svn commit: r367822 - head/lib/libc/string

[Ed Maste]

Author: emaste
Date: Thu Nov 19 00:02:12 2020
New Revision: 367822
URL: https://svnweb.freebsd.org/changeset/base/367822

Log:
  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
  MFC after:	2 weeks

Modified:
  head/lib/libc/string/memmem.c

Modified: head/lib/libc/string/memmem.c
==============================================================================
--- head/lib/libc/string/memmem.c	Wed Nov 18 22:01:34 2020	(r367821)
+++ head/lib/libc/string/memmem.c	Thu Nov 19 00:02:12 2020	(r367822)
@@ -153,8 +153,8 @@ twoway_memmem(const unsigned char *h, const unsigned c
 		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;