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authorThomas Rast <trast@student.ethz.ch>2012-04-06 23:01:23 +0200
committerJunio C Hamano <gitster@pobox.com>2012-04-09 17:03:25 -0700
commit6942efcfa9f3083e7c7863348fa5bb1350412595 (patch)
tree569985c285ef0acea5617a93ebdea46a299f46ee /xdiff
parente8dde3e5f9ddb7cf95a6ff3cea6cf07c3a2db80d (diff)
downloadgit-6942efcfa9f3083e7c7863348fa5bb1350412595.tar.gz
xdiff: load full words in the inner loop of xdl_hash_record
Redo the hashing loop in xdl_hash_record in a way that loads an entire 'long' at a time, using masking tricks to see when and where we found the terminating '\n'. I stole inspiration and code from the posts by Linus Torvalds around https://lkml.org/lkml/2012/3/2/452 https://lkml.org/lkml/2012/3/5/6 His method reads the buffers in sizeof(long) increments, and may thus overrun it by at most sizeof(long)-1 bytes before it sees the final newline (or hits the buffer length check). I considered padding out all buffers by a suitable amount to "catch" the overrun, but * this does not work for mmap()'d buffers: if you map 4096+8 bytes from a 4096 byte file, accessing the last 8 bytes results in a SIGBUS on my machine; and * it would also be extremely ugly because it intrudes deep into the unpacking machinery. So I adapted it to not read beyond the buffer at all. Instead, it reads the final partial word byte-by-byte and strings it together. Then it can use the same logic as before to finish the hashing. So far we enable this only on x86_64, where it provides nice speedup for diff-related work: Test origin/next tr/xdiff-fast-hash ----------------------------------------------------------------------------- 4000.1: log -3000 (baseline) 0.07(0.05+0.02) 0.08(0.06+0.02) +14.3% 4000.2: log --raw -3000 (tree-only) 0.37(0.33+0.04) 0.37(0.32+0.04) +0.0% 4000.3: log -p -3000 (Myers) 1.75(1.65+0.09) 1.60(1.49+0.10) -8.6% 4000.4: log -p -3000 --histogram 1.73(1.62+0.09) 1.58(1.49+0.08) -8.7% 4000.5: log -p -3000 --patience 2.11(2.00+0.10) 1.94(1.80+0.11) -8.1% Perhaps other platforms could also benefit. However it does NOT work on big-endian systems! [jc: minimum style and compilation fixes] Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'xdiff')
-rw-r--r--xdiff/xutils.c112
1 files changed, 112 insertions, 0 deletions
diff --git a/xdiff/xutils.c b/xdiff/xutils.c
index 0de084e53f..e05b5c96aa 100644
--- a/xdiff/xutils.c
+++ b/xdiff/xutils.c
@@ -20,6 +20,8 @@
*
*/
+#include <limits.h>
+#include <assert.h>
#include "xinclude.h"
@@ -276,6 +278,115 @@ static unsigned long xdl_hash_record_with_whitespace(char const **data,
return ha;
}
+#ifdef XDL_FAST_HASH
+
+#define ONEBYTES 0x0101010101010101ul
+#define NEWLINEBYTES 0x0a0a0a0a0a0a0a0aul
+#define HIGHBITS 0x8080808080808080ul
+
+/* Return the high bit set in the first byte that is a zero */
+static inline unsigned long has_zero(unsigned long a)
+{
+ return ((a - ONEBYTES) & ~a) & HIGHBITS;
+}
+
+#if __WORDSIZE == 64
+
+/*
+ * Jan Achrenius on G+: microoptimized version of
+ * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
+ * that works for the bytemasks without having to
+ * mask them first.
+ */
+static inline long count_masked_bytes(unsigned long mask)
+{
+ return mask * 0x0001020304050608 >> 56;
+}
+
+#else /* 32-bit case */
+
+/* Modified Carl Chatfield G+ version for 32-bit */
+static inline long count_masked_bytes(long mask)
+{
+ /*
+ * (a) gives us
+ * -1 (0, ff), 0 (ffff) or 1 (ffffff)
+ * (b) gives us
+ * 0 for 0, 1 for (ff ffff ffffff)
+ * (a+b+1) gives us
+ * correct 0-3 bytemask count result
+ */
+ long a = (mask - 256) >> 23;
+ long b = mask & 1;
+ return a + b + 1;
+}
+
+#endif
+
+unsigned long xdl_hash_record(char const **data, char const *top, long flags)
+{
+ unsigned long hash = 5381;
+ unsigned long a = 0, mask = 0;
+ char const *ptr = *data;
+ char const *end = top - sizeof(unsigned long) + 1;
+
+ if (flags & XDF_WHITESPACE_FLAGS)
+ return xdl_hash_record_with_whitespace(data, top, flags);
+
+ ptr -= sizeof(unsigned long);
+ do {
+ hash += hash << 5;
+ hash ^= a;
+ ptr += sizeof(unsigned long);
+ if (ptr >= end)
+ break;
+ a = *(unsigned long *)ptr;
+ /* Do we have any '\n' bytes in this word? */
+ mask = has_zero(a ^ NEWLINEBYTES);
+ } while (!mask);
+
+ if (ptr >= end) {
+ /*
+ * There is only a partial word left at the end of the
+ * buffer. Because we may work with a memory mapping,
+ * we have to grab the rest byte by byte instead of
+ * blindly reading it.
+ *
+ * To avoid problems with masking in a signed value,
+ * we use an unsigned char here.
+ */
+ const char *p;
+ for (p = top - 1; p >= ptr; p--)
+ a = (a << 8) + *((const unsigned char *)p);
+ mask = has_zero(a ^ NEWLINEBYTES);
+ if (!mask)
+ /*
+ * No '\n' found in the partial word. Make a
+ * mask that matches what we read.
+ */
+ mask = 1UL << (8 * (top - ptr) + 7);
+ }
+
+ /* The mask *below* the first high bit set */
+ mask = (mask - 1) & ~mask;
+ mask >>= 7;
+ hash += hash << 5;
+ hash ^= a & mask;
+
+ /* Advance past the last (possibly partial) word */
+ ptr += count_masked_bytes(mask);
+
+ if (ptr < top) {
+ assert(*ptr == '\n');
+ ptr++;
+ }
+
+ *data = ptr;
+
+ return hash;
+}
+
+#else /* XDL_FAST_HASH */
unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
unsigned long ha = 5381;
@@ -293,6 +404,7 @@ unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
return ha;
}
+#endif /* XDL_FAST_HASH */
unsigned int xdl_hashbits(unsigned int size) {
unsigned int val = 1, bits = 0;