faster strlen

[Eric Blake]

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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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