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/*
* Copyright (C) 2014, 2015 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
* THE POSSIBILITY OF SUCH DAMAGE.
*/
#ifndef CopyBarrier_h
#define CopyBarrier_h
#include "Heap.h"
namespace JSC {
enum class CopyState {
// The backing store is not planned to get copied in this epoch. If you keep a pointer to the backing
// store on the stack, it will not get copied. If you don't keep it on the stack, it may get copied
// starting at the next handshake (that is, it may transition from ToSpace to CopyPlanned, but
// CopyPlanned means ToSpace prior to the handshake that starts the copy phase).
ToSpace,
// The marking phase has selected this backing store to be copied. If we are not yet in the copying
// phase, this backing store is still in to-space. All that is needed in such a case is to mask off the
// low bits. If we are in the copying phase, this means that the object points to from-space. The
// barrier should first copy the object - or wait for copying to finish - before using the object.
CopyPlanned,
// The object is being copied right now. Anyone wanting to use the object must wait for the object to
// finish being copied. Notifications about copying use the ParkingLot combined with these bits. If the
// state is CopyingAndWaiting, then when the copying finishes, whatever thread was doing it will
// unparkAll() on the address of the CopyBarrierBase. So, to wait for copying to finish, CAS this to
// CopyingAndWaiting and then parkConditionally on the barrier address.
Copying,
// The object is being copied right now, and there are one or more threads parked. Those threads want
// to be unparked when copying is done. So, whichever thread does the copying needs to call unparkAll()
// on the barrier address after copying is done.
CopyingAndWaiting
};
class CopyBarrierBase {
public:
static const unsigned spaceBits = 3;
CopyBarrierBase()
: m_value(nullptr)
{
}
bool operator!() const { return !m_value; }
explicit operator bool() const { return m_value; }
void* getWithoutBarrier() const
{
return m_value;
}
// Use this version of get() if you only want to execute the barrier slow path if some condition
// holds, and you only want to evaluate that condition after first checking the barrier's
// condition. Usually, you just want to use get().
template<typename Functor>
void* getPredicated(const JSCell* owner, const Functor& functor) const
{
void* result = m_value;
if (UNLIKELY(bitwise_cast<uintptr_t>(result) & spaceBits)) {
if (functor())
return Heap::copyBarrier(owner, m_value);
}
return result;
}
// When we are in the concurrent copying phase, this method may lock the barrier object (i.e. the field
// pointing to copied space) and call directly into the owning object's copyBackingStore() method.
void* get(const JSCell* owner) const
{
return getPredicated(owner, [] () -> bool { return true; });
}
CopyState copyState() const
{
return static_cast<CopyState>(bitwise_cast<uintptr_t>(m_value) & spaceBits);
}
// This only works when you know that there is nobody else concurrently messing with this CopyBarrier.
// That's hard to guarantee, though there are a few unusual places where this ends up being safe.
// Usually you want to use CopyBarrier::weakCAS().
void setCopyState(CopyState copyState)
{
WTF::storeStoreFence();
uintptr_t value = bitwise_cast<uintptr_t>(m_value);
value &= ~static_cast<uintptr_t>(spaceBits);
value |= static_cast<uintptr_t>(copyState);
m_value = bitwise_cast<void*>(value);
}
void clear() { m_value = nullptr; }
protected:
CopyBarrierBase(VM& vm, const JSCell* owner, void* value)
{
this->set(vm, owner, value);
}
void set(VM& vm, const JSCell* owner, void* value)
{
this->m_value = value;
vm.heap.writeBarrier(owner);
}
void setWithoutBarrier(void* value)
{
this->m_value = value;
}
bool weakCASWithoutBarrier(
void* oldPointer, CopyState oldCopyState, void* newPointer, CopyState newCopyState)
{
uintptr_t oldValue = bitwise_cast<uintptr_t>(oldPointer) | static_cast<uintptr_t>(oldCopyState);
uintptr_t newValue = bitwise_cast<uintptr_t>(newPointer) | static_cast<uintptr_t>(newCopyState);
return WTF::weakCompareAndSwap(
&m_value, bitwise_cast<void*>(oldValue), bitwise_cast<void*>(newValue));
}
private:
mutable void* m_value;
};
template <typename T>
class CopyBarrier : public CopyBarrierBase {
public:
CopyBarrier()
{
}
CopyBarrier(VM& vm, const JSCell* owner, T& value)
: CopyBarrierBase(vm, owner, &value)
{
}
CopyBarrier(VM& vm, const JSCell* owner, T* value)
: CopyBarrierBase(vm, owner, value)
{
}
T* getWithoutBarrier() const
{
return bitwise_cast<T*>(CopyBarrierBase::getWithoutBarrier());
}
T* get(const JSCell* owner) const
{
return bitwise_cast<T*>(CopyBarrierBase::get(owner));
}
template<typename Functor>
T* getPredicated(const JSCell* owner, const Functor& functor) const
{
return bitwise_cast<T*>(CopyBarrierBase::getPredicated(owner, functor));
}
void set(VM& vm, const JSCell* owner, T* value)
{
CopyBarrierBase::set(vm, owner, value);
}
void setWithoutBarrier(T* value)
{
CopyBarrierBase::setWithoutBarrier(value);
}
bool weakCASWithoutBarrier(T* oldPointer, CopyState oldCopyState, T* newPointer, CopyState newCopyState)
{
return CopyBarrierBase::weakCASWithoutBarrier(oldPointer, oldCopyState, newPointer, newCopyState);
}
};
} // namespace JSC
#endif // CopyBarrier_h
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