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From: Xin LI <delphij@FreeBSD.org>
Date: Fri, 12 Mar 2010 21:14:56 +0000 (UTC)
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Subject: cvs commit: src/lib/libc/string strlen.c
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delphij     2010-03-12 21:14:56 UTC

  FreeBSD src repository

  Modified files:
    lib/libc/string      strlen.c 
  Log:
  SVN rev 205099 on 2010-03-12 21:14:56Z by delphij
  
  Two optimizations to MI strlen(3) inspired by David S. Miller's
  blog posting [1].
  
   - Use word-sized test for unaligned pointer before working
     the hard way.
  
     Memory page boundary is always integral multiple of a word
     alignment boundary.  Therefore, if we can access memory
     referenced by pointer p, then (p & ~word mask) must be also
     accessible.
  
   - Better utilization of multi-issue processor's ability of
     concurrency.
  
     The previous implementation utilized a formular that must be
     executed sequentially.  However, the ~, & and - operations can
     actually be caculated at the same time when the operand were
     different and unrelated.
  
     The original Hacker's Delight formular also offered consistent
     performance regardless whether the input would contain
     characters with their highest-bit set, as it catches real
     nul characters only.
  
  These two optimizations has shown further improvements over the
  previous implementation on microbenchmarks on i386 and amd64 CPU
  including Pentium 4, Core Duo 2 and i7.
  
  [1] http://vger.kernel.org/~davem/cgi-bin/blog.cgi/2010/03/08#strlen_1
  
  MFC after:      1 month
  
  Revision  Changes    Path
  1.8       +31 -12    src/lib/libc/string/strlen.c