[PATCH 2 of 2] bdiff: don't check border condition in loop
Gregory Szorc
gregory.szorc at gmail.com
Sat Nov 12 22:44:49 EST 2016
# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1479000803 28800
# Sat Nov 12 17:33:23 2016 -0800
# Node ID 812aa0657b1a5dc275be6bfd26c684cf9f937bfe
# Parent 99388e747e8f99c7bdcfd2592ed8fc45e885ee55
bdiff: don't check border condition in loop
This is pretty much a copy of d500ddae7494, just to a different loop.
The condition `p == plast` (`plast == a + len - 1`) was only true on
the final iteration of the loop. So it was wasteful to check for it
on every iteration. We decrease the iteration count by 1 and add an
explicit check for `p == plast` after the loop.
Again, we see modest wins.
>From the mozilla-unified repository:
$ perfbdiff -m 3041e4d59df2
! wall 0.035502 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)
! wall 0.030480 comb 0.030000 user 0.030000 sys 0.000000 (best of 100)
$ perfbdiff 0e9928989e9c --alldata --count 100
! wall 4.097394 comb 4.100000 user 4.100000 sys 0.000000 (best of 3)
! wall 3.597798 comb 3.600000 user 3.600000 sys 0.000000 (best of 3)
This 2nd example throws a total of ~3.3GB of data at bdiff. This
change increases the throughput from ~811 MB/s to ~924 MB/s.
diff --git a/mercurial/bdiff.c b/mercurial/bdiff.c
--- a/mercurial/bdiff.c
+++ b/mercurial/bdiff.c
@@ -57,14 +57,19 @@ int bdiff_splitlines(const char *a, ssiz
/* build the line array and calculate hashes */
hash = 0;
- for (p = a; p < a + len; p++) {
+ for (p = a; p < plast; p++) {
hash = HASH(hash, *p);
- if (*p == '\n' || p == plast) {
+ if (*p == '\n') {
FINALIZELINE(l, hash, p, b);
}
}
+ if (p == plast) {
+ hash = HASH(hash, *p);
+ FINALIZELINE(l, hash, p, b);
+ }
+
/* set up a sentinel */
l->hash = 0;
l->len = 0;
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