[GSOC Patch v4 0/4] Move generation, graph_pos to a slab

[Abhishek Kumar <abhishekkumar8222@gmail.com>]

The struct commit is used in many contexts. However, members
`generation` and `graph_pos` are only used for commit graph related
operations and otherwise waste memory.

This wastage would have been more pronounced as we transition to
generation nuber v2, which uses 64-bit generation number instead of
current 32-bits.

While the overall test suite runs as fast as master
(series: 26m48s, master: 27m34s, faster by 2.87%), certain commands
like `git merge-base --is-ancestor` were slowed by 40% as discovered
by Szeder Gábor [1]. After minimizing commit-slab access, the slow down
persists but is closer to 20%.

Derrick Stolee believes the slow down is attributable to the underlying
algorithm rather than the slowness of commit-slab access [2] and we will
follow-up in a later series.

Abhishek Kumar (4):
  object: drop parsed_object_pool->commit_count
  commit-graph: introduce commit_graph_data_slab
  commit: move members graph_pos, generation to a slab
  commit-graph: minimize commit_graph_data_slab access

 alloc.c                         |  18 +++--
 alloc.h                         |   2 +-
 blame.c                         |   2 +-
 blob.c                          |   2 +-
 bloom.c                         |   7 +-
 builtin/commit-graph.c          |   2 +-
 builtin/fsck.c                  |   2 +-
 commit-graph.c                  | 130 ++++++++++++++++++++++++--------
 commit-graph.h                  |  10 +++
 commit-reach.c                  |  69 ++++++++++-------
 commit.c                        |  12 +--
 commit.h                        |   2 -
 contrib/coccinelle/commit.cocci |  18 +++++
 object.c                        |   4 +-
 object.h                        |   3 +-
 refs.c                          |   2 +-
 revision.c                      |  20 +++--
 t/helper/test-reach.c           |   2 +-
 tag.c                           |   2 +-
 tree.c                          |   2 +-
 20 files changed, 217 insertions(+), 94 deletions(-)

-- 
2.27.0

Changes in v4:
- Fix segfault while initializing commit_graph_data_slab.
- Fix the "commit index should have unique values" more cleanly

Changes in v3:
- Introduce alloc commit to fix the failing diff-submodule test.
- Elaborate on perforamnce and the slow down in commit message

Changes in v2:
- Introduce struct commit_graph_data
- Merge `graph_pos`, `generation` slabs into a single,
  `commit_graph_data` slab.
- Use graph position for an intermediate check for generation, saving
  the cost of initializing generation numbers.
- Add an follow-up patch caching results of slab access in local
  variables.
- Move coccinelle transformation to commit.coccinelle instead of
  creating new scripts.
- Elaborate on removing default values from init_commit_node().
- Revert moving macro constants (e.g. COMMIT_NOT_FROM_GRAPH,
  GENERATION_NUMBER_ZERO0 from commit.h to commit-graph.h