[GSOC Patch v2 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 number v2, which uses 64-bite generation number instead of
current 32-bits.

Abhishek Kumar (4):
  commit-graph: introduce commit_graph_data_slab
  commit: move members graph_pos, generation to a slab
  commit-graph: use generation directly when writing commit-graph
  commit-graph: minimize commit_graph_data_slab access

 alloc.c                         |   2 -
 blame.c                         |   2 +-
 bloom.c                         |   7 +-
 commit-graph.c                  | 127 ++++++++++++++++++++++++--------
 commit-graph.h                  |  10 +++
 commit-reach.c                  |  69 ++++++++++-------
 commit.c                        |   8 +-
 contrib/coccinelle/commit.cocci |  18 +++++
 revision.c                      |  20 +++--
 9 files changed, 190 insertions(+), 73 deletions(-)

-- 
2.27.0

Thanks to Dr. Stolee, Dr. Narebski and Junio for their excellent
suggestions.

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_ZERO) from commit.h to commit-graph.h

About the failing diff-submodule related tests, I came up with a
plausible explanation but could be wrong on this:

Commit slabs rely on uniqueness of commit->index to access data. But
submodules are repositories on their own, alloc_commit_index(), which
relies on repository->parsed_objects->commit_count no longer returns
unique values.

A commit belong to super repo and another belonging to submodule might
have the same index but different generation and graph positions.

This could be fixed by defining commit index as maximum of commit index
of all repositories + 1 but I have no idea how that would impact other
code.

Thoughts on this?

Regards
Abhishek