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)
The 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.
#!/usr/bin/env python
#
# Based on python's Tools/scripts/md5sum.py
#
# This software may be used and distributed according to the terms
# of the PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2, which is
# GPL-compatible.
from __future__ import absolute_import
import os
import sys
try:
import hashlib
md5 = hashlib.md5
except ImportError:
import md5
md5 = md5.md5
try:
import msvcrt
msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
except ImportError:
pass
for filename in sys.argv[1:]:
try:
fp = open(filename, 'rb')
except IOError as msg:
sys.stderr.write('%s: Can\'t open: %s\n' % (filename, msg))
sys.exit(1)
m = md5()
try:
for data in iter(lambda: fp.read(8192), ''):
m.update(data)
except IOError as msg:
sys.stderr.write('%s: I/O error: %s\n' % (filename, msg))
sys.exit(1)
sys.stdout.write('%s %s\n' % (m.hexdigest(), filename))
sys.exit(0)