cmd/compile: introduce masked memeq

[josharian]

This is an idea to experiment with. I don't know whether it'll yield enough fruit to be worth the added code.

Consider this type:

type T struct {
	x uint64
	b bool
}

Arrays of T require generated alg routines. This is because x has a required alignment of 8, so there's a 7 byte gap between Ts in the array, so we can't use one big memeq call.

We could, however, introduce a masked memeq, to which the compiler passes not only the length of the memory to compare but also a repeating mask, probably of a fixed size. For T, the mask would be something like 1111111110000000 (nine 1s, seven 0s). When comparing memory the routine would load bytes from memory, mask out some of the bytes, and then do the comparison. This is the kind of thing that SIMD instructions excel at on many platforms.

This would also work for structs containing blank fields (fields named _).

This could also work for hash routines: Mask out bytes before feeding them to the regular hash routine.

We could also use this as the first step in other alg routines. For example, [8]string's equality function could start by calling memeqmask with a size of 8*8*2 (on 64 bit systems) and a mask of 1111111100000000 (eight 1s, eight 0s), to compare all the lengths. Then it could do its second loop to compare all the contents.

cc @martisch @randall77