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229 lines
9.8 KiB
229 lines
9.8 KiB
/*
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* Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.lang.invoke;
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/**
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* <p>
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* A {@code SwitchPoint} is an object which can publish state transitions to other threads.
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* A switch point is initially in the <em>valid</em> state, but may at any time be
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* changed to the <em>invalid</em> state. Invalidation cannot be reversed.
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* A switch point can combine a <em>guarded pair</em> of method handles into a
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* <em>guarded delegator</em>.
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* The guarded delegator is a method handle which delegates to one of the old method handles.
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* The state of the switch point determines which of the two gets the delegation.
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* <p>
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* A single switch point may be used to control any number of method handles.
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* (Indirectly, therefore, it can control any number of call sites.)
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* This is done by using the single switch point as a factory for combining
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* any number of guarded method handle pairs into guarded delegators.
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* <p>
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* When a guarded delegator is created from a guarded pair, the pair
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* is wrapped in a new method handle {@code M},
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* which is permanently associated with the switch point that created it.
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* Each pair consists of a target {@code T} and a fallback {@code F}.
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* While the switch point is valid, invocations to {@code M} are delegated to {@code T}.
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* After it is invalidated, invocations are delegated to {@code F}.
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* <p>
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* Invalidation is global and immediate, as if the switch point contained a
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* volatile boolean variable consulted on every call to {@code M}.
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* The invalidation is also permanent, which means the switch point
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* can change state only once.
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* The switch point will always delegate to {@code F} after being invalidated.
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* At that point {@code guardWithTest} may ignore {@code T} and return {@code F}.
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* <p>
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* Here is an example of a switch point in action:
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* <pre>{@code
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* MethodHandle MH_strcat = MethodHandles.lookup()
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* .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class));
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* SwitchPoint spt = new SwitchPoint();
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* assert(!spt.hasBeenInvalidated());
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* // the following steps may be repeated to re-use the same switch point:
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* MethodHandle worker1 = MH_strcat;
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* MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0);
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* MethodHandle worker = spt.guardWithTest(worker1, worker2);
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* assertEquals("method", (String) worker.invokeExact("met", "hod"));
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* SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });
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* assert(spt.hasBeenInvalidated());
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* assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));
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* }</pre>
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* <p style="font-size:smaller;">
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* <em>Discussion:</em>
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* Switch points are useful without subclassing. They may also be subclassed.
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* This may be useful in order to associate application-specific invalidation logic
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* with the switch point.
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* Notice that there is no permanent association between a switch point and
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* the method handles it produces and consumes.
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* The garbage collector may collect method handles produced or consumed
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* by a switch point independently of the lifetime of the switch point itself.
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* <p style="font-size:smaller;">
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* <em>Implementation Note:</em>
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* A switch point behaves as if implemented on top of {@link MutableCallSite},
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* approximately as follows:
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* <pre>{@code
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* public class SwitchPoint {
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* private static final MethodHandle
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* K_true = MethodHandles.constant(boolean.class, true),
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* K_false = MethodHandles.constant(boolean.class, false);
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* private final MutableCallSite mcs;
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* private final MethodHandle mcsInvoker;
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* public SwitchPoint() {
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* this.mcs = new MutableCallSite(K_true);
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* this.mcsInvoker = mcs.dynamicInvoker();
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* }
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* public MethodHandle guardWithTest(
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* MethodHandle target, MethodHandle fallback) {
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* // Note: mcsInvoker is of type ()boolean.
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* // Target and fallback may take any arguments, but must have the same type.
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* return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback);
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* }
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* public static void invalidateAll(SwitchPoint[] spts) {
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* List<MutableCallSite> mcss = new ArrayList<>();
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* for (SwitchPoint spt : spts) mcss.add(spt.mcs);
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* for (MutableCallSite mcs : mcss) mcs.setTarget(K_false);
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* MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));
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* }
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* }
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* }</pre>
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* @author Remi Forax, JSR 292 EG
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*/
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public class SwitchPoint {
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private static final MethodHandle
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K_true = MethodHandles.constant(boolean.class, true),
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K_false = MethodHandles.constant(boolean.class, false);
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private final MutableCallSite mcs;
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private final MethodHandle mcsInvoker;
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/**
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* Creates a new switch point.
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*/
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public SwitchPoint() {
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this.mcs = new MutableCallSite(K_true);
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this.mcsInvoker = mcs.dynamicInvoker();
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}
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/**
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* Determines if this switch point has been invalidated yet.
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*
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* <p style="font-size:smaller;">
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* <em>Discussion:</em>
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* Because of the one-way nature of invalidation, once a switch point begins
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* to return true for {@code hasBeenInvalidated},
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* it will always do so in the future.
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* On the other hand, a valid switch point visible to other threads may
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* be invalidated at any moment, due to a request by another thread.
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* <p style="font-size:smaller;">
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* Since invalidation is a global and immediate operation,
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* the execution of this query, on a valid switchpoint,
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* must be internally sequenced with any
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* other threads that could cause invalidation.
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* This query may therefore be expensive.
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* The recommended way to build a boolean-valued method handle
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* which queries the invalidation state of a switch point {@code s} is
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* to call {@code s.guardWithTest} on
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* {@link MethodHandles#constant constant} true and false method handles.
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*
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* @return true if this switch point has been invalidated
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*/
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public boolean hasBeenInvalidated() {
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return (mcs.getTarget() != K_true);
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}
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/**
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* Returns a method handle which always delegates either to the target or the fallback.
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* The method handle will delegate to the target exactly as long as the switch point is valid.
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* After that, it will permanently delegate to the fallback.
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* <p>
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* The target and fallback must be of exactly the same method type,
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* and the resulting combined method handle will also be of this type.
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*
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* @param target the method handle selected by the switch point as long as it is valid
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* @param fallback the method handle selected by the switch point after it is invalidated
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* @return a combined method handle which always calls either the target or fallback
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* @throws NullPointerException if either argument is null
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* @throws IllegalArgumentException if the two method types do not match
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* @see MethodHandles#guardWithTest
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*/
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public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {
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if (mcs.getTarget() == K_false)
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return fallback; // already invalid
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return MethodHandles.guardWithTest(mcsInvoker, target, fallback);
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}
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/**
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* Sets all of the given switch points into the invalid state.
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* After this call executes, no thread will observe any of the
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* switch points to be in a valid state.
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* <p>
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* This operation is likely to be expensive and should be used sparingly.
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* If possible, it should be buffered for batch processing on sets of switch points.
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* <p>
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* If {@code switchPoints} contains a null element,
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* a {@code NullPointerException} will be raised.
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* In this case, some non-null elements in the array may be
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* processed before the method returns abnormally.
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* Which elements these are (if any) is implementation-dependent.
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*
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* <p style="font-size:smaller;">
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* <em>Discussion:</em>
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* For performance reasons, {@code invalidateAll} is not a virtual method
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* on a single switch point, but rather applies to a set of switch points.
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* Some implementations may incur a large fixed overhead cost
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* for processing one or more invalidation operations,
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* but a small incremental cost for each additional invalidation.
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* In any case, this operation is likely to be costly, since
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* other threads may have to be somehow interrupted
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* in order to make them notice the updated switch point state.
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* However, it may be observed that a single call to invalidate
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* several switch points has the same formal effect as many calls,
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* each on just one of the switch points.
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*
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* <p style="font-size:smaller;">
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* <em>Implementation Note:</em>
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* Simple implementations of {@code SwitchPoint} may use
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* a private {@link MutableCallSite} to publish the state of a switch point.
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* In such an implementation, the {@code invalidateAll} method can
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* simply change the call site's target, and issue one call to
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* {@linkplain MutableCallSite#syncAll synchronize} all the
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* private call sites.
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*
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* @param switchPoints an array of call sites to be synchronized
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* @throws NullPointerException if the {@code switchPoints} array reference is null
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* or the array contains a null
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*/
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public static void invalidateAll(SwitchPoint[] switchPoints) {
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if (switchPoints.length == 0) return;
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MutableCallSite[] sites = new MutableCallSite[switchPoints.length];
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for (int i = 0; i < switchPoints.length; i++) {
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SwitchPoint spt = switchPoints[i];
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if (spt == null) break; // MSC.syncAll will trigger a NPE
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sites[i] = spt.mcs;
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spt.mcs.setTarget(K_false);
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}
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MutableCallSite.syncAll(sites);
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}
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}
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