https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96135
Bug ID: 96135 Summary: [9/10/11 regression] bswap not detected by bswap pass, unexpected results between optimization levels Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: tobi at gcc dot gnu.org Target Milestone: --- This is an odd one, and it seems different from the other bswap bugs that I could find in bugzilla. This is on x64. Compiler Explorer link is here: https://godbolt.org/z/arTf5T Full source code: ============================================================== constexpr long long bswap64(long long in) // unsigned long long behaves the same { union { long long v; char c[8]; } u{in}; union { char c[8]; long long v; } v{ u.c[7], u.c[6], u.c[5], u.c[4], u.c[3], u.c[2], u.c[1], u.c[0]}; return v.v; } long long f(long long i) { return bswap64(i); } constexpr long long bswapD(double x) { return bswap64(*(long long*)&x); } long long g(double x) { return bswapD(x); } =============================================================== There are three observations / bugs: 1) bswapD is never recognized as byte-swapping 2) bswap64 is optimized to bswap at -O2 but not at -O3 3) 131t.bswap never shows bswap, apparently the pass doesn't detect this way of writing bswap, leaving it to the RTL optimizers. Hence I classified this as tree-optimization bug. Verified at -O2 with 9.3, 10.1 and trunk on the compiler explorer. I'm flagging this as a regression because at -O2 gcc 8.3 detects bswap in both cases, but I'm guessing that this is by some accident. In 7.5 neither function is compiled as bswap.