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https://github.com/golang/go
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f03c9202c4
Recognize loops of the form
for i := range a {
a[i] = zero
}
in which the evaluation of a is free from side effects.
Replace these loops with calls to memclr.
This occurs in the stdlib in 18 places.
The motivating example is clearing a byte slice:
benchmark old ns/op new ns/op delta
BenchmarkGoMemclr5 3.31 3.26 -1.51%
BenchmarkGoMemclr16 13.7 3.28 -76.06%
BenchmarkGoMemclr64 50.8 4.14 -91.85%
BenchmarkGoMemclr256 157 6.02 -96.17%
Update #5373.
Change-Id: I99d3e6f5f268e8c6499b7e661df46403e5eb83e4
Reviewed-on: https://go-review.googlesource.com/2520
Reviewed-by: Keith Randall <khr@golang.org>
72 lines
1.4 KiB
Go
72 lines
1.4 KiB
Go
// run
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Ensure that zeroing range loops have the requisite side-effects.
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package main
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import (
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"fmt"
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"os"
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)
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func check(n int) {
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// When n == 0, i is untouched by the range loop.
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// Picking an initial value of -1 for i makes the
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// "want" calculation below correct in all cases.
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i := -1
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s := make([]byte, n)
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for i = range s {
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s[i] = 0
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}
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if want := n - 1; i != want {
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fmt.Printf("index after range with side-effect = %d want %d\n", i, want)
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os.Exit(1)
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}
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i = n + 1
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// i is shadowed here, so its value should be unchanged.
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for i := range s {
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s[i] = 0
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}
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if want := n + 1; i != want {
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fmt.Printf("index after range without side-effect = %d want %d\n", i, want)
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os.Exit(1)
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}
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// Index variable whose evaluation has side-effects
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var x int
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f := func() int {
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x++
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return 0
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}
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var a [1]int
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for a[f()] = range s {
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s[a[f()]] = 0
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}
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if want := n * 2; x != want {
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fmt.Printf("index function calls = %d want %d\n", x, want)
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os.Exit(1)
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}
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// Range expression whose evaluation has side-effects
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x = 0
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b := [1][]byte{s}
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for i := range b[f()] {
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b[f()][i] = 0
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}
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if want := n + 1; x != n+1 {
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fmt.Printf("range expr function calls = %d want %d\n", x, want)
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os.Exit(1)
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}
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}
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func main() {
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check(0)
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check(1)
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check(15)
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}
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