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Russ Cox authored
The old code generated for a bounds check was CMP JLT ok CALL panicindex ok: ... The new code is (once the linker finishes with it): CMP JGE panic ... panic: CALL panicindex which moves the calls out of line, putting more useful code in each cache line. This matters especially in tight loops, such as in Fannkuch. The benefit is more modest elsewhere, but real. From test/bench/go1, amd64: benchmark old ns/op new ns/op delta BenchmarkBinaryTree17 6096092000 6088808000 -0.12% BenchmarkFannkuch11 6151404000 4020463000 -34.64% BenchmarkGobDecode 28990050 28894630 -0.33% BenchmarkGobEncode 12406310 12136730 -2.17% BenchmarkGzip 179923 179903 -0.01% BenchmarkGunzip 11219 11130 -0.79% BenchmarkJSONEncode 86429350 86515900 +0.10% BenchmarkJSONDecode 334593800 315728400 -5.64% BenchmarkRevcomp25M 1219763000 1180767000 -3.20% BenchmarkTemplate 492947600 483646800 -1.89% And 386: benchmark old ns/op new ns/op delta BenchmarkBinaryTree17 6354902000 6243000000 -1.76% BenchmarkFannkuch11 8043769000 7326965000 -8.91% BenchmarkGobDecode 19010800 18941230 -0.37% BenchmarkGobEncode 14077500 13792460 -2.02% BenchmarkGzip 194087 193619 -0.24% BenchmarkGunzip 12495 12457 -0.30% BenchmarkJSONEncode 125636400 125451400 -0.15% BenchmarkJSONDecode 696648600 685032800 -1.67% BenchmarkRevcomp25M 2058088000 2052545000 -0.27% BenchmarkTemplate 602140000 589876800 -2.04% To implement this, two new instruction forms: JLT target // same as always JLT $0, target // branch expected not taken JLT $1, target // branch expected taken The linker could also emit the prediction prefixes, but it does not: expected taken branches are reversed so that the expected case is not taken (as in example above), and the default expectaton for such a jump is not taken already. R=golang-dev, gri, r, dave CC=golang-dev https://golang.org/cl/6248049fefae6ee
