http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32629
--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> 2012-06-11 08:39:45 UTC --- On Sat, 9 Jun 2012, hubicka at ucw dot cz wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32629 > > --- Comment #2 from Jan Hubicka <hubicka at ucw dot cz> 2012-06-09 22:17:07 > UTC --- > > I suppose doing the $0x0 optimization should be done post-reload. > I was wondering how to implement this nice for some years already.... I don't > see how this can be done without specialized pass, really, and the interface > is > probably going to be bit weird, since it is very weird property of x86 > instruction set that there are no stores with short immediate... I wonder if we can use a peephole2 and DF information (and update it on-the-fly). Thus, when seeing mov $0, ... mov $0, ... transform it incrementally to xor %eax, %eax mov %eax, ... mov $0, ... and then have a 2nd peephole2 with higher priority that looks for a register with zero content (eh ... that's the interesting part ;)) and do xor %eax, %eax mov %eax, ... mov %eax, ... I suppose we have the first peephole already - though we'd end up with xor %eax, %eax mov %eax, ... xor %eax, %eax mov %eax, ... and rely on postreload-cse to clean that up (which isn't run after the peephole2 in postreload queue ...) Richard.
