faster strlen

[Jeff Johnston]

Patch checked in.

-- Jeff J.

Eric Blake wrote:
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> In the same vein as strchr.  OK to apply?
>
> 2008-05-22  Eric Blake  <ebb9@byu.net>
>
>     Optimize the generic and x86 strlen.
>     * libc/string/strlen.c (strlen) [!__OPTIMIZE_SIZE__]: Pre-align
>     data so unaligned searches aren't penalized.
>     * libc/machine/i386/strlen.S (strlen) [!__OPTIMIZE_SIZE__]: Word
>     operations are faster than repnz byte searches.
>
>
>
> I'm attaching my test program as well.  I compiled my changes to strlen.S
> as the function strlen1, so I could compare to the pre-patch strlen on
> cygwin.  When run as:
>
> $ for i in `seq 0 16` ; do
> | for j in `seq 0 $i` ; do
> | ./foo $i 1 $j 0
> | done; done
>
> it proves that strlen handles any alignment for both the start of the
> string as well as any alignment of the NUL at the end of the string (ie.
> 100% coverage of the code).
>
> When run with longer strings and multiple iterations, I got the following
> timings:
>
> $ ./foo
> usage: ./foo size repeat offset [func]
> $ time ./foo 1000000 1000 0
>
> real    0m2.071s
> user    0m1.999s
> sys    0m0.046s
>
> $ time ./foo 1000000 1000 1
>
> real    0m2.065s
> user    0m1.921s
> sys    0m0.031s
>
> # Pre-patch assembly, uses 'repnz scasb', which is dirt slow
> # at least alignment was not an issue with byte-wise algorithm
>
> $ time ./foo 1000000 1000 0 0
>
> real    0m0.707s
> user    0m0.624s
> sys    0m0.046s
>
> $ time ./foo 1000000 1000 1 0
>
> real    0m0.704s
> user    0m0.655s
> sys    0m0.046s
>
> # patched assembly, 3x faster, alignment still doesn't matter
>
> $ time ./foo 1000000 1000 0 1
>
> real    0m0.702s
> user    0m0.655s
> sys    0m0.030s
>
> $ time ./foo 1000000 1000 1 1
>
> real    0m0.724s
> user    0m0.686s
> sys    0m0.031s
>
> # patched C code, marginally slower than patched assembly
>
> - --
> Don't work too hard, make some time for fun as well!
>
> Eric Blake             ebb9@byu.net
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