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.
$ hg init a
$ cd a
$ touch empty1
$ hg add empty1
$ hg commit -m 'add empty1'
$ touch empty2
$ hg add empty2
$ hg commit -m 'add empty2'
$ hg up -C 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ touch empty3
$ hg add empty3
$ hg commit -m 'add empty3'
created new head
$ hg heads
changeset: 2:a1cb177e0d44
tag: tip
parent: 0:1e1d9c4e5b64
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: add empty3
changeset: 1:097d2b0e17f6
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: add empty2
$ hg merge 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
Before changeset 05257fd28591, we didn't notice the
empty file that came from rev 1:
$ hg status
M empty2
$ hg commit -m merge
$ hg manifest --debug tip
b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty1
b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty2
b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty3
$ cd ..