Re: [PATCH 0/3] switch to tombstone-free khashl table

> Fortunately, this set of changes is unintrusive; but I'm
> hoping to have more time to make deeper changes this year.
>
> Eric Wong (3):
>   list-objects-filter: use kh_size API
>   treewide: switch to khashl for memory savings
>   khashl: fix ensemble lookups on empty table
>
>  builtin/fast-import.c       |   2 +-
>  builtin/fsmonitor--daemon.c |   4 +-
>  delta-islands.c             |   4 +-
>  khash.h                     | 338 -----------------------
>  khashl.h                    | 522 ++++++++++++++++++++++++++++++++++++
>  list-objects-filter.c       |   2 +-
>  object-store-ll.h           |   2 +-
>  object-store.h              |   7 +-
>  oidset.h                    |   2 +-
>  pack-bitmap.h               |   2 +-
>  10 files changed, 535 insertions(+), 350 deletions(-)
>  delete mode 100644 khash.h
>  create mode 100644 khashl.h
>
> Range-diff:
> -:  ---------- > 1:  3bf3148cab list-objects-filter: use kh_size API
> 1:  e74965907e ! 2:  09900edb48 treewide: switch to khashl for memory savings

>     @@ Commit message
>      
>          khashl is an updated version of khash with less memory overhead
>          (one bit/bucket instead of two) than the original khash and
>     -    similar overall performance.  Insertions are simpler (linear
>     -    probing) but deletions may be slightly slower[1].  Of course,
>     -    the majority of hash tables in git do not delete individual
>     -    elements.
>     +    similar overall performance.  According to its author,
>     +    insertions are simpler (linear probing) but deletions may be
>     +    slightly slower[1].  Of course, the majority of hash tables in
>     +    git do not delete individual elements.
>      
>          Overall memory usage did not decrease much, as the hash tables
>          and elements we store in them are big and currently dwarf the
>          overhead of the khash internals.  Only around 10 MB in
>     -    allocations (not peak use) is saved when doing a no-op `git gc'
>     -    of a Linux kernel object store with thousands of refs and
>     -    islands.
>     +    allocations (and a few dozen KB peak use out of ~6 GB) is saved
>     +    when doing a no-op `git gc' of a Linux kernel object store with
>     +    thousands of refs and islands.
>      
>          A summary of differences I've found from khash to khashl:
>      
>     @@ Commit message
>          * flesh out KHASHL_{SET,MAP}_INIT wrappers with *_clear, *_resize,
>            and *_release functions
>      
>     +    * sparse fixes from Junio and Jeff
>     +
>          [1] https://attractivechaos.wordpress.com/2019/12/28/deletion-from-hash-tables-without-tombstones/
>          [2] git clone https://github.com/attractivechaos/klib.git
>              2895a16cb55e (support an ensemble of hash tables, 2023-12-18)
>     @@ Commit message
>            typedef) and was the only place where I had to change a definition.
>      
>          Signed-off-by: Eric Wong <e@80x24.org>
>     +    Helped-by: Junio C Hamano <gitster@pobox.com>
>     +    Helped-by: Jeff King <peff@peff.net>
>      
>       ## builtin/fast-import.c ##
>      @@
>     @@ khashl.h (new)
>      +#define __KHASHL_IMPL_GET(SCOPE, HType, prefix, khkey_t, __hash_fn, __hash_eq) \
>      +	SCOPE khint_t prefix##_getp_core(const HType *h, const khkey_t *key, khint_t hash) { \
>      +		khint_t i, last, n_buckets, mask; \
>     -+		if (h->keys == 0) return 0; \
>     ++		if (!h->keys) return 0; \
>      +		n_buckets = (khint_t)1U << h->bits; \
>      +		mask = n_buckets - 1U; \
>      +		i = last = __kh_h2b(hash, h->bits); \
>     @@ khashl.h (new)
>      +
>      +#define __KHASHL_IMPL_RESIZE(SCOPE, HType, prefix, khkey_t, __hash_fn, __hash_eq) \
>      +	SCOPE void prefix##_resize(HType *h, khint_t new_n_buckets) { \
>     -+		khint32_t *new_used = 0; \
>     ++		khint32_t *new_used = NULL; \
>      +		khint_t j = 0, x = new_n_buckets, n_buckets, new_bits, new_mask; \
>      +		while ((x >>= 1) != 0) ++j; \
>      +		if (new_n_buckets & (new_n_buckets - 1)) ++j; \
>     @@ khashl.h (new)
>      +#define __KHASHL_IMPL_DEL(SCOPE, HType, prefix, khkey_t, __hash_fn) \
>      +	SCOPE int prefix##_del(HType *h, khint_t i) { \
>      +		khint_t j = i, k, mask, n_buckets; \
>     -+		if (h->keys == 0) return 0; \
>     ++		if (!h->keys) return 0; \
>      +		n_buckets = (khint_t)1U<<h->bits; \
>      +		mask = n_buckets - 1U; \
>      +		while (1) { \
> 2:  744e1b7198 = 3:  bfb20eae37 khashl: fix ensemble lookups on empty table