Sorry, it is the other way around.
Total number of zero-extension instructions before : 5814
Total number of zero-extension instructions after : 1456
Thanks for pointing it.
On Wed, Sep 23, 2009 at 4:10 PM, Ramana Radhakrishnan
<raman...@gmail.com> wrote:
>>
>> 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
>>>>
>>>
>>
>
