http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50168
--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-08-24 09:48:20 UTC --- (In reply to comment #7) > Hm, so the best thing we can do is a peephole recognizing that > > 10: 48 0f bc ff bsf %rdi,%rdi > 14: 48 63 ff movslq %edi,%rdi > > when rdi is zero it will stay so and the movslq is redundant? I don't think so. While the SandyBridge CPU seems to behave that way, there is no such guarantee in the Intel manuals which say that the destination value is undefined if zero-flag is cleared (i.e. the source was zero) and thus some older or future CPUs might behave differently.
