Remove the fallback that switched paint_down_to_common() from
generation ordering to commit-date ordering when the commit-graph
lacks corrected commit dates (v1 graph with topo levels only).
The fallback was added in 091f4cf3 (commit: don't use generation
numbers if not needed, 2018-08-30) to avoid a performance
regression on the Linux kernel repo where v1 topo levels caused
"git merge-base v4.8 v4.9" to walk 636k commits instead of 167k.
A side branch with a low topo level stayed in the queue behind a
long chain, preventing early STALE propagation.
Side-exhaustion (added in the previous commits) solves this
differently by terminating the walk as soon as one paint side
empties from the queue, preventing the deep walk regardless of
queue ordering. Benchmarks of "git merge-base --all v4.8 v4.9"
on the Linux kernel repo show that side-exhaustion reduces the
step count far below what the date-ordering fallback achieved:
steps time
no graph, baseline: 167,413 3.25 s
v1 graph, baseline: 167,413 0.25 s
v2 graph, baseline: 167,441 0.29 s
v1 graph, this series: 5,725 0.02 s
v2 graph, this series: 3,887 0.01 s
With generation ordering always active, the existing min_generation
check in paint_queue_get() can safely terminate once the walk
crosses below the caller's generation floor. The date ordering
fallback broke this invariant: a commit could have a finite topo
level while the queue was date-ordered, causing the early exit to
fire before all merge bases were found.
With the fallback removed, gen_ordered is always true and can be
dropped. The topo_ceiling field (introduced earlier) already
handles V1_MAX saturation, so the early exit gates need no further
changes.
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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