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xdiff: reduce indent heuristic overhead 

Skip searching for better indentation heuristics if we'd slide a hunk more
than its size. This is the easiest fix proposed in the analysis[1] in
response to a patch that mercurial took for xdiff to limit searching
by a constant. Using a performance test as:

     #!python
     open('a', 'w').write(" \n" * 1000000)
     open('b', 'w').write(" \n" * 1000001)

This patch reduces the execution of "git diff --no-index a b" from
0.70s to 0.31s. However limiting the sliding to the size of the diff hunk,
which was proposed as a solution (that I found easiest to implement for
now) is not optimal for cases like

     open('a', 'w').write(" \n" * 1000000)
     open('b', 'w').write(" \n" * 2000000)

as then we'd still slide 1000000 times.

In addition to limiting the sliding to size of the hunk, also limit by a
constant. Choose 100 lines as the constant as that fits more than a screen,
which really means that the diff sliding is probably not providing a lot
of benefit anyway.

[1] https://public-inbox.org/git/72ac1ac2-f567-f241-41d6-d0f83072e0b3@alum.mit.edu/

Reported-by: Jun Wu <quark@fb.com>
Analysis-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Stefan Beller 2018-07-27 15:23:56 -07:00 committed by Junio C Hamano
parent 53f9a3e157
commit 301ef85401
1 changed files with 11 additions and 1 deletions
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12
xdiff/xdiffi.c
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@ -591,6 +591,11 @@ static void measure_split(const xdfile_t *xdf, long split,
*/
#define INDENT_WEIGHT 60
/*
* How far do we slide a hunk at most?
*/
#define INDENT_HEURISTIC_MAX_SLIDING 100
/*
* Compute a badness score for the hypothetical split whose measurements are
* stored in m. The weight factors were determined empirically using the tools and
@ -903,7 +908,12 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
long shift, best_shift = -1;
struct split_score best_score;
for (shift = earliest_end; shift <= g.end; shift++) {
shift = earliest_end;
if (g.end - groupsize - 1 > shift)
shift = g.end - groupsize - 1;
if (g.end - INDENT_HEURISTIC_MAX_SLIDING > shift)
shift = g.end - INDENT_HEURISTIC_MAX_SLIDING;
for (; shift <= g.end; shift++) {
struct split_measurement m;
struct split_score score = {0, 0};