@leroysheep / BookAlchemy / commits / 716766765c

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

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>

Henrique Ferreiro committed Jul 8, 2026 at 21:42 UTC 716766765c945c9ca62f24f6debc3f255ff3b6a6
2 files changed +35 -1
t/perf/p0009-diff-pathspec.sh
+32
new file mode 100755 index 0000000000..6079db52c2 --- /dev/null +++ b/t/perf/p0009-diff-pathspec.sh @@ -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
unpack-trees.c
+3 -1
index f38c761ab9..4ecc07e4a1 100644 --- a/unpack-trees.c +++ b/unpack-trees.c @@ -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;