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Watchpoint.h
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/*
* Copyright (C) 2012, 2013 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. ``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
* 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 Watchpoint_h
#define Watchpoint_h
#include <wtf/SentinelLinkedList.h>
#include <wtf/ThreadSafeRefCounted.h>
namespace JSC {
class Watchpoint : public BasicRawSentinelNode<Watchpoint> {
public:
Watchpoint()
{
}
virtual ~Watchpoint();
void fire() { fireInternal(); }
protected:
virtual void fireInternal() = 0;
};
enum WatchpointState {
ClearWatchpoint,
IsWatched,
IsInvalidated
};
class InlineWatchpointSet;
class WatchpointSet : public ThreadSafeRefCounted<WatchpointSet> {
friend class LLIntOffsetsExtractor;
public:
WatchpointSet(WatchpointState);
~WatchpointSet(); // Note that this will not fire any of the watchpoints; if you need to know when a WatchpointSet dies then you need a separate mechanism for this.
// It is safe to call this from another thread. It may return an old
// state. Guarantees that if *first* read the state() of the thing being
// watched and it returned IsWatched and *second* you actually read its
// value then it's safe to assume that if the state being watched changes
// then also the watchpoint state() will change to IsInvalidated.
WatchpointState state() const
{
WTF::loadLoadFence();
WatchpointState result = static_cast<WatchpointState>(m_state);
WTF::loadLoadFence();
return result;
}
// It is safe to call this from another thread. It may return true
// even if the set actually had been invalidated, but that ought to happen
// only in the case of races, and should be rare. Guarantees that if you
// call this after observing something that must imply that the set is
// invalidated, then you will see this return false. This is ensured by
// issuing a load-load fence prior to querying the state.
bool isStillValid() const
{
return state() != IsInvalidated;
}
// Like isStillValid(), may be called from another thread.
bool hasBeenInvalidated() const { return !isStillValid(); }
// As a convenience, this will ignore 0. That's because code paths in the DFG
// that create speculation watchpoints may choose to bail out if speculation
// had already been terminated.
void add(Watchpoint*);
// Force the watchpoint set to behave as if it was being watched even if no
// watchpoints have been installed. This will result in invalidation if the
// watchpoint would have fired. That's a pretty good indication that you
// probably don't want to set watchpoints, since we typically don't want to
// set watchpoints that we believe will actually be fired.
void startWatching()
{
ASSERT(state() != IsInvalidated);
m_state = IsWatched;
}
void fireAll()
{
if (state() != IsWatched)
return;
fireAllSlow();
}
void touch()
{
if (state() == ClearWatchpoint)
startWatching();
else
fireAll();
}
void invalidate()
{
if (state() == IsWatched)
fireAll();
m_state = IsInvalidated;
}
int8_t* addressOfState() { return &m_state; }
int8_t* addressOfSetIsNotEmpty() { return &m_setIsNotEmpty; }
JS_EXPORT_PRIVATE void fireAllSlow(); // Call only if you've checked isWatched.
private:
void fireAllWatchpoints();
friend class InlineWatchpointSet;
SentinelLinkedList<Watchpoint, BasicRawSentinelNode<Watchpoint>> m_set;
int8_t m_state;
int8_t m_setIsNotEmpty;
};
// InlineWatchpointSet is a low-overhead, non-copyable watchpoint set in which
// it is not possible to quickly query whether it is being watched in a single
// branch. There is a fairly simple tradeoff between WatchpointSet and
// InlineWatchpointSet:
//
// Do you have to emit JIT code that rapidly tests whether the watchpoint set
// is being watched? If so, use WatchpointSet.
//
// Do you need multiple parties to have pointers to the same WatchpointSet?
// If so, use WatchpointSet.
//
// Do you have to allocate a lot of watchpoint sets? If so, use
// InlineWatchpointSet unless you answered "yes" to the previous questions.
//
// InlineWatchpointSet will use just one pointer-width word of memory unless
// you actually add watchpoints to it, in which case it internally inflates
// to a pointer to a WatchpointSet, and transfers its state to the
// WatchpointSet.
class InlineWatchpointSet {
WTF_MAKE_NONCOPYABLE(InlineWatchpointSet);
public:
InlineWatchpointSet(WatchpointState state)
: m_data(encodeState(state))
{
}
~InlineWatchpointSet()
{
if (isThin())
return;
freeFat();
}
// It is safe to call this from another thread. It may return false
// even if the set actually had been invalidated, but that ought to happen
// only in the case of races, and should be rare.
bool hasBeenInvalidated() const
{
WTF::loadLoadFence();
uintptr_t data = m_data;
if (isFat(data)) {
WTF::loadLoadFence();
return fat(data)->hasBeenInvalidated();
}
return decodeState(data) == IsInvalidated;
}
// Like hasBeenInvalidated(), may be called from another thread.
bool isStillValid() const
{
return !hasBeenInvalidated();
}
void add(Watchpoint*);
void startWatching()
{
if (isFat()) {
fat()->startWatching();
return;
}
ASSERT(decodeState(m_data) != IsInvalidated);
m_data = encodeState(IsWatched);
}
void fireAll()
{
if (isFat()) {
fat()->fireAll();
return;
}
if (decodeState(m_data) == ClearWatchpoint)
return;
m_data = encodeState(IsInvalidated);
WTF::storeStoreFence();
}
void touch()
{
if (isFat()) {
fat()->touch();
return;
}
if (decodeState(m_data) == ClearWatchpoint)
m_data = encodeState(IsWatched);
else
m_data = encodeState(IsInvalidated);
WTF::storeStoreFence();
}
private:
static const uintptr_t IsThinFlag = 1;
static const uintptr_t StateMask = 6;
static const uintptr_t StateShift = 1;
static bool isThin(uintptr_t data) { return data & IsThinFlag; }
static bool isFat(uintptr_t data) { return !isThin(data); }
static WatchpointState decodeState(uintptr_t data)
{
ASSERT(isThin(data));
return static_cast<WatchpointState>((data & StateMask) >> StateShift);
}
static uintptr_t encodeState(WatchpointState state)
{
return (state << StateShift) | IsThinFlag;
}
bool isThin() const { return isThin(m_data); }
bool isFat() const { return isFat(m_data); };
static WatchpointSet* fat(uintptr_t data)
{
return bitwise_cast<WatchpointSet*>(data);
}
WatchpointSet* fat()
{
ASSERT(isFat());
return fat(m_data);
}
const WatchpointSet* fat() const
{
ASSERT(isFat());
return fat(m_data);
}
WatchpointSet* inflate()
{
if (LIKELY(isFat()))
return fat();
return inflateSlow();
}
JS_EXPORT_PRIVATE WatchpointSet* inflateSlow();
JS_EXPORT_PRIVATE void freeFat();
uintptr_t m_data;
};
} // namespace JSC
#endif // Watchpoint_h