git: 13bc5a7358e5 - stable/12 - libc: optimize memmem two-way bad character shift
Ed Maste
emaste at FreeBSD.org
Fri Jan 15 14:26:57 UTC 2021
The branch stable/12 has been updated by emaste:
URL: https://cgit.FreeBSD.org/src/commit/?id=13bc5a7358e5a39feec12b77b097c70dd111305e
commit 13bc5a7358e5a39feec12b77b097c70dd111305e
Author: Ed Maste <emaste at FreeBSD.org>
AuthorDate: 2020-11-19 00:02:12 +0000
Commit: Ed Maste <emaste at FreeBSD.org>
CommitDate: 2021-01-15 14:25:35 +0000
libc: optimize memmem two-way bad character shift
first, the condition (mem && k < p) is redundant, because mem being
nonzero implies the needle is periodic with period exactly p, in which
case any byte that appears in the needle must appear in the last p
bytes of the needle, bounding the shift (k) by p.
second, the whole point of replacing the shift k by mem (=l-p) is to
prevent shifting by less than mem when discarding the memory on shift,
in which case linear time could not be guaranteed. but as written, the
check also replaced shifts greater than mem by mem, reducing the
benefit of the shift. there is no possible benefit to this reduction of
the shift; since mem is being cleared, the full shift is valid and
more optimal. so only replace the shift by mem when it would be less
than mem.
musl commits:
8f5a820d147da36bcdbddd201b35d293699dacd8
122d67f846cb0be2c9e1c3880db9eb9545bbe38c
Obtained from: musl
(cherry picked from commit 7dbcd06e63101d51e6a777f7315cfde794411e53)
---
lib/libc/string/memmem.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/lib/libc/string/memmem.c b/lib/libc/string/memmem.c
index 27863b9db623..9e7bf94b1464 100644
--- a/lib/libc/string/memmem.c
+++ b/lib/libc/string/memmem.c
@@ -153,8 +153,8 @@ twoway_memmem(const unsigned char *h, const unsigned char *z,
if (BITOP(byteset, h[l - 1], &)) {
k = l - shift[h[l - 1]];
if (k) {
- if (mem0 && mem && k < p)
- k = l - p;
+ if (k < mem)
+ k = mem;
h += k;
mem = 0;
continue;
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