>
> GCC bootstrap :
>
> Total number of zero-extension instructions before : 1456
> Total number of zero-extension instructions after : 5814
> No impact on boot-strap time.
You sure you have these numbers the right way around ? Shouldn't the
number of zero-extension instructions after the patch be less than the
number of zero-extension instructions before or is this a regression
?
Thanks,
Ramana
>
>
> I have attached the latest patch :
>
>
> On Sun, Aug 9, 2009 at 2:15 PM, Richard Guenther
> <richard.guent...@gmail.com> wrote:
>> On Sat, Aug 8, 2009 at 11:59 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.
>>
>> 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.
>>> tmsri...@google.com
>>>
>>
>
