Re: [PATCH] S390: Refactor ifunc handling

[Stefan Liebler <stli at linux dot ibm dot com>]

On 11/30/2018 07:31 PM, Adhemerval Zanella wrote:
> On 30/11/2018 13:57, Stefan Liebler wrote:
> > This patch series is mainly refactoring the s390 specific ifunc handling.
> > The idea is to omit ifunc variants or ifunc at all if the used compile
> > options are already building for newer cpus. The glibc internal calls
> > and ld.so will then use "newer" ifunc variants as before.
> >
> > In case of the memcpy, memset and memcmp functions, the newest ifunc variant
> > is for z196 and there are two further ones for older cpus, but the current usual
> > compile options are e.g. building for zEC12.
> > In case of the string / wcsmbs functions, there are variants for z13 and
> > "before z13".  After switching to z13 as default cpu level, there won't
> > be IFUNC symbols in s390 libc.so.
> >
> > Furthermore new z13 specific ifunc variants are introduced for
> > memmove, strstr and memmem.
> >
> > Some functions like the mem* functions are duplicated twice for 31 and 64 bit.
> > In fact they are nearly the same. Thus those implementations are now unified
> > and adjusted in order to be usable for 31 and 64bit.
> >
> > I've build and tested these patches with different -march levels
> > and with / without multiarch and checked the symbols with readelf - e.g. if
> > IFUNC is used or not and if the __GI_ symbols are targeting the correct
> > ifunc variant.
> >
> > If no one objects, I plan to commit this series in the next one or two weeks.
>
> The only issue I have for this change is it would require another
> ABI variant for testing and validation, which would require more
> coverage from build-many-glibcs.py (similar to armv7 for instance).
The ABI itself is not changed, but you are right tests could be done for 
e.g. -march=zEC12 and -march=z13

> The gains I see is a slight reduction is loading time (due no ifunc)
> and less code side. Is is what is driving you for this change? Does
> it worth the extra testing and validation it might require?
The current s390_vx_libc_ifunc macros are not flexible and do not allow 
future enhancements as they only use the *_c or *_vx variant.

As soon as future ditros will use z13 as default architecture,
vector instructions can be used by default.  But the current __GI_* 
functions won't use the vector variants.  Instead the fallback is always 
used for internal calls and the vector variants are always used for 
external calls via IFUNC.

Bye.
Stefan