cmd/gc: elide write barrier for x = x[0:y] and x = append(x, ...)
Both of these forms can avoid writing to the base pointer in x
(in the slice, always, and in the append, most of the time).
For Go 1.5, will need to change the compilation of x = x[0:y]
to avoid writing to the base pointer, so that the elision is safe,
and will need to change the compilation of x = append(x, ...)
to write to the base pointer (through a barrier) only when
growing the underlying array, so that the general elision is safe.
For Go 1.4, elide the write barrier always, a change that should
have equivalent performance characteristics but is much
simpler and therefore safer.
benchmark old ns/op new ns/op delta
BenchmarkBinaryTree17
3910526122 3918802545 +0.21%
BenchmarkFannkuch11
3747650699 3732600693 -0.40%
BenchmarkFmtFprintfEmpty 106 98.7 -6.89%
BenchmarkFmtFprintfString 280 269 -3.93%
BenchmarkFmtFprintfInt 296 282 -4.73%
BenchmarkFmtFprintfIntInt 467 470 +0.64%
BenchmarkFmtFprintfPrefixedInt 418 398 -4.78%
BenchmarkFmtFprintfFloat 574 535 -6.79%
BenchmarkFmtManyArgs 1768 1818 +2.83%
BenchmarkGobDecode
14916799 14925182 +0.06%
BenchmarkGobEncode
14110076 13358298 -5.33%
BenchmarkGzip
546609795 542630402 -0.73%
BenchmarkGunzip
136270657 136496277 +0.17%
BenchmarkHTTPClientServer 126574 125245 -1.05%
BenchmarkJSONEncode
30006238 27862354 -7.14%
BenchmarkJSONDecode
106020889 102664600 -3.17%
BenchmarkMandelbrot200
5793550 5818320 +0.43%
BenchmarkGoParse
5437608 5463962 +0.48%
BenchmarkRegexpMatchEasy0_32 192 179 -6.77%
BenchmarkRegexpMatchEasy0_1K 462 460 -0.43%
BenchmarkRegexpMatchEasy1_32 168 153 -8.93%
BenchmarkRegexpMatchEasy1_1K 1420 1280 -9.86%
BenchmarkRegexpMatchMedium_32 338 286 -15.38%
BenchmarkRegexpMatchMedium_1K 107435 98027 -8.76%
BenchmarkRegexpMatchHard_32 5941 4846 -18.43%
BenchmarkRegexpMatchHard_1K 185965 153830 -17.28%
BenchmarkRevcomp
795497458 798447829 +0.37%
BenchmarkTemplate
132091559 134938425 +2.16%
BenchmarkTimeParse 604 608 +0.66%
BenchmarkTimeFormat 551 548 -0.54%
LGTM=r
R=r, dave
CC=golang-codereviews, iant, khr, rlh
https://golang.org/cl/
159960043