https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127984
Bug ID: 127984
Summary: wrong code at -O2 and -O3 for a bit-reflected CRC loop
of 24 iterations
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kyuwoncho18 at gmail dot com
Target Milestone: ---
Target: x86_64-pc-linux-gnu
__attribute__((noipa)) unsigned f(unsigned crc) {
for (int i = 0; i < 24; i++) {
if (crc & 1) crc = (crc >> 1) ^ 0xEDB88320; else crc >>= 1;
}
return crc;
}
int main(void) {
if (f(0xFFFFFFFF) != 0xBE26ED)
__builtin_abort();
}
godbolt: https://godbolt.org/z/9os41K4Mb
With -O0, -O1 or -Os, f returns 0x00be26ed (24 shift steps) and the program
exits normally, but with -O2 or -O3 it returns 0xdebb20e3, the value after 32
steps, and the program aborts. -fno-optimize-crc avoids it.
Disclaimer: This test case was generated by a fuzzer, and the report was
written with LLM assistance.
