Date: Thu, 19 Nov 2020 00:02:13 +0000 (UTC) From: Ed Maste <emaste@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r367822 - head/lib/libc/string Message-ID: <202011190002.0AJ02DJ3064717@repo.freebsd.org>
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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;
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