perf: increase min buckets on very small types

[morrisonlevi]

Consider HashSet<u8> on x86_64 with SSE with various bucket sizes and how many bytes the allocation ends up being:

buckets	capacity	allocated bytes
4	3	36
8	7	40
16	14	48
28	32	80

In general, doubling the number of buckets should roughly double the number of bytes used. However, for small bucket sizes for these small TableLayouts (4 -> 8, 8 -> 16), it doesn't happen. This is an edge case which happens because of padding of the control bytes and adding the Group::WIDTH. Taking the buckets from 4 to 16 (4x) only takes the allocated bytes from 36 to 48 (~1.3x).

This platform isn't the only one with edges. Here's aarch64 on an M1 for the same HashSet<u8>:

buckets	capacity	allocated bytes
4	3	20
8	7	24
16	14	40

Notice doubling from 4 to 8 buckets only lead to 4 more bytes (20 -> 24) instead of roughly doubling.

Generalized, buckets * table_layout.size needs to be at least as big as table_layout.ctrl_align. For the cases I listed above, we'd get these new minimum bucket sizes:

• x86_64 with SSE: 16
• aarch64: 8

This is a niche optimization. However, it also removes possible undefined behavior edge case in resize operations. In addition, it would be a useful property when utilizing over-sized allocations (see #523).

[Amanieu]

Also see #47 for previous discussion of this problem.

[morrisonlevi]

It could also be written this way. Not sure what is better:

Suggested change

	let cap = cap.max(match (Group::WIDTH, table_layout.size) {
	(16, 0..=1) => 14,
	(16, 2..=3) => 7,
	(8, 0..=1) => 7,
	_ => 3,
	});
	let cap = cap.max(match table_layout.size {
	0..=1 if Group::WIDTH == 16 => 14,
	2..=3 if Group::WIDTH == 16 => 7,
	0..=1 if Group::WIDTH == 8 => 7,
	_ => 3,
	});

[Amanieu]

What I had in mind was to move the max check inside the if cap < 8 block entirely (and changing the bound to 15). Then just have hard-coded selection depending on the requested capacity and the group width/item size.

[morrisonlevi]

It feels weird to me. It'll be brittle if we ever add 256 width types, for instance. But I can try doing what you suggest.