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

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 24 November 2016 at 17:48, Richard Biener <rguenther@suse.de> wrote:
> 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.
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) ?

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 <rguenther@suse.de>
> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)