Re: Request for code review - (ZEE patch : Redundant Zero extension elimination)

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Aug 8, 2009 at 2:59 PM, Sriraman Tallam <tmsriram@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
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653

-- 
H.J.