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

[Richard Guenther <richard dot guenther at gmail dot com>]

On Sat, Aug 8, 2009 at 11:59 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.

The patch misses testcases.  Why does zee run after register allocation?
Your examples suggest that it will free hard registers so doing it before
regalloc looks odd.

What is the compile-time impact of your patch on say, gcc bootstrap?
How many percent of instructions are removed as useless zero-extensions
during gcc bootstrap?  How much do CSiBE numbers improve?

Thanks,
Richard.

>
> 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.
>
> Motivating Example I :
> ----------------------------------
> For this program :
> **********************************************
> bad_code.c
>
> int mask[1000];
>
> int foo(unsigned x)
> {
> ?if (x < 10)
> ? ?x = x * 45;
> ?else
> ? ?x = x * 78;
> ?return mask[x];
> }
> **********************************************
>
> $ gcc -O2 bad_code.c
> ?........
> ?400315: ? ? ? b8 4e 00 00 00 ? ? ? ? ? ?mov ? ?$0x4e,%eax
> ?40031a: ? ? ? 0f af f8 ? ? ? ? ? ? ? ? ? ? ? ?imul ? %eax,%edi
> ?40031d: ? ? ? 89 ff ? ? ? ? ? ? ? ? ? ? ? ? ? ? mov ? ?%edi,%edi
> ---> Useless zero extend.
> ?40031f: ? ? ? 8b 04 bd 60 19 40 00 ? ?mov ? ?0x401960(,%rdi,4),%eax
> ?400326: ? ? ? c3 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? retq
> ?......
> ?400330: ? ? ? ba 2d 00 00 00 ? ? ? ? ?mov ? ?$0x2d,%edx
> ?400335: ? ? ? 0f af fa ? ? ? ? ? ? ? ? ? ? ?imul ? %edx,%edi
> ?400338: ? ? ? 89 ff ? ? ? ? ? ? ? ? ? ? ? ? ? mov ? ?%edi,%edi ?--->
> Useless zero extend.
> ?40033a: ? ? ? 8b 04 bd 60 19 40 00 ? ?mov ? ?0x401960(,%rdi,4),%eax
> ?400341: ? ? ? c3 ? ? ? ? ? ? ? ? ? ? ?retq
>
> $ gcc -O2 -fzee bad_code.c
> ?......
> ?400315: ? ? ? 6b ff 4e ? ? ? ? ? ? ? ?imul ? $0x4e,%edi,%edi
> ?400318: ? ? ? 8b 04 bd 40 19 40 00 ? ?mov ? ?0x401940(,%rdi,4),%eax
> ?40031f: ? ? ? c3 ? ? ? ? ? ? ? ? ? ? ?retq
> ?400320: ? ? ? 6b ff 2d ? ? ? ? ? ? ? ?imul ? $0x2d,%edi,%edi
> ?400323: ? ? ? 8b 04 bd 40 19 40 00 ? ?mov ? ?0x401940(,%rdi,4),%eax
> ?40032a: ? ? ? c3 ? ? ? ? ? ? ? ? ? ? ?retq
>
>
>
> Thanks,
>
> Sriraman M Tallam.
> Google, Inc.
> tmsriram@google.com
>