Revert "argon2: align blocks to 128-byte boundaries for modern x86-64…

… and aarch64"

This reverts commit 1342037.

For reasons not yet understood, aligning blocks to 128-byte boundaries
results in worse performance than using 64-byte alignment.

Looking into this with perf suggests that it is *not* a cache problem,
but rather that the generated code is different and results in
substantially more instructions being executed when the blocks are
128-byte aligned.

For now, revert the alignment back to 64 bytes. While we're at it, also
remove the comment that suggests alignment is only needed to prevent
false sharing: it's possible that other places in the crate, which I
haven't checked, required (for correctness or best performance) the
64-byte alignment we're reverting back to.

It's worth noting that false sharing isn't generally a major issue in
Argon2: due to how memory is accessed, only the first and last few words
of a segment can (and most of the time probably still won't) experience
some false sharing with reads from other lanes.

Finally, changing the alignment with 1342037 would have a major
SemVer change:

https://doc.rust-lang.org/cargo/reference/semver.html#repr-align-n-change

argon2/src/block.rs

@@ -46,12 +46,8 @@ macro_rules! permute {
 }
 
 /// Structure for the (1 KiB) memory block implemented as 128 64-bit words.
-//
-// Blocks are 128-byte aligned to prevent false sharing. The specific alignment value is based on
-// the notes in `crossbeam-utils::CachePadded` for modern architectures, which either use 128-byte
-// cache lines (aarch64) or pull 64-byte cache lines in pairs (x86-64).
 #[derive(Copy, Clone, Debug)]
-#[repr(align(128))]
+#[repr(align(64))]
 pub struct Block([u64; Self::SIZE / 8]);
 
 impl Block {