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, 12 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index be1957a5e9..13ed1b1d29 100644
--- a/Makefile
+++ b/Makefile
@@ -288,6 +288,11 @@ all::
 # dependency rules.
 #
 # Define NATIVE_CRLF if your platform uses CRLF for line endings.
+#
+# Define XDL_FAST_HASH to use an alternative line-hashing method in
+# the diff algorithm.  It gives a nice speedup if your processor has
+# fast unaligned word loads.  Does NOT work on big-endian systems!
+# Enabled by default on x86_64.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -864,6 +869,9 @@ EXTLIBS =
 # because maintaining the nesting to match is a pain.  If
 # we had "elif" things would have been much nicer...
 
+ifeq ($(uname_M),x86_64)
+	XDL_FAST_HASH = YesPlease
+endif
 ifeq ($(uname_S),OSF1)
 	# Need this for u_short definitions et al
 	BASIC_CFLAGS += -D_OSF_SOURCE
@@ -1737,6 +1745,10 @@ ifndef NO_MSGFMT_EXTENDED_OPTIONS
 	MSGFMT += --check --statistics
 endif
 
+ifneq (,$(XDL_FAST_HASH))
+	BASIC_CFLAGS += -DXDL_FAST_HASH
+endif
+
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK=NoThanks
 endif