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Keep track of how many uses each Value has. Each appearance in Value.Args and in Block.Control counts once. The number of uses of a value is generically useful to constrain rewrite rules. For instance, we might want to prevent merging index operations into loads if the same index expression is used lots of times. But I have one use in particular for which the use count is required. We must make sure we don't combine ops with loads if the load has more than one use. Otherwise, we may split a single load into multiple loads and that breaks perceived behavior in the presence of races. In particular, the load of m.state in sync/mutex.go:Lock can't be done twice. (I have a separate CL which triggers the mutex failure. This CL has a test which demonstrates a similar failure.) Change-Id: Icaafa479239f48632a069d0c3f624e6ebc6b1f0e Reviewed-on: https://go-review.googlesource.com/20790 Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Todd Neal <todd@tneal.org>
46 lines
798 B
Go
46 lines
798 B
Go
// run
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// Copyright 2016 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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// Check that we do loads exactly once. The SSA backend
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// once tried to do the load in f twice, once sign extended
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// and once zero extended. This can cause problems in
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// racy code, particularly sync/mutex.
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package main
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func f(p *byte) bool {
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x := *p
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a := int64(int8(x))
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b := int64(uint8(x))
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return a == b
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}
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func main() {
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var x byte
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const N = 1000000
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c := make(chan struct{})
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go func() {
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for i := 0; i < N; i++ {
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x = 1
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}
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c <- struct{}{}
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}()
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go func() {
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for i := 0; i < N; i++ {
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x = 2
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}
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c <- struct{}{}
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}()
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for i := 0; i < N; i++ {
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if !f(&x) {
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panic("non-atomic load!")
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}
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}
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<-c
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<-c
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}
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