cmd/compile: avoid mapaccess at m[k]=append(m[k]..
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:Matthew Dempsky <mdempsky@google.com> Run-TryBot: Matthew Dempsky <mdempsky@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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test/codegen/mapaccess.go
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