On Wed, Sep 23, 2009 at 3:57 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Sat, Aug 8, 2009 at 2:59 PM, Sriraman Tallam <tmsri...@google.com> wrote: >> Hi, >> >> Here is a patch to eliminate redundant zero-extension instructions >> on x86_64. >> >> Tested: Ran the gcc regresssion testsuite on x86_64-linux and verified >> that the results are the same with/without this patch. >> >> >> Problem Description : >> --------------------------------- >> >> This pass is intended to be applicable only to targets that implicitly >> zero-extend 64-bit registers after writing to their lower 32-bit half. >> For instance, x86_64 zero-extends the upper bits of a register >> implicitly whenever an instruction writes to its lower 32-bit half. >> For example, the instruction *add edi,eax* also zero-extends the upper >> 32-bits of rax after doing the addition. These zero extensions come >> for free and GCC does not always exploit this well. That is, it has >> been observed that there are plenty of cases where GCC explicitly >> zero-extends registers for x86_64 that are actually useless because >> these registers were already implicitly zero-extended in a prior >> instruction. This pass tries to eliminate such useless zero extension >> instructions. >> > > Does this fix: > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17387
Yes, this patch fixes this problem. All the mov %eax, %eax are removed. > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653 No, this patch does not fix this problem. > > -- > H.J. >
