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

["bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #57 from Rich Felker <bugdal at aerifal dot cx> ---
I think one could reasonably envision an implementation that does some sort of
vector loads/stores where, due to some performance constraint or avoiding
special casing for possible page boundary past the end of the copy, it only
wants to load N bits at a time, but the efficient store instruction always
stores a full vector of 2N bits. Of course, one could also argue quite
reasonably that this is a weird enough thing to do that the implementation
should then just check for src==dest and early-out.

I'm far less concerned about whether such mechanical breakage exists, and more
concerned about the consequences of LTO/whole-program-analysis where something
in the translation process can see the violated restrict qualifier, infer UB,
and blow everything up.

The change being requested here is really one of removing the restrict
qualification from the arguments and making a custom weaker condition. This may
in turn have consequences on what types of transformations are possible.