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Re: [PATCH 3/4] S390: Do not call memcpy, memcmp, memset within libc.so via ifunc-plt.
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: libc-alpha at sourceware dot org
- Date: Wed, 4 May 2016 12:22:06 -0300
- Subject: Re: [PATCH 3/4] S390: Do not call memcpy, memcmp, memset within libc.so via ifunc-plt.
- Authentication-results: sourceware.org; auth=none
- References: <1461672469-2107-1-git-send-email-stli at linux dot vnet dot ibm dot com> <1461672469-2107-3-git-send-email-stli at linux dot vnet dot ibm dot com> <571F6EA0 dot 7080201 at linaro dot org> <nfpsdv$aac$1 at ger dot gmane dot org>
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.