cmd/gc: cache itab lookup in convT2I.
There may be further savings if convT2I can avoid the function call
if the cache is good and T is uintptr-shaped, a la convT2E, but that
will be a follow-up CL.
src/pkg/runtime:
benchmark old ns/op new ns/op delta
BenchmarkConvT2ISmall 43 15 -64.01%
BenchmarkConvT2IUintptr 45 14 -67.48%
BenchmarkConvT2ILarge 130 101 -22.31%
test/bench/go1:
benchmark old ns/op new ns/op delta
BenchmarkBinaryTree17
8588997000 8499058000 -1.05%
BenchmarkFannkuch11
5300392000 5358093000 +1.09%
BenchmarkGobDecode
30295580 31040190 +2.46%
BenchmarkGobEncode
18102070 17675650 -2.36%
BenchmarkGzip
774191400 771591400 -0.34%
BenchmarkGunzip
245915100 247464100 +0.63%
BenchmarkJSONEncode
123577000 121423050 -1.74%
BenchmarkJSONDecode
451969800 596256200 +31.92%
BenchmarkMandelbrot200
10060050 10072880 +0.13%
BenchmarkParse
10989840 11037710 +0.44%
BenchmarkRevcomp
1782666000 1716864000 -3.69%
BenchmarkTemplate
798286600 723234400 -9.40%
R=rsc, bradfitz, go.peter.90, daniel.morsing, dave, uriel
CC=golang-dev
https://golang.org/cl/
6337058