strconv: reduce buffer size for multi-precision decimals.
The longest numbers we have to represent are the smallest denormals.
Their decimal mantissa is not longer than 5^1100. Taking into
account some extra size for in-place operations, 800 digits are
enough. This saves time used for zero intiialization of extra
bytes.
old ns/op new ns/op delta
strconv_test.BenchmarkAtof64Decimal 521 334 -35.9%
strconv_test.BenchmarkAtof64Float 572 391 -31.6%
strconv_test.BenchmarkAtof64FloatExp 10242 10036 -2.0%
strconv_test.BenchmarkAtof64Big 4229 4029 -4.7%
strconv_test.BenchmarkFormatFloatDecimal 1396 934 -33.1%
strconv_test.BenchmarkFormatFloat 4295 3341 -22.2%
strconv_test.BenchmarkFormatFloatExp 12035 11181 -7.1%
strconv_test.BenchmarkFormatFloatBig 4213 3229 -23.4%
strconv_test.BenchmarkAppendFloatDecimal 1031 600 -41.8%
strconv_test.BenchmarkAppendFloat 3971 3044 -23.3%
strconv_test.BenchmarkAppendFloatExp 11699 11003 -5.9%
strconv_test.BenchmarkAppendFloatBig 3836 2915 -24.0%
R=golang-dev, bradfitz, rsc
CC=golang-dev, remy
https://golang.org/cl/
5491064