https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114449
Bug ID: 114449
Summary: bswap64 not optimized
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: pali at kernel dot org
Target Milestone: ---
https://godbolt.org/z/dc3br9dYT
gcc 13.2 with -O3 does not detect straightforward code for bswap64
functionality. It generates unoptimized code.
uint64_t bswap64_1(uint64_t num) {
uint64_t ret = 0;
for (size_t i = 0; i < sizeof(num); i++) {
ret |= ((num >> (8*(sizeof(num)-1-i))) & 0xff) << (8*i);
}
return ret;
}
Rewriting the code to manually unpack the loop cause that gcc produces
optimized code with single "bswap" instruction on x86-64.
uint64_t bswap64_2(uint64_t num) {
uint64_t ret = 0;
ret |= (((num >> 56) & 0xff) << 0);
ret |= (((num >> 48) & 0xff) << 8);
ret |= (((num >> 40) & 0xff) << 16);
ret |= (((num >> 32) & 0xff) << 24);
ret |= (((num >> 24) & 0xff) << 32);
ret |= (((num >> 16) & 0xff) << 40);
ret |= (((num >> 8) & 0xff) << 48);
ret |= (((num >> 0) & 0xff) << 56);
return ret;
}
Additional -funroll-all-loops argument for the first example does not help and
still produces unoptimized code.
