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unpack-trees: avoid quadratic index scan in next_cache_entry()

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Diffing the working tree against a commit with a pathspec can take
time quadratic in the size of the index when the pathspec matches a
subtree whose entries are the first entries of the index.  Fix it by
having next_cache_entry() record how far it scanned in cache_bottom,
so repeated calls no longer rescan the growing prefix of
already-unpacked entries.  On a Chromium checkout (~500k index
entries),

	git diff HEAD -- .agents/OWNERS

took about 8 minutes before this change and 0.07 seconds after it.
The same diff without the commit, without the pathspec, or with
--cached was already instant.

Add p0009-diff-pathspec.sh, which builds a 10,000-entry index whose
first path lives in a subtree (100,000 entries under --long-tests),
to guard against the regression.  Comparing v2.55.0 with this change
using GIT_TEST_LONG=t:

Test                            v2.55.0           HEAD
------------------------------------------------------------------------
0009.2: diff pathspec subtree   7.16(7.12+0.01)   0.02(0.01+0.00) -99.7%

Signed-off-by: Henrique Ferreiro <hferreiro@igalia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Henrique Ferreiro
2026-07-08 21:42:13 +00:00
committed by Junio C Hamano
parent 67ad42147a
commit 716766765c
2 changed files with 35 additions and 1 deletions
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32
t/perf/p0009-diff-pathspec.sh Executable file
View File

@@ -0,0 +1,32 @@
#!/bin/sh
test_description='Tests performance of diffing the working tree with a pathspec'
. ./perf-lib.sh
test_perf_fresh_repo
count=10000
if test_have_prereq EXPENSIVE
then
count=100000
fi
# The entries exist only in the index, which is enough to
# exercise the index scan.
test_expect_success 'setup' '
blob=$(echo content | git hash-object -w --stdin) &&
{
printf "100644 $blob\taaa/file\n" &&
printf "100644 $blob\tf%s\n" $(test_seq $count)
} | git update-index --index-info &&
git commit -q -m initial &&
mkdir -p aaa &&
echo content >aaa/file
'
test_perf 'diff pathspec subtree' '
git diff HEAD -- aaa/file
'
test_done

4
unpack-trees.c
View File

@@ -671,8 +671,10 @@ static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
while (pos < index->cache_nr) {
struct cache_entry *ce = index->cache[pos];
if (!(ce->ce_flags & CE_UNPACKED))
if (!(ce->ce_flags & CE_UNPACKED)) {
o->internal.cache_bottom = pos;
return ce;
}
pos++;
}
return NULL;