[Bug middle-end/32667] block copy with exact overlap is expanded as memcpy

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667

--- Comment #47 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #46)
> So yes, the most clean solution would be to have __forgiving_memcpy
> possibly also allowing NULL pointers when n == 0 besides allowing
> the exact overlap.  Its prototype wouldn't have restrict then or
> nonnull.

That is basically what I've argued above (however it is called), and completely
agree also on the NULL pointers when n == 0 case for it, I think that one came
up in the Honza vs. Jonathan libstdc++ discussions again recently.
And, ideally it could be implemented in libc as an alias to memcpy if e.g. the
assembly written memcpy satisfies all the requirements in it, but valgrind
could implement it separately and do there if (n == 0 || dst == src) return
dst; return memcpy (dst, src, n);
Of course, it would take some time, because it needs to be in libc first, gcc
needs to key on the versions which have it.  But then possibly in such case
could also fold
if (n != 0)
  memcpy (dst, src, n);
into
  __forgiving_memcpy (dst, src, n);
or __builtin_* variants thereof.