https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123039
Bug ID: 123039
Summary: crc loop recognition fails at -O3
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: camel-cdr at protonmail dot com
Target Milestone: ---
I noticed that GCC fails to recognize CRC loops, e.g.:
uint32_t
crc32(uint8_t *src, size_t n, uint32_t crc)
{
const uint32_t POLY = 0x04C11DB7;
for (size_t i = 0; i < n; ++i) {
crc = crc ^ ((uint64_t)src[i] << 24);
for (size_t j = 0; j < 8; ++j)
crc = (crc & 1u<<31) ? (crc << 1) ^ POLY : (crc << 1);
}
return crc;
}
pinskia pointed out that this seems to be an -O3 problem, as the loop is
recognized with -O2: https://godbolt.org/z/9E18h191v
