• Vladimir Kuzmin's avatar
    cmd/compile: avoid mapaccess at m[k]=append(m[k].. · c12b185a
    Vladimir Kuzmin authored
    Currently rvalue m[k] is transformed during walk into:
    
            tmp1 := *mapaccess(m, k)
            tmp2 := append(tmp1, ...)
            *mapassign(m, k) = tmp2
    
    However, this is suboptimal, as we could instead produce just:
            tmp := mapassign(m, k)
            *tmp := append(*tmp, ...)
    
    Optimization is possible only if during Order it may tell that m[k] is
    exactly the same at left and right part of assignment. It doesn't work:
    1) m[f(k)] = append(m[f(k)], ...)
    2) sink, m[k] = sink, append(m[k]...)
    3) m[k] = append(..., m[k],...)
    
    Benchmark:
    name                           old time/op    new time/op    delta
    MapAppendAssign/Int32/256-8      33.5ns ± 3%    22.4ns ±10%  -33.24%  (p=0.000 n=16+18)
    MapAppendAssign/Int32/65536-8    68.2ns ± 6%    48.5ns ±29%  -28.90%  (p=0.000 n=20+20)
    MapAppendAssign/Int64/256-8      34.3ns ± 4%    23.3ns ± 5%  -32.23%  (p=0.000 n=17+18)
    MapAppendAssign/Int64/65536-8    65.9ns ± 7%    61.2ns ±19%   -7.06%  (p=0.002 n=18+20)
    MapAppendAssign/Str/256-8         116ns ±12%      79ns ±16%  -31.70%  (p=0.000 n=20+19)
    MapAppendAssign/Str/65536-8       134ns ±15%     111ns ±45%  -16.95%  (p=0.000 n=19+20)
    
    name                           old alloc/op   new alloc/op   delta
    MapAppendAssign/Int32/256-8       47.0B ± 0%     46.0B ± 0%   -2.13%  (p=0.000 n=19+18)
    MapAppendAssign/Int32/65536-8     27.0B ± 0%     20.7B ±30%  -23.33%  (p=0.000 n=20+20)
    MapAppendAssign/Int64/256-8       47.0B ± 0%     46.0B ± 0%   -2.13%  (p=0.000 n=20+17)
    MapAppendAssign/Int64/65536-8     27.0B ± 0%     27.0B ± 0%     ~     (all equal)
    MapAppendAssign/Str/256-8         94.0B ± 0%     78.0B ± 0%  -17.02%  (p=0.000 n=20+16)
    MapAppendAssign/Str/65536-8       54.0B ± 0%     54.0B ± 0%     ~     (all equal)
    
    Fixes #24364
    Updates #5147
    
    Change-Id: Id257d052b75b9a445b4885dc571bf06ce6f6b409
    Reviewed-on: https://go-review.googlesource.com/100838Reviewed-by: 's avatarMatthew Dempsky <mdempsky@google.com>
    Run-TryBot: Matthew Dempsky <mdempsky@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    c12b185a
walk.go 95.5 KB