pseudo-merge: implement support for finding existing merges

This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.

Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).

Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.

This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:

  - non-matching pseudo-merge bitmaps are rejected quickly as soon as
    they differ in a single bit

  - already-matched pseudo-merge bitmaps are discarded from subsequent
    rounds of search

  - the number of pseudo-merges is generally small, even for large
    repositories

In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index 47250398aa..6e8060f8a0 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -19,6 +19,10 @@
 #include "tree-walk.h"
 #include "pseudo-merge.h"
 #include "oid-array.h"
+#include "config.h"
+#include "alloc.h"
+#include "refs.h"
+#include "strmap.h"
 
 struct bitmapped_commit {
 	struct commit *commit;
@@ -465,6 +469,7 @@ static int fill_bitmap_tree(struct bitmap_writer *writer,
 }
 
 static int reused_bitmaps_nr;
+static int reused_pseudo_merge_bitmaps_nr;
 
 static int fill_bitmap_commit(struct bitmap_writer *writer,
 			      struct bb_commit *ent,
@@ -490,7 +495,7 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
 			struct bitmap *remapped = bitmap_new();
 
 			if (commit->object.flags & BITMAP_PSEUDO_MERGE)
-				old = NULL;
+				old = pseudo_merge_bitmap_for_commit(old_bitmap, c);
 			else
 				old = bitmap_for_commit(old_bitmap, c);
 			/*
@@ -501,7 +506,10 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
 			if (old && !rebuild_bitmap(mapping, old, remapped)) {
 				bitmap_or(ent->bitmap, remapped);
 				bitmap_free(remapped);
-				reused_bitmaps_nr++;
+				if (commit->object.flags & BITMAP_PSEUDO_MERGE)
+					reused_pseudo_merge_bitmaps_nr++;
+				else
+					reused_bitmaps_nr++;
 				continue;
 			}
 			bitmap_free(remapped);
@@ -631,6 +639,9 @@ int bitmap_writer_build(struct bitmap_writer *writer,
 			    the_repository);
 	trace2_data_intmax("pack-bitmap-write", the_repository,
 			   "building_bitmaps_reused", reused_bitmaps_nr);
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "building_bitmaps_pseudo_merge_reused",
+			   reused_pseudo_merge_bitmaps_nr);
 
 	stop_progress(&writer->progress);