[PATCH] S390: Refactor ifunc handling

[Stefan Liebler]

On 12/11/2018 03:46 PM, Stefan Liebler wrote:
> Hi all,
> 
> after all the comments, we've also re-discussed the approach internally.
> We still see more advantages (apart from performance-perspective) than 
> disadvantages and thus plan to commit the patches if there are no strong 
> objections.
> 
> I've summarized some points:
> -Reducing code size as not used ifunc variants are omitted. E.g. when 
> building with march >= zEC12 we have only one variant for memset, 
> memcmp, memcpy instead of three for each. When building with march >= 
> z13 we have only the vector variants for the string functions.
> 
> -Ifunc symbols are avoided at all (if we have only one remaining ifunc 
> variant). In the most tools like binutils, gdb and co, ifunc symbols are 
> implemented as separate more complex code paths. Thus we think it is 
> better to have less ifuncs. E.g. setting a breakpoint on an ifunc'ed 
> symbol is more complicated than for an usual symbol.
> 
> -Automatically using the best available ifunc variant as glibc internal 
> symbol (depending on the used gcc-options) without adding the 
> plut-stub-overhead. The same applies to --disable-multiarch and ld.so.
> 
> -Introducing ifunc'ed glibc internal symbol is not possible on s390-32 
> as a non plt-relocation is generated by gcc but an implicit plt-stub is 
> used. The plt-stub needs the GOT-pointer in r12 which was not loaded by 
> gcc.
> 
> -Calling a non-ifunc'ed glibc internal symbols is just a brasl to this 
> symbol. There are no special hacks. What shall go wrong? Whereas calling 
> an ifunc'ed glibc internal symbol involves the additional special plt-stub.
> 
> -As the code is shared between s390-32 and s390-64 we do not want to 
> introduce ifunc'ed glibc internal symbols for e.g. s390-64 and 
> non-ifunc'ed ones for s390-32.
> 
> -Testing glibc needs to be done for all march-levels - with or without 
> these patches. Thus we don't see the more testing effort.
> 
> -Testing the implementation of all different ifunc variants of a 
> specific function is still possible with ifunc-impl-list mechanism if 
> march-level is set to the oldest march-level.
> 
> Bye
> Stefan
> 

Committed.