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Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>, nd <nd at arm dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 5 May 2016 07:45:12 -0700
- Subject: Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
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On Thu, May 5, 2016 at 7:15 AM, Adhemerval Zanella
> On 05/05/2016 10:37, H.J. Lu wrote:
>> On Wed, May 4, 2016 at 1:58 PM, Adhemerval Zanella
>> <firstname.lastname@example.org> wrote:
>>> 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
>> Please don't enable it for x86. Calling memcpy means we have to
>> save and restore 2 registers for no good reasons.
> Yes, direct call will require save and restore the size for further add
> and this is true for most architectures. My question is if does this
> really matter in currently GLIBC internal usage and on programs that
> might use it compared against the burden of keeping the various
> string*.h header in check for multiple architectures or adding this
> logic (mempcpy transformation to memcpy) on compiler.
What burden? There is nothing to do in glibc for x86. GCC can
inline mempcpy for x86.