> On Mon, 26 Oct 2020, Jan Hubicka wrote:
>
> > Hi,
> > while looking for special cases of buitins I noticed that tree-ssa-ccp
> > can use EAF_RETURNS_ARG. I wonder if same should be done by value
> > numbering and other propagators....
>
> The issue is that changing
>
> q = memcpy (p, r);
> .. use q ...
>
> to
>
> memcpy (p, r);
> .. use p ..
>
> is bad for RA so we generally do not want to copy-propagate
> EAF_RETURNS_ARG. We eventually do want to optimize a following
>
>
> if (q == p)
>
> of course. And we eventually want to do the _reverse_ transform,
> replacing
>
> memcpy (p, r)
> .. use p ..
>
> with
>
> tem = memcpy (p, r)
> .. use tem ..
>
> ISTR playing with patches doing all of the above, would need to dig
> them out again. There's also a PR about this I think.
>
> Bernd added some code to RTL call expansion, not sure exactly
> what it does...
It adds copy intstruction to call fusage, so RTL backend now about the
equivalence.
void *
test(void *a, void *b, int l)
{
__builtin_memcpy (a,b,l);
return a;
}
eliminates the extra copy. So I would say that we should not be affraid
to propagate in gimple world. It is a minor thing I guess though.
(my interest is mostly to get rid of unnecesary special casing of
builtins, as these special cases are clearly not well maintained
because almost no one knows about them:)
Honza
