1// run
2
3// Copyright 2015 The Go Authors. All rights reserved.
4// Use of this source code is governed by a BSD-style
5// license that can be found in the LICENSE file.
6
7package main
8
9import (
10	"fmt"
11	"runtime"
12)
13
14const N = 100000
15
16func main() {
17	// Allocate more Ps than processors.  This raises
18	// the chance that we get interrupted by the OS
19	// in exactly the right (wrong!) place.
20	p := runtime.NumCPU()
21	runtime.GOMAXPROCS(2 * p)
22
23	// Allocate some pointers.
24	ptrs := make([]*int, p)
25	for i := 0; i < p; i++ {
26		ptrs[i] = new(int)
27	}
28
29	// Arena where we read and write pointers like crazy.
30	collider := make([]*int, p)
31
32	done := make(chan struct{}, 2*p)
33
34	// Start writers.  They alternately write a pointer
35	// and nil to a slot in the collider.
36	for i := 0; i < p; i++ {
37		i := i
38		go func() {
39			for j := 0; j < N; j++ {
40				// Write a pointer using memmove.
41				copy(collider[i:i+1], ptrs[i:i+1])
42				// Write nil using memclr.
43				// (This is a magic loop that gets lowered to memclr.)
44				r := collider[i : i+1]
45				for k := range r {
46					r[k] = nil
47				}
48			}
49			done <- struct{}{}
50		}()
51	}
52	// Start readers.  They read pointers from slots
53	// and make sure they are valid.
54	for i := 0; i < p; i++ {
55		i := i
56		go func() {
57			for j := 0; j < N; j++ {
58				var ptr [1]*int
59				copy(ptr[:], collider[i:i+1])
60				if ptr[0] != nil && ptr[0] != ptrs[i] {
61					panic(fmt.Sprintf("bad pointer read %p!", ptr[0]))
62				}
63			}
64			done <- struct{}{}
65		}()
66	}
67	for i := 0; i < 2*p; i++ {
68		<-done
69	}
70}
71