Source/JavaScriptCore/heap/CellState.h

/*
 * Copyright (C) 2015 Apple Inc. All rights reserved.
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#ifndef CellState_h
#define CellState_h

namespace JSC {

enum class CellState : uint8_t {
    // The object is black as far as this GC is concerned. When not in GC, this just means that it's an
    // old gen object. Note that we deliberately arrange OldBlack to be zero, so that the store barrier on
    // a target object "from" is just:
    //
    // if (!from->cellState())
    //     slowPath(from);
    //
    // There is a bunch of code in the LLInt and JITs that rely on this being the case. You'd have to
    // change a lot of code if you ever wanted the store barrier to be anything but a non-zero check on
    // cellState.
    OldBlack = 0,
    
    // The object is in eden. During GC, this means that the object has not been marked yet.
    NewWhite = 1,

    // The object is grey - i.e. it will be scanned - but it either belongs to old gen (if this is eden
    // GC) or it is grey a second time in this current GC (because a concurrent store barrier requested
    // re-greying).
    OldGrey = 2,

    // The object is grey - i.e. it will be scanned - and this is the first time in this GC that we are
    // going to scan it. If this is an eden GC, this also means that the object is in eden.
    NewGrey = 3
};

} // namespace JSC

#endif // CellState_h