kernel
/
git
mirror of https://git.kernel.org/pub/scm/git/git.git
1
0
Fork 0
Code Releases Activity

xdiff: disable cleanup_records heuristic with --minimal 

The cleanup_records function marks some lines as changed before running
the actual diff algorithm. For most lines, this is a good performance
optimization, but it also marks lines that are surrounded by many
changed lines as changed as well. This can cause redundant changes and
longer-than-necessary diffs.

Whether this results in better-looking diffs is subjective. However, the
--minimal flag explicitly requests the shortest possible diff.

The change results in shorter diffs in about 1.3% of all diffs in Git's
history. Performance wise, I have measured the impact on
"git log -p -3000 --minimal > /dev/null". With this change, I get
  Time (mean ± σ): 2.363 s ±  0.023 s (25 runs)
and without this patch I measured
  Time (mean ± σ): 2.362 s ±  0.035 s (25 runs).
As the difference is well within the margin of error, this does not seem
to have an impact on performance.

Signed-off-by: Niels Glodny <n.glodny@campus.lmu.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
seen
Niels Glodny 2025-04-29 16:09:49 +02:00 committed by Junio C Hamano
parent f65182a99e
commit 03f2915541
3 changed files with 18 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
t/meson.build
View File

@ -501,6 +501,7 @@ integration_tests = [
't4068-diff-symmetric-merge-base.sh',
't4069-remerge-diff.sh',
't4070-diff-pairs.sh',
't4071-diff-minimal.sh',
't4100-apply-stat.sh',
't4101-apply-nonl.sh',
't4102-apply-rename.sh',

14
t/t4071-diff-minimal.sh Executable file
View File

@ -0,0 +1,14 @@
#!/bin/sh

test_description='minimal diff algorithm'

. ./test-lib.sh

test_expect_success 'minimal diff should not mark changes between changed lines' '
test_write_lines x x x x >pre &&
test_write_lines x x x A B C D x E F G >post &&
test_expect_code 1 git diff --no-index --minimal pre post >diff &&
test_grep ! ^[+-]x diff
'

test_done

5
xdiff/xprepare.c
View File

@ -368,6 +368,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
xrecord_t **recs;
xdlclass_t *rcrec;
char *dis, *dis1, *dis2;
int need_min = !!(cf->flags & XDF_NEED_MINIMAL);

if (!XDL_CALLOC_ARRAY(dis, xdf1->nrec + xdf2->nrec + 2))
return -1;
@ -379,7 +380,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
for (i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart]; i <= xdf1->dend; i++, recs++) {
rcrec = cf->rcrecs[(*recs)->ha];
nm = rcrec ? rcrec->len2 : 0;
dis1[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
dis1[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;
}

if ((mlim = xdl_bogosqrt(xdf2->nrec)) > XDL_MAX_EQLIMIT)
@ -387,7 +388,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
for (i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart]; i <= xdf2->dend; i++, recs++) {
rcrec = cf->rcrecs[(*recs)->ha];
nm = rcrec ? rcrec->len1 : 0;
dis2[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
dis2[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;
}

for (nreff = 0, i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart];