https://openjdk.java.net/ 
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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 * 2 along with this work; if not, write to the Free Software Foundation,
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package jdk.nashorn.internal.runtime;

import static jdk.nashorn.internal.codegen.CompilerConstants.staticCall;
import static jdk.nashorn.internal.codegen.CompilerConstants.virtualCall;
import static jdk.nashorn.internal.lookup.Lookup.MH;
import static jdk.nashorn.internal.runtime.UnwarrantedOptimismException.INVALID_PROGRAM_POINT;
import static jdk.nashorn.internal.runtime.linker.NashornCallSiteDescriptor.getProgramPoint;
import static jdk.nashorn.internal.runtime.logging.DebugLogger.quote;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.SwitchPoint;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.logging.Level;
import jdk.dynalink.CallSiteDescriptor;
import jdk.dynalink.DynamicLinker;
import jdk.dynalink.linker.GuardedInvocation;
import jdk.dynalink.linker.LinkRequest;
import jdk.nashorn.internal.lookup.Lookup;
import jdk.nashorn.internal.lookup.MethodHandleFactory;
import jdk.nashorn.internal.runtime.linker.NashornCallSiteDescriptor;
import jdk.nashorn.internal.runtime.logging.DebugLogger;
import jdk.nashorn.internal.runtime.logging.Loggable;
import jdk.nashorn.internal.runtime.logging.Logger;


Each context owns one of these. This is basically table of accessors for global properties. A global constant is evaluated to a MethodHandle.constant for faster access and to avoid walking to proto chain looking for it. We put a switchpoint on the global setter, which invalidates the method handle constant getters, and reverts to the standard access strategy However, there is a twist - while certain globals like "undefined" and "Math" are usually never reassigned, a global value can be reset once, and never again. This is a rather common pattern, like: x = function(something) { ... Thus everything registered as a global constant gets an extra chance. Set once, reregister the switchpoint. Set twice or more - don't try again forever, or we'd just end up relinking our way into megamorphism. Also it has to be noted that this kind of linking creates a coupling between a Global and the call sites in compiled code belonging to the Context. For this reason, the linkage becomes incorrect as soon as the Context has more than one Global. The invalidateForever() is invoked by the Context to invalidate all linkages and turn off the functionality of this object as soon as the Context's Context.newGlobal() is invoked for second time. We can extend this to ScriptObjects in general (GLOBAL_ONLY=false), which requires a receiver guard on the constant getter, but it currently leaks memory and its benefits have not yet been investigated property. As long as all Globals in a Context share the same GlobalConstants instance, we need synchronization whenever we access it. 
/**
 * Each context owns one of these. This is basically table of accessors
 * for global properties. A global constant is evaluated to a MethodHandle.constant
 * for faster access and to avoid walking to proto chain looking for it.
 *
 * We put a switchpoint on the global setter, which invalidates the
 * method handle constant getters, and reverts to the standard access strategy
 *
 * However, there is a twist - while certain globals like "undefined" and "Math"
 * are usually never reassigned, a global value can be reset once, and never again.
 * This is a rather common pattern, like:
 *
 * x = function(something) { ...
 *
 * Thus everything registered as a global constant gets an extra chance. Set once,
 * reregister the switchpoint. Set twice or more - don't try again forever, or we'd
 * just end up relinking our way into megamorphism.
 *
 * Also it has to be noted that this kind of linking creates a coupling between a Global
 * and the call sites in compiled code belonging to the Context. For this reason, the
 * linkage becomes incorrect as soon as the Context has more than one Global. The
 * {@link #invalidateForever()} is invoked by the Context to invalidate all linkages and
 * turn off the functionality of this object as soon as the Context's {@link Context#newGlobal()} is invoked
 * for second time.
 *
 * We can extend this to ScriptObjects in general (GLOBAL_ONLY=false), which requires
 * a receiver guard on the constant getter, but it currently leaks memory and its benefits
 * have not yet been investigated property.
 *
 * As long as all Globals in a Context share the same GlobalConstants instance, we need synchronization
 * whenever we access it.
 */
@Logger(name="const")
public final class GlobalConstants implements Loggable {

    
Should we only try to link globals as constants, and not generic script objects.
Script objects require a receiver guard, which is memory intensive, so this is currently
disabled. We might implement a weak reference based approach to this later.

/**
     * Should we only try to link globals as constants, and not generic script objects.
     * Script objects require a receiver guard, which is memory intensive, so this is currently
     * disabled. We might implement a weak reference based approach to this later.
     */
    public static final boolean GLOBAL_ONLY = true;

    private static final MethodHandles.Lookup LOOKUP = MethodHandles.lookup();

    private static final MethodHandle INVALIDATE_SP  = virtualCall(LOOKUP, GlobalConstants.class, "invalidateSwitchPoint", Object.class, Object.class, Access.class).methodHandle();
    private static final MethodHandle RECEIVER_GUARD = staticCall(LOOKUP, GlobalConstants.class, "receiverGuard", boolean.class, Access.class, Object.class, Object.class).methodHandle();

    
Logger for constant getters 
/** Logger for constant getters */
    private final DebugLogger log;

    
Access map for this global - associates a symbol name with an Access object, with getter
and invalidation information

/**
     * Access map for this global - associates a symbol name with an Access object, with getter
     * and invalidation information
     */
    private final Map<Object, Access> map = new HashMap<>();

    private final AtomicBoolean invalidatedForever = new AtomicBoolean(false);

    
Constructor - used only by global

Params:
  • log – logger, or null if none
/**
     * Constructor - used only by global
     * @param log logger, or null if none
     */
    public GlobalConstants(final DebugLogger log) {
        this.log = log == null ? DebugLogger.DISABLED_LOGGER : log;
    }

    @Override
    public DebugLogger getLogger() {
        return log;
    }

    @Override
    public DebugLogger initLogger(final Context context) {
        return DebugLogger.DISABLED_LOGGER;
    }

    
Information about a constant access and its potential invalidations

/**
     * Information about a constant access and its potential invalidations
     */
    private static class Access {
        
name of symbol 
/** name of symbol */
        private final String name;

        
switchpoint that invalidates the getters and setters for this access 
/** switchpoint that invalidates the getters and setters for this access */
        private SwitchPoint sp;

        
invalidation count for this access, i.e. how many times has this property been reset 
/** invalidation count for this access, i.e. how many times has this property been reset */
        private int invalidations;

        
has a guard guarding this property getter failed? 
/** has a guard guarding this property getter failed? */
        private boolean guardFailed;

        private static final int MAX_RETRIES = 2;

        private Access(final String name, final SwitchPoint sp) {
            this.name      = name;
            this.sp        = sp;
        }

        private boolean hasBeenInvalidated() {
            return sp.hasBeenInvalidated();
        }

        private boolean guardFailed() {
            return guardFailed;
        }

        private void failGuard() {
            invalidateOnce();
            guardFailed = true;
        }

        private void newSwitchPoint() {
            assert hasBeenInvalidated();
            sp = new SwitchPoint();
        }

        private void invalidate(final int count) {
            if (!sp.hasBeenInvalidated()) {
                SwitchPoint.invalidateAll(new SwitchPoint[] { sp });
                invalidations += count;
            }
        }

        
Invalidate the access, but do not contribute to the invalidation count

/**
         * Invalidate the access, but do not contribute to the invalidation count
         */
        private void invalidateUncounted() {
            invalidate(0);
        }

        
Invalidate the access, and contribute 1 to the invalidation count

/**
         * Invalidate the access, and contribute 1 to the invalidation count
         */
        private void invalidateOnce() {
            invalidate(1);
        }

        
Invalidate the access and make sure that we never try to turn this into
a MethodHandle.constant getter again

/**
         * Invalidate the access and make sure that we never try to turn this into
         * a MethodHandle.constant getter again
         */
        private void invalidateForever() {
            invalidate(MAX_RETRIES);
        }

        
Are we allowed to relink this as constant getter, even though it
it has been reset

Returns:true if we can relink as constant, one retry is allowed
/**
         * Are we allowed to relink this as constant getter, even though it
         * it has been reset
         * @return true if we can relink as constant, one retry is allowed
         */
        private boolean mayRetry() {
            return invalidations < MAX_RETRIES;
        }

        @Override
        public String toString() {
            return "[" + quote(name) + " <id=" + Debug.id(this) + "> inv#=" + invalidations + '/' + MAX_RETRIES + " sp_inv=" + sp.hasBeenInvalidated() + ']';
        }

        String getName() {
            return name;
        }

        SwitchPoint getSwitchPoint() {
            return sp;
        }
    }

    
To avoid an expensive global guard "is this the same global", similar to the
receiver guard on the ScriptObject level, we invalidate all getters once
when we switch globals. This is used from the class cache. We _can_ reuse
the same class for a new global, but the builtins and global scoped variables
will have changed.

/**
     * To avoid an expensive global guard "is this the same global", similar to the
     * receiver guard on the ScriptObject level, we invalidate all getters once
     * when we switch globals. This is used from the class cache. We _can_ reuse
     * the same class for a new global, but the builtins and global scoped variables
     * will have changed.
     */
    public void invalidateAll() {
        if (!invalidatedForever.get()) {
            log.info("New global created - invalidating all constant callsites without increasing invocation count.");
            synchronized (this) {
                for (final Access acc : map.values()) {
                    acc.invalidateUncounted();
                }
            }
        }
    }

    
To avoid an expensive global guard "is this the same global", similar to the
receiver guard on the ScriptObject level, we invalidate all getters when the
second Global is created by the Context owning this instance. After this
method is invoked, this GlobalConstants instance will both invalidate all the
switch points it produced, and it will stop handing out new method handles
altogether.

/**
     * To avoid an expensive global guard "is this the same global", similar to the
     * receiver guard on the ScriptObject level, we invalidate all getters when the
     * second Global is created by the Context owning this instance. After this
     * method is invoked, this GlobalConstants instance will both invalidate all the
     * switch points it produced, and it will stop handing out new method handles
     * altogether.
     */
    public void invalidateForever() {
        if (invalidatedForever.compareAndSet(false, true)) {
            log.info("New global created - invalidating all constant callsites.");
            synchronized (this) {
                for (final Access acc : map.values()) {
                    acc.invalidateForever();
                }
                map.clear();
            }
        }
    }

    
Invalidate the switchpoint of an access - we have written to
the property

Params:
  • obj – receiver
  • acc – access
Returns:receiver, so this can be used as param filter
/**
     * Invalidate the switchpoint of an access - we have written to
     * the property
     *
     * @param obj receiver
     * @param acc access
     *
     * @return receiver, so this can be used as param filter
     */
    @SuppressWarnings("unused")
    private synchronized Object invalidateSwitchPoint(final Object obj, final Access acc) {
        if (log.isEnabled()) {
            log.info("*** Invalidating switchpoint " + acc.getSwitchPoint() + " for receiver=" + obj + " access=" + acc);
        }
        acc.invalidateOnce();
        if (acc.mayRetry()) {
            if (log.isEnabled()) {
                log.info("Retry is allowed for " + acc + "... Creating a new switchpoint.");
            }
            acc.newSwitchPoint();
        } else {
            if (log.isEnabled()) {
                log.info("This was the last time I allowed " + quote(acc.getName()) + " to relink as constant.");
            }
        }
        return obj;
    }

    private Access getOrCreateSwitchPoint(final String name) {
        Access acc = map.get(name);
        if (acc != null) {
            return acc;
        }
        final SwitchPoint sp = new SwitchPoint();
        map.put(name, acc = new Access(name, sp));
        return acc;
    }

    
Called from script object on property deletion to erase a property
that might be linked as MethodHandle.constant and force relink

Params:
  • name – name of property
/**
     * Called from script object on property deletion to erase a property
     * that might be linked as MethodHandle.constant and force relink
     * @param name name of property
     */
    void delete(final Object name) {
        if (!invalidatedForever.get()) {
            synchronized (this) {
                final Access acc = map.get(name);
                if (acc != null) {
                    acc.invalidateForever();
                }
            }
        }
    }

    
Receiver guard is used if we extend the global constants to script objects in general.
As the property can have different values in different script objects, while Global is
by definition a singleton, we need this for ScriptObject constants (currently disabled)
TODO: Note - this seems to cause memory leaks. Use weak references? But what is leaking seems
to be the Access objects, which isn't the case for Globals. Weird.

Params:
  • acc –            access
  • boundReceiver –  the receiver bound to the callsite
  • receiver –       the receiver to check against
Returns:true if this receiver is still the one we bound to the callsite
/**
     * Receiver guard is used if we extend the global constants to script objects in general.
     * As the property can have different values in different script objects, while Global is
     * by definition a singleton, we need this for ScriptObject constants (currently disabled)
     *
     * TODO: Note - this seems to cause memory leaks. Use weak references? But what is leaking seems
     * to be the Access objects, which isn't the case for Globals. Weird.
     *
     * @param acc            access
     * @param boundReceiver  the receiver bound to the callsite
     * @param receiver       the receiver to check against
     *
     * @return true if this receiver is still the one we bound to the callsite
     */
    @SuppressWarnings("unused")
    private static boolean receiverGuard(final Access acc, final Object boundReceiver, final Object receiver) {
        final boolean id = receiver == boundReceiver;
        if (!id) {
            acc.failGuard();
        }
        return id;
    }

    private static boolean isGlobalSetter(final ScriptObject receiver, final FindProperty find) {
        if (find == null) {
            return receiver.isScope();
        }
        return find.getOwner().isGlobal();
    }

    
Augment a setter with switchpoint for invalidating its getters, should the setter be called

Params:
  • find –    property lookup
  • inv –     normal guarded invocation for this setter, as computed by the ScriptObject linker
  • desc –    callsite descriptor
  • request – link request
Returns:null if failed to set up constant linkage
/**
     * Augment a setter with switchpoint for invalidating its getters, should the setter be called
     *
     * @param find    property lookup
     * @param inv     normal guarded invocation for this setter, as computed by the ScriptObject linker
     * @param desc    callsite descriptor
     * @param request link request
     *
     * @return null if failed to set up constant linkage
     */
    GuardedInvocation findSetMethod(final FindProperty find, final ScriptObject receiver, final GuardedInvocation inv, final CallSiteDescriptor desc, final LinkRequest request) {
        if (invalidatedForever.get() || (GLOBAL_ONLY && !isGlobalSetter(receiver, find))) {
            return null;
        }

        final String name = NashornCallSiteDescriptor.getOperand(desc);

        synchronized (this) {
            final Access acc  = getOrCreateSwitchPoint(name);

            if (log.isEnabled()) {
                log.fine("Trying to link constant SETTER ", acc);
            }

            if (!acc.mayRetry() || invalidatedForever.get()) {
                if (log.isEnabled()) {
                    log.fine("*** SET: Giving up on " + quote(name) + " - retry count has exceeded " + DynamicLinker.getLinkedCallSiteLocation());
                }
                return null;
            }

            if (acc.hasBeenInvalidated()) {
                log.info("New chance for " + acc);
                acc.newSwitchPoint();
            }

            assert !acc.hasBeenInvalidated();

            // if we haven't given up on this symbol, add a switchpoint invalidation filter to the receiver parameter
            final MethodHandle target           = inv.getInvocation();
            final Class<?>     receiverType     = target.type().parameterType(0);
            final MethodHandle boundInvalidator = MH.bindTo(INVALIDATE_SP,  this);
            final MethodHandle invalidator      = MH.asType(boundInvalidator, boundInvalidator.type().changeParameterType(0, receiverType).changeReturnType(receiverType));
            final MethodHandle mh               = MH.filterArguments(inv.getInvocation(), 0, MH.insertArguments(invalidator, 1, acc));

            assert inv.getSwitchPoints() == null : Arrays.asList(inv.getSwitchPoints());
            log.info("Linked setter " + quote(name) + " " + acc.getSwitchPoint());
            return new GuardedInvocation(mh, inv.getGuard(), acc.getSwitchPoint(), inv.getException());
        }
    }

    
Try to reuse constant method handles for getters

Params:
  • c – constant value
Returns:method handle (with dummy receiver) that returns this constant
/**
     * Try to reuse constant method handles for getters
     * @param c constant value
     * @return method handle (with dummy receiver) that returns this constant
     */
    public static MethodHandle staticConstantGetter(final Object c) {
        return MH.dropArguments(JSType.unboxConstant(c), 0, Object.class);
    }

    private MethodHandle constantGetter(final Object c) {
        final MethodHandle mh = staticConstantGetter(c);
        if (log.isEnabled()) {
            return MethodHandleFactory.addDebugPrintout(log, Level.FINEST, mh, "getting as constant");
        }
        return mh;
    }

    
Try to turn a getter into a MethodHandle.constant, if possible

Params:
  • find –      property lookup
  • receiver –  receiver
  • desc –      callsite descriptor
Returns:resulting getter, or null if failed to create constant
/**
     * Try to turn a getter into a MethodHandle.constant, if possible
     *
     * @param find      property lookup
     * @param receiver  receiver
     * @param desc      callsite descriptor
     *
     * @return resulting getter, or null if failed to create constant
     */
    GuardedInvocation findGetMethod(final FindProperty find, final ScriptObject receiver, final CallSiteDescriptor desc) {
        // Only use constant getter for fast scope access, because the receiver may change between invocations
        // for slow-scope and non-scope callsites.
        // Also return null for user accessor properties as they may have side effects.
        if (invalidatedForever.get() || !NashornCallSiteDescriptor.isFastScope(desc)
                || (GLOBAL_ONLY && !find.getOwner().isGlobal())
                || find.getProperty() instanceof UserAccessorProperty) {
            return null;
        }

        final boolean  isOptimistic = NashornCallSiteDescriptor.isOptimistic(desc);
        final int      programPoint = isOptimistic ? getProgramPoint(desc) : INVALID_PROGRAM_POINT;
        final Class<?> retType      = desc.getMethodType().returnType();
        final String   name         = NashornCallSiteDescriptor.getOperand(desc);

        synchronized (this) {
            final Access acc = getOrCreateSwitchPoint(name);

            log.fine("Starting to look up object value " + name);
            final Object c = find.getObjectValue();

            if (log.isEnabled()) {
                log.fine("Trying to link constant GETTER " + acc + " value = " + c);
            }

            if (acc.hasBeenInvalidated() || acc.guardFailed() || invalidatedForever.get()) {
                if (log.isEnabled()) {
                    log.info("*** GET: Giving up on " + quote(name) + " - retry count has exceeded " + DynamicLinker.getLinkedCallSiteLocation());
                }
                return null;
            }

            final MethodHandle cmh = constantGetter(c);

            MethodHandle mh;
            MethodHandle guard;

            if (isOptimistic) {
                if (JSType.getAccessorTypeIndex(cmh.type().returnType()) <= JSType.getAccessorTypeIndex(retType)) {
                    //widen return type - this is pessimistic, so it will always work
                    mh = MH.asType(cmh, cmh.type().changeReturnType(retType));
                } else {
                    //immediately invalidate - we asked for a too wide constant as a narrower one
                    mh = MH.dropArguments(MH.insertArguments(JSType.THROW_UNWARRANTED.methodHandle(), 0, c, programPoint), 0, Object.class);
                }
            } else {
                //pessimistic return type filter
                mh = Lookup.filterReturnType(cmh, retType);
            }

            if (find.getOwner().isGlobal()) {
                guard = null;
            } else {
                guard = MH.insertArguments(RECEIVER_GUARD, 0, acc, receiver);
            }

            if (log.isEnabled()) {
                log.info("Linked getter " + quote(name) + " as MethodHandle.constant() -> " + c + " " + acc.getSwitchPoint());
                mh = MethodHandleFactory.addDebugPrintout(log, Level.FINE, mh, "get const " + acc);
            }

            return new GuardedInvocation(mh, guard, acc.getSwitchPoint(), null);
        }
    }
}