Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 04/05/2016 17:51, Wilco Dijkstra wrote:
> Adhemerval Zanella wrote:
>>
>> But my point is all the architectures which provide an optimized mempcpy is
>> though either 1. jump directly to optimized memcpy (s390 case for this patchset),
>> 2. clonning the same memcpy implementation and adjusting the pointers (x86_64) or
>> 3. using a similar strategy for both implementations (powerpc).
> 
> Indeed, which of those are used doesn't matter much.
> 
>> So for this change I am proposing compiler support won't be required because both
>> memcpy and __mempcpy will be transformed to memcpy + s.  Based on assumption that
>> memcpy is fast as mempcpy implementation I think there is no need to just add
>> this micro-optimization to only s390, but rather make is general.
> 
> GLIBC already has this optimization in the generic string header, it's just that s390 wants
> to do something different again. As long as GCC isn't fixed this isn't possible to support
> s390 without this header workaround. And we need GCC to improve so things work
> better for all the other C libraries...

But the current one at string/string.h is only enabled with !defined _HAVE_STRING_ARCH_mempcpy,
so if a port actually adds a mempcpy one it won't be enabled.  What I am trying to argue it
to just remove the !defined _HAVE_STRING_ARCH_mempcpy and enable it as default for all
ports.

> 
> In an ideal world mempcy and friends should never have been introduced as micro
> optimizations, they just cause performance and maintenance headaches for no actual
> benefit...
> 

Agreed ...

> Wilco
>