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

The memory improvement is minor, but any memory reduction at all is welcome at this point. Fortunately, this set of changes is unintrusive.

I have some other ideas that I'll hopefully get to implement before swapping kills all my SSDs (see bottom).

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

TODO (some ideas stolen from khashl):

* obj_hash can probably use a 0.75 load factor (instead of 0.5)
  to save memory and not slow down too much.  oid_* khash has
  always had 0.77 which was fine and now khashl has 0.75.
  0.75 is cheaper to compute via (shifts + ORs) than 0.77.

* obj_hash can use `i = (i + 1) & mask' like khashl does to
  avoid branches in linear probe loops

* obj_hash can use realloc and copy in-place resize logic khashl does.
  khashl uses the ->used bitmap for this, but it should be
  possible to do for obj_hash w/o additional allocations
  via pointer tagging

(not khashl inspired):

* keep an identity pool of OIDs separately (similar to how Perl5
  pools all hash keys) and only use tagged pointers for OIDs.
  Pointer tagging can be used to distinguish between 7 hash
  functions, leaving us room for 5 more in addition to SHA-(1|256).
  This change will be a large effort (with a hopefully large savings).
  If we ever need more than 7 hash functions, we can switch to
  storing hash type information in slabs that can be looked up
  using address ranges (AFAIK, jemalloc does this).