Minor performance impact running sequentially:

benchmark                      old ns/op    new ns/op    delta
BenchmarkString10Base2               389          391   +0.51%
BenchmarkString100Base2             1530         1534   +0.26%
BenchmarkString1000Base2           11789        11787   -0.02%
BenchmarkString10000Base2         111443       112030   +0.53%
BenchmarkString100000Base2       1017483      1015347   -0.21%
BenchmarkString10Base8               339          344   +1.47%
BenchmarkString100Base8              753          756   +0.40%
BenchmarkString1000Base8            4618         4641   +0.50%
BenchmarkString10000Base8          43217        43534   +0.73%
BenchmarkString100000Base8        397518       400602   +0.78%
BenchmarkString10Base10              630          630   +0.00%
BenchmarkString100Base10            1975         1960   -0.76%
BenchmarkString1000Base10          10179        10174   -0.05%
BenchmarkString10000Base10         44527        44416   -0.25%
BenchmarkString100000Base10     14404694     14425308   +0.14%
BenchmarkString10Base16              283          288   +1.77%
BenchmarkString100Base16             597          598   +0.17%
BenchmarkString1000Base16           3189         3186   -0.09%
BenchmarkString10000Base16         29403        29364   -0.13%
BenchmarkString100000Base16       265657       265587   -0.03%

Note that due to other improvements (faster assembly routines,
better code generation by compiler), these benchmarks now run
up to 37% faster than they used to at the last time measured (1/9/2012).

Minor performance impact for StringPiParallel running in parallel:

Current CL but with Lock/Unlock commented out (removed):

BenchmarkStringPiParallel	    5000	    343581 ns/op
BenchmarkStringPiParallel-2	   10000	    184511 ns/op
BenchmarkStringPiParallel-3	   10000	    129768 ns/op
BenchmarkStringPiParallel-4	   10000	    102326 ns/op

Current CL:

BenchmarkStringPiParallel	    5000	    345169 ns/op
BenchmarkStringPiParallel-2	   10000	    185827 ns/op
BenchmarkStringPiParallel-3	   10000	    131168 ns/op
BenchmarkStringPiParallel-4	   10000	    102353 ns/op

Fixes #4218.

R=dvyukov, michael.jones, dave
CC=golang-dev
https://golang.org/cl/6643053