runtime: map[int64]struct{} requires 16 bytes per slot

[prattmic]

With swissmaps in 1.24, a map[int64]struct{} requires 16 bytes of space per slot, rather than the expected 8 bytes.

This is an unfortunate side effect of the way the storage is defined internally https://cs.opensource.google/go/go/+/master:src/cmd/compile/internal/reflectdata/map_swiss.go;l=30

	// type group struct {
	//     ctrl uint64
	//     slots [abi.SwissMapGroupSlots]struct {
	//         key  keyType
	//         elem elemType
	//     }
	// }

elemType is struct{}. The struct size rules in the compiler say that if struct ends in a zero-size type, that field is given 1 byte of space (in case someone creates a pointer to the last field, we don't want that to point past the end of allocation).
Then, keyType needs 8-byte alignment, so the last field actually ends up using a full 8-bytes.

This is the most extreme case of KVKVKVKV wasting space due to alignment requirements. We could probably fix this specific case by deconstructing the array+struct in this internal definition, then we'd only need the extra 8 bytes for the last slot.

Note that #70835 considers changing the layout entirely (putting all keys together), which would also fix this.

[thediveo]

Is this basically "as bad" as the also common map[X]bool, where the bool actually isn't used but just because is easier to type/read than struct{}?

[prattmic]

Yes, both now allocate exactly the same amount of memory.

[zigo101]

Will

struct {
	elem elemType
	key  keyType
}

fix this?

[valyala]

This may be the reason of performance drop for map[uint64]struct{} maps with millions of items when migrating from Go1.23 to Go1.24 - https://x.com/valyala/status/1879988053076504761

[jub0bs]

Relevant discussion on Gophers Slack: https://gophers.slack.com/archives/C0VP8EF3R/p1737383438473369

[jub0bs]

@prattmic Any thoughts on @zigo101's comment? Flipping the key and elem fields in the declaration of the slot struct type sounds promising and simple enough, but perhaps I'm failing to anticipate the ramifications of such a change.