runtime/InferredValue.h

   1 /*
   2  * Copyright (C) 2015 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
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   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
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  23  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  24  */
  25
  26 #ifndef InferredValue_h
  27 #define InferredValue_h
  28
  29 #include "JSCell.h"
  30 #include "Watchpoint.h"
  31 #include "WriteBarrier.h"
  32
  33 namespace JSC {
  34
  35 // Allocate one of these if you'd like to infer a constant value. Writes to the value should use
  36 // notifyWrite(). So long as exactly one value had ever been written and invalidate() has never been
  37 // called, and you register a watchpoint, you can rely on the inferredValue() being the one true
  38 // value.
  39 //
  40 // Commonly used for inferring singletons - in that case each allocation does notifyWrite(). But you
  41 // can use it for other things as well.
  42
  43 class InferredValue : public JSCell {
  44 public:
  45     typedef JSCell Base;
  46
  47     static InferredValue* create(VM&);
  48
  49     static const bool needsDestruction = true;
  50     static void destroy(JSCell*);
  51
  52     static Structure* createStructure(VM&, JSGlobalObject*, JSValue prototype);
  53
  54     static void visitChildren(JSCell*, SlotVisitor&);
  55
  56     DECLARE_INFO;
  57
  58     // For the purpose of deciding whether or not to watch this variable, you only need
  59     // to inspect inferredValue(). If this returns something other than the empty
  60     // value, then it means that at all future safepoints, this watchpoint set will be
  61     // in one of these states:
  62     //
  63     //    IsWatched: in this case, the variable's value must still be the
  64     //        inferredValue.
  65     //
  66     //    IsInvalidated: in this case the variable's value may be anything but you'll
  67     //        either notice that it's invalidated and not install the watchpoint, or
  68     //        you will have been notified that the watchpoint was fired.
  69     JSValue inferredValue() { return m_value.get(); }
  70
  71     // Forwards some WatchpointSet methods.
  72     WatchpointState state() const { return m_set.state(); }
  73     bool isStillValid() const { return m_set.isStillValid(); }
  74     bool hasBeenInvalidated() const { return m_set.hasBeenInvalidated(); }
  75     void add(Watchpoint* watchpoint) { m_set.add(watchpoint); }
  76
  77     void notifyWrite(VM& vm, JSValue value, const FireDetail& detail)
  78     {
  79         if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
  80             return;
  81         notifyWriteSlow(vm, value, detail);
  82     }
  83
  84     void notifyWrite(VM& vm, JSValue value, const char* reason)
  85     {
  86         if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
  87             return;
  88         notifyWriteSlow(vm, value, reason);
  89     }
  90
  91     void invalidate(const FireDetail& detail)
  92     {
  93         m_value.clear();
  94         m_set.invalidate(detail);
  95     }
  96
  97     static const unsigned StructureFlags = StructureIsImmortal | Base::StructureFlags;
  98
  99     // We could have used Weak<>. But we want arbitrary JSValues, not just cells. It's also somewhat
 100     // convenient to have eager notification of death.
 101     //
 102     // Also note that this should be a private class, but it isn't because Windows.
 103     class ValueCleanup : public UnconditionalFinalizer {
 104         WTF_MAKE_FAST_ALLOCATED;
 105
 106     public:
 107         ValueCleanup(InferredValue*);
 108         virtual ~ValueCleanup();
 109
 110     protected:
 111         void finalizeUnconditionally() override;
 112
 113     private:
 114         InferredValue* m_owner;
 115     };
 116
 117 private:
 118     InferredValue(VM&);
 119     ~InferredValue();
 120
 121     JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const FireDetail&);
 122     JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const char* reason);
 123
 124     friend class ValueCleanup;
 125
 126     InlineWatchpointSet m_set;
 127     WriteBarrier<Unknown> m_value;
 128     std::unique_ptr<ValueCleanup> m_cleanup;
 129 };
 130
 131 // FIXME: We could have an InlineInferredValue, which only allocates the InferredValue object when
 132 // a notifyWrite() transitions us towards watching, and then clears the reference (allowing the object
 133 // to die) when we get invalidated.
 134
 135 } // namespace JSC
 136
 137 #endif // InferredValue_h
 138