https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106513
Bug ID: 106513 Summary: bswap is incorrectly generated Product: gcc Version: 13.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: kristerw at gcc dot gnu.org Target Milestone: --- GCC may incorrectly generate bswap instructions for code not doing a correct swap. This can be seen by running the function from testsuite/gcc.dg/pr40501.c as typedef long int int64_t; __attribute__((noinline)) int64_t swap64 (int64_t n) { return (((n & (((int64_t) 0xff) )) << 56) | ((n & (((int64_t) 0xff) << 8)) << 40) | ((n & (((int64_t) 0xff) << 16)) << 24) | ((n & (((int64_t) 0xff) << 24)) << 8) | ((n & (((int64_t) 0xff) << 32)) >> 8) | ((n & (((int64_t) 0xff) << 40)) >> 24) | ((n & (((int64_t) 0xff) << 48)) >> 40) | ((n & (((int64_t) 0xff) << 56)) >> 56)); } int main (void) { volatile int64_t n = 0x8000000000000000l; if (swap64(n) != 0xffffffffffffff80l) __builtin_abort (); return 0; } This fails at -Os and higher optimization levels.