1/*
2 * Copyright (C) 2015-2017 Apple Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
14 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
17 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
18 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
19 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
20 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
21 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
23 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#pragma once
27
28#include "IsoSubspace.h"
29#include "JSCast.h"
30#include "VM.h"
31#include "Watchpoint.h"
32#include "WriteBarrier.h"
33
34namespace JSC {
35
36// Allocate one of these if you'd like to infer a constant value. Writes to the value should use
37// notifyWrite(). So long as exactly one value had ever been written and invalidate() has never been
38// called, and you register a watchpoint, you can rely on the inferredValue() being the one true
39// value.
40//
41// Commonly used for inferring singletons - in that case each allocation does notifyWrite(). But you
42// can use it for other things as well.
43
44class InferredValue final : public JSCell {
45public:
46 typedef JSCell Base;
47
48 template<typename CellType, SubspaceAccess mode>
49 static IsoSubspace* subspaceFor(VM& vm)
50 {
51 return vm.inferredValueSpace<mode>();
52 }
53
54 static InferredValue* create(VM&);
55
56 static const bool needsDestruction = true;
57 static void destroy(JSCell*);
58
59 static Structure* createStructure(VM&, JSGlobalObject*, JSValue prototype);
60
61 static void visitChildren(JSCell*, SlotVisitor&);
62
63 DECLARE_INFO;
64
65 // For the purpose of deciding whether or not to watch this variable, you only need
66 // to inspect inferredValue(). If this returns something other than the empty
67 // value, then it means that at all future safepoints, this watchpoint set will be
68 // in one of these states:
69 //
70 // IsWatched: in this case, the variable's value must still be the
71 // inferredValue.
72 //
73 // IsInvalidated: in this case the variable's value may be anything but you'll
74 // either notice that it's invalidated and not install the watchpoint, or
75 // you will have been notified that the watchpoint was fired.
76 JSValue inferredValue() { return m_value.get(); }
77
78 // Forwards some WatchpointSet methods.
79 WatchpointState state() const { return m_set.state(); }
80 bool isStillValid() const { return m_set.isStillValid(); }
81 bool hasBeenInvalidated() const { return m_set.hasBeenInvalidated(); }
82 void add(Watchpoint* watchpoint) { m_set.add(watchpoint); }
83
84 void notifyWrite(VM& vm, JSValue value, const FireDetail& detail)
85 {
86 if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
87 return;
88 notifyWriteSlow(vm, value, detail);
89 }
90
91 void notifyWrite(VM& vm, JSValue value, const char* reason)
92 {
93 if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
94 return;
95 notifyWriteSlow(vm, value, reason);
96 }
97
98 void invalidate(VM& vm, const FireDetail& detail)
99 {
100 m_value.clear();
101 m_set.invalidate(vm, detail);
102 }
103
104 static const unsigned StructureFlags = Base::StructureFlags | StructureIsImmortal;
105
106 void finalizeUnconditionally(VM&);
107
108private:
109 InferredValue(VM&);
110 ~InferredValue();
111
112 JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const FireDetail&);
113 JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const char* reason);
114
115 InlineWatchpointSet m_set;
116 WriteBarrier<Unknown> m_value;
117};
118
119// FIXME: We could have an InlineInferredValue, which only allocates the InferredValue object when
120// a notifyWrite() transitions us towards watching, and then clears the reference (allowing the object
121// to die) when we get invalidated.
122
123} // namespace JSC
124