Source/JavaScriptCore/heap/ReleaseHeapAccessScope.h

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#pragma once

#include "Heap.h"

namespace JSC {

// Almost all of the VM's code runs with "heap access". This means that the GC thread believes that
// the VM is messing with the heap in a way that would be unsafe for certain phases of the collector,
// like the weak reference fixpoint, stack scanning, and changing barrier modes. However, many long
// running operations inside the VM don't require heap access. For example, memcpying a typed array
// if a reference to it is on the stack is totally fine without heap access. Blocking on a futex is
// also fine without heap access. Releasing heap access for long-running code (in the case of futex
// wait, possibly infinitely long-running) ensures that the GC can finish a collection cycle while
// you are waiting.
class ReleaseHeapAccessScope {
public:
    ReleaseHeapAccessScope(Heap& heap)
        : m_heap(heap)
    {
        m_heap.releaseAccess();
    }
    
    ~ReleaseHeapAccessScope()
    {
        m_heap.acquireAccess();
    }

private:
    Heap& m_heap;
};

class ReleaseHeapAccessIfNeededScope {
public:
    ReleaseHeapAccessIfNeededScope(Heap& heap)
        : m_heap(heap)
    {
        hadHeapAccess = m_heap.hasAccess();
        if (hadHeapAccess)
            m_heap.releaseAccess();
    }

    ~ReleaseHeapAccessIfNeededScope()
    {
        if (hadHeapAccess)
            m_heap.acquireAccess();
    }

private:
    Heap& m_heap;
    bool hadHeapAccess { false };
};

} // namespace JSC