[PATCH 3/4] S390: Do not call memcpy, memcmp, memset within libc.so via ifunc-plt.

[Stefan Liebler]

On 05/04/2016 05:22 PM, Adhemerval Zanella wrote:
>
>
> On 27/04/2016 05:14, Stefan Liebler wrote:
>> On 04/26/2016 03:35 PM, Adhemerval Zanella wrote:
>>>
>>>
>>> On 26/04/2016 09:07, Stefan Liebler wrote:
>>>> On s390, the memcpy, memcmp, memset functions are IFUNC symbols,
>>>> which are created with s390_libc_ifunc-macro.
>>>> This macro creates a __GI_ symbol which is set to the
>>>> ifunced symbol. Thus calls within libc.so to e.g. memcpy
>>>> result in a call to *ABS*+0x954c0@plt stub and afterwards
>>>> to the resolved memcpy-ifunc-variant.
>>>>
>>>> This patch sets the __GI_ symbol to the default-ifunc-variant
>>>> to avoid the plt call. The __GI_ symbols are now created at the
>>>> default variant of ifunced function.
>>>
>>> Is the internal ifunc plt usage leading to a failure in s390/s390x
>>> (as for powerpc32 and i686) or is it an optimization fix?
>>
>> No it does not lead to a failure on s390.
>> It is an optimization to avoid the extra call to the plt-stub if called within libc.so.
>>
>
> Right because on 19c4bec0f43599eecc2f32de96ae179cd7d64053 I did the
> exact opposite because for POWER it shows that the gains of using
> optimized version over default one shows a good improvement in
> algorithms that use these symbol internally (like regex).
>
> It might be the case where the default s390 version is good enough
> and shows no performance difference.
>

Hi Adhemerval,

I've retested the patchset in context of regex with these functions 
called with/without ifunc-plt.
The version without ifunc-plt is slightly faster.

But you are right. This decision has to be redetermined with further 
variants or newer machines.

In case of the string-functions, calling them via ifunc within libc.so 
could gain improvement.

Bye
Stefan