https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838
--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to Wilco from comment #15) > It would make more sense to move x86 backends to CTZ_DEFINED_VALUE_AT_ZERO > == 2 so that you always get the same result even when you don't have tzcnt. > A conditional move would be possible, so it adds an extra 2 instructions at > worst (ie. still significantly faster than doing the table lookup, multiply > etc). And it could be optimized when you know CLZ/CTZ input is non-zero. Conditional moves are a lottery on x86, in many cases very bad idea. And when people actually use __builtin_clz*, they state that they don't care about the 0 value, so emitting terribly performing code for it just in case would be wrong. If forwprop emits the conditional in separate blocks for the CTZ_DVAZ!=2 case, on targets where conditional moves are beneficial for it it can also emit them, or emit the jump which say on x86 will be most likely faster than cmov.
