The compiler can't currently figure out that it can eliminate both c.s
loads (using store to load forwarding) in the second line of the
following code:
The compiler eliminates the second load of c.s[j] (using the original
value of c.s[i]), however the load of c.s[i] remains because the compiler
doesn't know that c.s[i] and c.s[j] either overlap completely or not at
all.
Introducing temporaries to make this explicit improves the performance
of the generic code slightly, the goal being to remove the assembly in
this package in the future. This change also hoists a bounds check out
of the main loop which gives a slight performance boost and also makes
the behaviour identical to the assembly implementation when len(dst) <
len(src).