changeset 30318:e1d6aa0e4c3a

bdiff: replace hash algorithm

This patch replaces lyhash with the hash algorithm used by diffutils.
The algorithm has its origins in Git commit 2e9d1410, which is all the
way back from 1992. The license header in the code at that revision
in GPL v2.

I have not performed an extensive analysis of the distribution
(and therefore buckets) of hash output. However, `hg perfbdiff`
gives some clear wins. I'd like to think that if it is good enough
for diffutils it is good enough for us?

From the mozilla-unified repository:

$ perfbdiff -m 3041e4d59df2
! wall 0.053271 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
! wall 0.035827 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)

$ perfbdiff 0e9928989e9c --alldata --count 100
! wall 6.204277 comb 6.200000 user 6.200000 sys 0.000000 (best of 3)
! wall 4.309710 comb 4.300000 user 4.300000 sys 0.000000 (best of 3)

From the hg repo:

$ perfbdiff 35000 --alldata --count 1000
! wall 0.660358 comb 0.660000 user 0.660000 sys 0.000000 (best of 15)
! wall 0.534092 comb 0.530000 user 0.530000 sys 0.000000 (best of 19)

Looking at the generated assembly and statistical profiler output
from the kernel level, I believe there is room to make this function
even faster. Namely, we're still consuming data character by character
instead of at the word level. This translates to more loop iterations
and more instructions.

At this juncture though, the real performance killer is that we're
hashing every line. We should get a significant speedup if we change
the algorithm to find the longest prefix, longest suffix, treat those
as single "lines" and then only do the line splitting and hashing on
the parts that are different. That will require a lot of C code,
however. I'm optimistic this approach could result in a ~2x speedup.

--- a/mercurial/bdiff.c	Fri Nov 04 21:44:25 2016 -0700
+++ b/mercurial/bdiff.c	Sun Nov 06 18:51:57 2016 -0800
@@ -17,6 +17,10 @@
 #include "bitmanipulation.h"
 #include "bdiff.h"
 
+/* Hash implementation from diffutils */
+#define ROL(v, n) ((v) << (n) | (v) >> (sizeof(v) * CHAR_BIT - (n)))
+#define HASH(h, c) ((c) + ROL(h ,7))
+
 struct pos {
 	int pos, len;
 };
@@ -44,8 +48,7 @@
 	/* build the line array and calculate hashes */
 	hash = 0;
 	for (p = a; p < a + len; p++) {
-		/* Leonid Yuriev's hash */
-		hash = (hash * 1664525) + (unsigned char)*p + 1013904223;
+		hash = HASH(hash, *p);
 
 		if (*p == '\n' || p == plast) {
 			l->hash = hash;