@matheusbsbs / matheus-bagliano / commits / 03f2915541

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>

Niels Glodny committed Apr 29, 2025 at 16:09 UTC 03f2915541a4923c5733e505a42e77031eb9494c

3 files changed +18 -2

t/meson.build

@@ -501,6 +501,7 @@ integration_tests = [

501 't4068-diff-symmetric-merge-base.sh',

502 't4069-remerge-diff.sh',

503 't4070-diff-pairs.sh',

504 + 't4071-diff-minimal.sh',

505 't4100-apply-stat.sh',

506 't4101-apply-nonl.sh',

507 't4102-apply-rename.sh',

t/t4071-diff-minimal.sh new

+14

@@ -0,0 +1,14 @@

1 +#!/bin/sh

2 +

3 +test_description='minimal diff algorithm'

4 +

5 +. ./test-lib.sh

6 +

7 +test_expect_success 'minimal diff should not mark changes between changed lines' '

8 + test_write_lines x x x x >pre &&

9 + test_write_lines x x x A B C D x E F G >post &&

10 + test_expect_code 1 git diff --no-index --minimal pre post >diff &&

11 + test_grep ! ^[+-]x diff

12 +'

13 +

14 +test_done

xdiff/xprepare.c

+3 -2

@@ -368,6 +368,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd

368 xrecord_t **recs;

369 xdlclass_t *rcrec;

370 char *dis, *dis1, *dis2;

371 + int need_min = !!(cf->flags & XDF_NEED_MINIMAL);

372

373 if (!XDL_CALLOC_ARRAY(dis, xdf1->nrec + xdf2->nrec + 2))

374 return -1;

@@ -379,7 +380,7 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd

380 for (i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart]; i <= xdf1->dend; i++, recs++) {

381 rcrec = cf->rcrecs[(*recs)->ha];

382 nm = rcrec ? rcrec->len2 : 0;

382 - dis1[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;

383 + dis1[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;

384 }

385

386 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

388 for (i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart]; i <= xdf2->dend; i++, recs++) {

389 rcrec = cf->rcrecs[(*recs)->ha];

390 nm = rcrec ? rcrec->len1 : 0;

390 - dis2[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;

391 + dis2[i] = (nm == 0) ? 0: (nm >= mlim && !need_min) ? 2: 1;

392 }

393

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