint64_t)x[i] << i*8; return y;
}
produces
get_le64:
.LFB11:
.cfi_startproc
movq (%rdi), %rax
ret
the unroll heuristics do not anticipate that later bswap/load detection will
merge all the loads and thus not grow code too much.
