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
--- 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;
}
}