[sha2] TODO: replace after sha2-asm rework

[mattcollier]

Is there an issue to track this TODO? Can some additional description be added?

hashes/sha2/src/sha512.rs

Lines 235 to 236 in 39a55cf

	// TODO: replace after sha2-asm rework
	fn compress(state: &mut [u64; 8], blocks: &[[u8; 128]]) {

[tarcieri]

@newpavlov ?

[newpavlov]

The idea was to change signatures of the asm crates to accept &[[u8; 128]] instead of the current single-block &[u8; 128]. Effectively it's a workaround around the noalias LLVM bug, currently after each block was processed the state is saved needlessly, instead of staying in registers. The only workaround which I have found was to copy state to a local variable and work with it, thus the need for the multi-block compression.

The ASM crates currently have a low priority for us and potential performance improvements are quite small, so I probably simply will remove this comment.