Support for memcpy with equal source and destination

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Ralf,

> However, at that point it seems unclear why that branch should live inside 
> `memcpy`, rather than being performed by the caller. The entire argument made 
> all along by compiler developers (as I understood it) was that the existing 
> `memcpy` are already working fine for the src==dest case; if new branches need 
> to be added, that's a different discussion.

Existing implementations don't need (or want) any extra branches. We also don't
want to complicate inline memcpy expansions. They all work fine if src==dst.

Paul's example was to show that with restrict you could write a conformant
C implementation. I think it is sufficient to cast away restrict without adding an
extra branch (the existing generic C version does this).

>> When n is zero, this implementation also supports NULL dest or src, though 
>> that's a separate issue.
>
> Yeah I'd like to see that guarantee as well, if possible. :)

That should also work fine on existing implementations.

Cheers,
Wilco