http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50168
--- Comment #3 from Gunther Piez <gpiez at web dot de> 2011-08-23 21:54:40 UTC --- On 23.08.2011 19:58, jakub at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50168 > > Jakub Jelinek <jakub at gcc dot gnu.org> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |uros at gcc dot gnu.org > > --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-08-23 > 17:58:52 UTC --- > Those aren't equivalent unfortunately, because bsf and bsr insns on x86 have > undefined value if the source is zero. While __builtin_c[lt]z* documentation > says that the result is undefined in that case, I wonder if it would be fine > even if long l = (int) __builtin_c[lt]z* (x); gave a value that wasn't > actually > sign-extended to 64 bits. > The combiner already simplifies zero or sign extension of popcount/parity/ffs > and, if ctz or clz value is defined at zero, also those, but if it is > undefined > it assumes anything in any of the bits and thus can't optimize the sign/zero > extension away. With -mbmi it will be optimized just fine, because for tzcnt > (and lzcnt for -mlzcnt) insns are well defined even for source operand zero. >
