On Thu, 24 Nov 2016, Prathamesh Kulkarni wrote:
> On 24 November 2016 at 17:48, Richard Biener <rguent...@suse.de> wrote:
> > On Thu, 24 Nov 2016, Prathamesh Kulkarni wrote:
> >
> >> On 24 November 2016 at 14:07, Richard Biener <rguent...@suse.de> wrote:
> >> > On Thu, 24 Nov 2016, Prathamesh Kulkarni wrote:
> >> >
> >> >> Hi,
> >> >> Consider following test-case:
> >> >>
> >> >> void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
> >> >> {
> >> >> __builtin_memcpy (a1, a2, a3);
> >> >> return a1;
> >> >> }
> >> >>
> >> >> return a1 can be considered equivalent to return value of memcpy,
> >> >> and the call could be emitted as a tail-call.
> >> >> gcc doesn't emit the above call to memcpy as a tail-call,
> >> >> but if it is changed to:
> >> >>
> >> >> void *t1 = __builtin_memcpy (a1, a2, a3);
> >> >> return t1;
> >> >>
> >> >> Then memcpy is emitted as a tail-call.
> >> >> The attached patch tries to handle the former case.
> >> >>
> >> >> Bootstrapped+tested on x86_64-unknown-linux-gnu.
> >> >> Cross tested on arm*-*-*, aarch64*-*-*
> >> >> Does this patch look OK ?
> >> >
> >> > +/* Return arg, if function returns it's argument or NULL if it doesn't.
> >> > */
> >> > +tree
> >> > +gimple_call_return_arg (gcall *call_stmt)
> >> > +{
> >> >
> >> >
> >> > Please just inline it at the single use - the name is not terribly
> >> > informative.
> >> >
> >> > I'm not sure you can rely on code-generation working if you not
> >> > effectively change the IL to
> >> >
> >> > a1 = __builtin_memcpy (a1, a2, a3);
> >> > return a1;
> >> >
> >> > someone more familiar with RTL expansion plus tail call emission on
> >> > RTL needs to chime in.
> >> Well I was trying to copy-propagate function's argument into uses of
> >> it's return value if
> >> function returned that argument, so the assignment to lhs of call
> >> could be made redundant.
> >>
> >> eg:
> >> void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
> >> {
> >> void *t1 = __builtin_memcpy (a1, a2, a3);
> >> return t1;
> >> }
> >>
> >> After patch, copyprop transformed it into:
> >> t1 = __builtin_memcpy (a1, a2, a3);
> >> return a1;
> >
> > But that's a bad transform -- if we know that t1 == a1 then it's
> > better to use t1 as that's readily available in the return register
> > while the register for a1 might have been clobbered and thus we
> > need to spill it for the later return.
> Oh I didn't realize this could possibly pessimize RA.
> For test-case:
>
> void *t1 = memcpy (dest, src, n);
> if (t1 != dest)
> __builtin_abort ();
>
> we could copy-propagate t1 into cond_expr and make the condition redundant.
> However I suppose this particular case could be handled with VRP instead
> (t1 and dest should be marked equivalent) ?
Yeah, exposing this to value-numbering in general can enable some
optimizations (but I wouldn't put it in copyprop). Note it's then
difficult to avoid copy-propgating things...
The user can also write
void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
{
__builtin_memcpy (a1, a2, a3);
return a1;
}
so it's good to improve code-gen for that (for the tailcall issue).
Richard.
> Thanks,
> Prathamesh
> >
> >> But this now interferes with tail-call optimization, because it is not
> >> able to emit memcpy
> >> as tail-call anymore due to which the patch regressed 20050503-1.c.
> >> I am not sure how to workaround this.
> >>
> >> Thanks,
> >> Prathamesh
> >> >
> >> > Richard.
> >>
> >
> > --
> > Richard Biener <rguent...@suse.de>
> > SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
> > 21284 (AG Nuernberg)
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
