[PATCH] Prefer mempcpy to memcpy on x86_64 target (PR middle-end/81657).

Jakub Jelinek jakub@redhat.com
Tue Mar 13 08:35:00 GMT 2018


On Tue, Mar 13, 2018 at 09:24:11AM +0100, Martin Liška wrote:
> On 03/12/2018 10:39 AM, Marc Glisse wrote:
> > On Mon, 12 Mar 2018, Martin Liška wrote:
> > 
> > > This is fix for the PR that introduces a new target macro. Using the macro
> > > one can say that a target has a fast mempcpy and thus it's preferred to be used
> > > if possible.
> > > 
> > > Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
> > > I also tested on x86_64-linux-gnu.
> > > 
> > > Ready to be installed?
> > > Martin
> > > 
> > > gcc/ChangeLog:
> > > 
> > > 2018-03-08  Martin Liska  <mliska@suse.cz>
> > > 
> > >     PR middle-end/81657
> > >     * builtins.c (expand_builtin_memory_copy_args): Add new
> > >     arguments.
> > >     * config/i386/i386.h (TARGET_HAS_FAST_MEMPCPY_ROUTINE):
> > >     New macro.
> > 
> > Shouldn't the macro be defined in a more specific case, for instance glibc on x86? Or do all known libc on x86 happen to provide a fast mempcpy?
> 
> That's Marc a very good question. Do we already have a glibc-related target macros/hooks?
> If so, I would add this as one of these.

Yes, see e.g. TARGET_LIBC_HAS_FUNCTION target hook,
where in particular linux_libc_has_function deals with various C libraries.
Of course, in this case you need another target hook, that is dependent both
on the target backend and C library.

It would be nice to make the target hook a little bit more generic as well,
e.g. pass it enum builtin_function and query if it is fast, slow or
unknown, or even some kind of cost, where the caller could ask for cost of
BUILT_IN_MEMCPY and BUILT_IN_MEMPCPY and decide based on the relative costs.

	Jakub



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