https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95529
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Uroš Bizjak from comment #4) > (In reply to Gabriel Ravier from comment #2) > > If using `-mbmi`, shouldn't GCC be able to assume the target is, in fact, a > > BMI-capable CPU ? I understand that this bug report may be invalid for `bsf` > > (which would mean Clang has invalid behaviour, which seems odd but ok), but > > should I reopen this report/make a new report for `-mbmi` ? > > We can do a peephole that would convert REP BSF + TEST to BSF. However, on > BMI capable targets, REP BSF decodes as TZCNT, so the question is if one BSF > is faster than TZCNT + TEST? I would expect so, yes. With -mbmi and TZCNT we could also use the carry flag to elide the test. > (Please note that the conversion to CMOVE comes a bit late in the pass > sequence, so we can't convert TZCNT + TEST + CMOVE to TZCNT + CMOVC.)
