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

[Martin Liška <mliska at suse dot cz>]

On 03/13/2018 09:32 AM, Jakub Jelinek wrote:
> 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.

Let me start with simple return enum value of FAST,SLOW,UNKNOWN. I've added new hook
definition to gcc/config/gnu-user.h that will point to gnu_libc_function_implementation.
I would like to implement the function in gcc/targhooks.c, but I don't know how to
make ifdef according to target?

One another issue is that built_in_function is enum defined in tree.h. Thus I'll replace the
callback argument with int, that will be casted. One last issue: am I right that I'll have to define
TARGET_LIBC_FUNCTION_IMPLEMENTATION in each config file (similar to no_c99_libc_has_function)?

Thanks,
Martin

> 	Jakub