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[PATCH 0/6] Compute and consume generation numbers

This is the first of several "small" patches that follow the serialized
Git commit graph patch (ds/commit-graph).

As described in Documentation/technical/commit-graph.txt, the generation
number of a commit is one more than the maximum generation number among
its parents (trivially, a commit with no parents has generation number

This series makes the computation of generation numbers part of the
commit-graph write process.

Finally, generation numbers are used to order commits in the priority
queue in paint_down_to_common(). This allows a constant-time check in
queue_has_nonstale() instead of the previous linear-time check.

This does not have a significant performance benefit in repositories
of normal size, but in the Windows repository, some merge-base
calculations improve from 3.1s to 2.9s. A modest speedup, but provides
an actual consumer of generation numbers as a starting point.

A more substantial refactoring of revision.c is required before making
'git log --graph' use generation numbers effectively.

This patch series depends on v7 of ds/commit-graph.

Derrick Stolee (6):
  object.c: parse commit in graph first
  commit: add generation number to struct commmit
  commit-graph: compute generation numbers
  commit: sort by generation number in paint_down_to_common()
  commit.c: use generation number to stop merge-base walks
  commit-graph.txt: update design doc with generation numbers

 Documentation/technical/commit-graph.txt |  7 +---
 alloc.c                                  |  1 +
 commit-graph.c                           | 48 +++++++++++++++++++++
 commit.c                                 | 53 ++++++++++++++++++++----
 commit.h                                 |  7 +++-
 object.c                                 |  4 +-
 6 files changed, 104 insertions(+), 16 deletions(-)