Mercurial > hg > compact-paper
changeset 36674:c420792217c8
xdiff: reduce indent heuristic overhead
Adds some threshold to avoid expensive cases, like:
```
#!python
open('a', 'w').write(" \n" * 1000000)
open('b', 'w').write(" \n" * 1000001)
```
The indent heuristic is O(N * 20) (N = 1000000) for the above case, and
makes diff much slower.
Before this patch (system git 2.14.2):
```
git diff --no-indent-heuristic a b 0.21s user 0.03s system 100% cpu 0.239 total
git diff --indent-heuristic a b 0.77s user 0.02s system 99% cpu 0.785 total
```
After this patch (git 2fc74f41, with xdiffi.c patched):
```
# with the changed xdiffi.c
git diff --indent-heuristic a b 0.16s user 0.01s system 90% cpu 0.188 total
git diff --no-indent-heuristic a b 0.18s user 0.01s system 99% cpu 0.192 total
```
Now turning on indent-heuristic has no visible impact on performance.
Differential Revision: https://phab.mercurial-scm.org/D2601
| author | Jun Wu <quark@fb.com> |
|---|---|
| date | Sat, 03 Mar 2018 12:39:11 -0800 |
| parents | b3c9c483cac9 |
| children | 430fdb717549 |
| files | mercurial/thirdparty/xdiff/xdiffi.c |
| diffstat | 1 files changed, 35 insertions(+), 3 deletions(-) [+] |
line wrap: on
line diff
--- a/mercurial/thirdparty/xdiff/xdiffi.c Sat Mar 03 12:38:41 2018 -0800 +++ b/mercurial/thirdparty/xdiff/xdiffi.c Sat Mar 03 12:39:11 2018 -0800 @@ -802,6 +802,14 @@ } /* + * For indentation heuristic, skip searching for better slide position after + * checking MAX_BORING lines without finding an improvement. This defends the + * indentation heuristic logic against pathological cases. The value is not + * picked scientifically but should be good enough. + */ +#define MAX_BORING 100 + +/* * Move back and forward change groups for a consistent and pretty diff output. * This also helps in finding joinable change groups and reducing the diff * size. @@ -897,19 +905,43 @@ long shift, best_shift = -1; struct split_score best_score; - for (shift = earliest_end; shift <= g.end; shift++) { + /* + * This is O(N * MAX_BLANKS) (N = shift-able lines). + * Even with MAX_BLANKS bounded to a small value, a + * large N could still make this loop take several + * times longer than the main diff algorithm. The + * "boring" value is to help cut down N to something + * like (MAX_BORING + groupsize). + * + * Scan from bottom to top. So we can exit the loop + * without compromising the assumption "for a same best + * score, pick the bottommost shift". + */ + int boring = 0; + for (shift = g.end; shift >= earliest_end; shift--) { struct split_measurement m; struct split_score score = {0, 0}; + int cmp; measure_split(xdf, shift, &m); score_add_split(&m, &score); measure_split(xdf, shift - groupsize, &m); score_add_split(&m, &score); - if (best_shift == -1 || - score_cmp(&score, &best_score) <= 0) { + + if (best_shift == -1) { + cmp = -1; + } else { + cmp = score_cmp(&score, &best_score); + } + if (cmp < 0) { + boring = 0; best_score.effective_indent = score.effective_indent; best_score.penalty = score.penalty; best_shift = shift; + } else { + boring += 1; + if (boring >= MAX_BORING) + break; } }