[tree-tailcall] Check if function returns it's argument

[Richard Biener]

On Thu, 24 Nov 2016, Prathamesh Kulkarni wrote:

> On 24 November 2016 at 14:07, Richard Biener <rguenther@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.
 
> 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 <rguenther@suse.de>
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