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>

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;