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Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
- From: Stefan Liebler <stli at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org
- Date: Mon, 9 May 2016 16:15:21 +0200
- Subject: Re: [PATCH 4/4] S390: Implement mempcpy with help of memcpy. [BZ #19765]
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On 05/05/2016 06:36 PM, H.J. Lu wrote:
On Thu, May 5, 2016 at 9:34 AM, Adhemerval Zanella
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
On 05/05/2016 10:37, H.J. Lu wrote:
On Wed, May 4, 2016 at 1:58 PM, Adhemerval Zanella
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.
I believe most of those micro-optimizations belong to GCC, not glibc.
Of course, we should keep the existing ones for older GCCs. We
should avoid adding new ones.
Does this mean, the proposed way is to not add a macro for mempcpy in
sysdeps/s390/bits/string.h or e.g. for another architecture?
If _HAVE_STRING_ARCH_mempcpy is not defined for s390, then it will
always use the macro defined in string/string.h, which inlines memcpy +
n and the mempcpy-function is not called. The memcpy is transformed to
mvc, mvhi for e.g. constant lengths. Otherwise, the memcpy-function is
called and the length will be added after the call.
According to PR70140
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70140), the macro in
string/string.h will be removed after this bug is fixed in GCC?
Then I think the decision, if mempcpy or memcpy is called or an inline
version is emitted, will be done per architecture backend?
If this is all true, then the mempcpy-function will be called in future
if it makes sense!?
If _HAVE_STRING_ARCH_mempcpy will be defined for s390, then
mempcpy-function is always called - even for cases with constant length
where mvc or mvhi is emitted.
This is definitely not the intention of this patch.
After fixing PR70140, GCC will correctly handle the constant length cases!?