This is the mail archive of the
mailing list for the glibc project.
Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- 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 13:34:23 -0300
- Subject: Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
- Authentication-results: sourceware.org; auth=none
- References: <AM3PR08MB00888058CAFD723F21D2342D837B0 at AM3PR08MB0088 dot eurprd08 dot prod dot outlook dot com> <572A3B9C dot 3080803 at linaro dot org> <AM3PR08MB00882DA2CEDFC95A1BB79776837B0 at AM3PR08MB0088 dot eurprd08 dot prod dot outlook dot com> <572A6271 dot 6050802 at linaro dot org> <CAMe9rOrcKzp_bk__3iYxROBdi9=DOSw3HzrZsUrEYCSXXVtCmg at mail dot gmail dot com> <572B559B dot 5080301 at linaro dot org> <CAMe9rOrJgz6Xk6Wu9EdUp4gNn7E7zWkROH-ApZb7ZXteUcMjUg at mail dot gmail dot com>
> On May 5, 2016, at 11:45, H.J. Lu <firstname.lastname@example.org> wrote:
> On Thu, May 5, 2016 at 7:15 AM, Adhemerval Zanella
> <email@example.com> wrote:
>>> 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.
In fact I am objecting all the bits GLIBC added on string*.h that only adds complexity for some micro-optimizations. For x86 I do agree that transforming mempcpy to memcpy is no the best strategy.
My rationale is to avoid add even more arch-specific bits in installed headers to add such optimizations.