• Russ Cox's avatar
    cmd/6g, cmd/8g: move panicindex calls out of line · fefae6ee
    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/6248049
    fefae6ee
y.tab.h 3.21 KB