1 /*
  2  * Copyright (c) 2008, 2023, Oracle and/or its affiliates. All rights reserved.
  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  4  *
  5  * This code is free software; you can redistribute it and/or modify it
  6  * under the terms of the GNU General Public License version 2 only, as
  7  * published by the Free Software Foundation.  Oracle designates this
  8  * particular file as subject to the "Classpath" exception as provided
  9  * by Oracle in the LICENSE file that accompanied this code.
 10  *
 11  * This code is distributed in the hope that it will be useful, but WITHOUT
 12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 14  * version 2 for more details (a copy is included in the LICENSE file that
 15  * accompanied this code).
 16  *
 17  * You should have received a copy of the GNU General Public License version
 18  * 2 along with this work; if not, write to the Free Software Foundation,
 19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 20  *
 21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 22  * or visit www.oracle.com if you need additional information or have any
 23  * questions.
 24  */
 25 
 26 package java.lang.invoke;
 27 
 28 import jdk.internal.misc.VM;
 29 import jdk.internal.ref.CleanerFactory;
 30 import sun.invoke.util.Wrapper;
 31 
 32 import java.lang.invoke.MethodHandles.Lookup;
 33 import java.lang.reflect.Field;
 34 
 35 import static java.lang.invoke.MethodHandleNatives.Constants.*;
 36 import static java.lang.invoke.MethodHandleStatics.TRACE_METHOD_LINKAGE;
 37 import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
 38 
 39 /**
 40  * The JVM interface for the method handles package is all here.
 41  * This is an interface internal and private to an implementation of JSR 292.
 42  * <em>This class is not part of the JSR 292 standard.</em>
 43  * @author jrose
 44  */
 45 class MethodHandleNatives {
 46 
 47     private MethodHandleNatives() { } // static only
 48 
 49     /// MemberName support
 50 
 51     static native void init(MemberName self, Object ref);
 52     static native void expand(MemberName self);
 53     static native MemberName resolve(MemberName self, Class<?> caller, int lookupMode,
 54             boolean speculativeResolve) throws LinkageError, ClassNotFoundException;
 55 
 56     /// Field layout queries parallel to jdk.internal.misc.Unsafe:
 57     static native long objectFieldOffset(MemberName self);  // e.g., returns vmindex
 58     static native long staticFieldOffset(MemberName self);  // e.g., returns vmindex
 59     static native Object staticFieldBase(MemberName self);  // e.g., returns clazz
 60     static native Object getMemberVMInfo(MemberName self);  // returns {vmindex,vmtarget}
 61 
 62     /// CallSite support
 63 
 64     /** Tell the JVM that we need to change the target of a CallSite. */
 65     static native void setCallSiteTargetNormal(CallSite site, MethodHandle target);
 66     static native void setCallSiteTargetVolatile(CallSite site, MethodHandle target);
 67 
 68     static native void copyOutBootstrapArguments(Class<?> caller, int[] indexInfo,
 69                                                  int start, int end,
 70                                                  Object[] buf, int pos,
 71                                                  boolean resolve,
 72                                                  Object ifNotAvailable);
 73 
 74     /** Represents a context to track nmethod dependencies on CallSite instance target. */
 75     static class CallSiteContext implements Runnable {
 76         //@Injected JVM_nmethodBucket* vmdependencies;
 77         //@Injected jlong last_cleanup;
 78 
 79         static CallSiteContext make(CallSite cs) {
 80             final CallSiteContext newContext = new CallSiteContext();
 81             // CallSite instance is tracked by a Cleanable which clears native
 82             // structures allocated for CallSite context. Though the CallSite can
 83             // become unreachable, its Context is retained by the Cleanable instance
 84             // (which is referenced from Cleaner instance which is referenced from
 85             // CleanerFactory class) until cleanup is performed.
 86             CleanerFactory.cleaner().register(cs, newContext);
 87             return newContext;
 88         }
 89 
 90         @Override
 91         public void run() {
 92             MethodHandleNatives.clearCallSiteContext(this);
 93         }
 94     }
 95 
 96     /** Invalidate all recorded nmethods. */
 97     private static native void clearCallSiteContext(CallSiteContext context);
 98 
 99     private static native void registerNatives();
100     static {
101         registerNatives();
102     }
103 
104     /**
105      * Compile-time constants go here. This collection exists not only for
106      * reference from clients, but also for ensuring the VM and JDK agree on the
107      * values of these constants (see {@link #verifyConstants()}).
108      */
109     static class Constants {
110         Constants() { } // static only
111 
112         static final int
113             MN_IS_METHOD           = 0x00010000, // method (not constructor)
114             MN_IS_CONSTRUCTOR      = 0x00020000, // constructor
115             MN_IS_FIELD            = 0x00040000, // field
116             MN_IS_TYPE             = 0x00080000, // nested type
117             MN_CALLER_SENSITIVE    = 0x00100000, // @CallerSensitive annotation detected
118             MN_TRUSTED_FINAL       = 0x00200000, // trusted final field
119             MN_REFERENCE_KIND_SHIFT = 24, // refKind
120             MN_REFERENCE_KIND_MASK = 0x0F000000 >> MN_REFERENCE_KIND_SHIFT;
121 
122         /**
123          * Constant pool reference-kind codes, as used by CONSTANT_MethodHandle CP entries.
124          */
125         static final byte
126             REF_NONE                    = 0,  // null value
127             REF_getField                = 1,
128             REF_getStatic               = 2,
129             REF_putField                = 3,
130             REF_putStatic               = 4,
131             REF_invokeVirtual           = 5,
132             REF_invokeStatic            = 6,
133             REF_invokeSpecial           = 7,
134             REF_newInvokeSpecial        = 8,
135             REF_invokeInterface         = 9,
136             REF_LIMIT                  = 10;
137 
138         /**
139          * Flags for Lookup.ClassOptions
140          */
141         static final int
142             NESTMATE_CLASS            = 0x00000001,
143             HIDDEN_CLASS              = 0x00000002,
144             STRONG_LOADER_LINK        = 0x00000004,
145             ACCESS_VM_ANNOTATIONS     = 0x00000008;
146 
147         /**
148          * Lookup modes
149          */
150         static final int
151             LM_MODULE        = Lookup.MODULE,
152             LM_UNCONDITIONAL = Lookup.UNCONDITIONAL,
153             LM_TRUSTED       = -1;
154 
155     }
156 
157     static boolean refKindIsValid(int refKind) {
158         return (refKind > REF_NONE && refKind < REF_LIMIT);
159     }
160     static boolean refKindIsField(byte refKind) {
161         assert(refKindIsValid(refKind));
162         return (refKind <= REF_putStatic);
163     }
164     static boolean refKindIsGetter(byte refKind) {
165         assert(refKindIsValid(refKind));
166         return (refKind <= REF_getStatic);
167     }
168     static boolean refKindIsSetter(byte refKind) {
169         return refKindIsField(refKind) && !refKindIsGetter(refKind);
170     }
171     static boolean refKindIsMethod(byte refKind) {
172         return !refKindIsField(refKind) && (refKind != REF_newInvokeSpecial);
173     }
174     static boolean refKindIsConstructor(byte refKind) {
175         return (refKind == REF_newInvokeSpecial);
176     }
177     static boolean refKindHasReceiver(byte refKind) {
178         assert(refKindIsValid(refKind));
179         return (refKind & 1) != 0;
180     }
181     static boolean refKindIsStatic(byte refKind) {
182         return !refKindHasReceiver(refKind) && (refKind != REF_newInvokeSpecial);
183     }
184     static boolean refKindDoesDispatch(byte refKind) {
185         assert(refKindIsValid(refKind));
186         return (refKind == REF_invokeVirtual ||
187                 refKind == REF_invokeInterface);
188     }
189     static {
190         final int HR_MASK = ((1 << REF_getField) |
191                              (1 << REF_putField) |
192                              (1 << REF_invokeVirtual) |
193                              (1 << REF_invokeSpecial) |
194                              (1 << REF_invokeInterface)
195                             );
196         for (byte refKind = REF_NONE+1; refKind < REF_LIMIT; refKind++) {
197             assert(refKindHasReceiver(refKind) == (((1<<refKind) & HR_MASK) != 0)) : refKind;
198         }
199     }
200     static String refKindName(byte refKind) {
201         assert(refKindIsValid(refKind));
202         return switch (refKind) {
203             case REF_getField         -> "getField";
204             case REF_getStatic        -> "getStatic";
205             case REF_putField         -> "putField";
206             case REF_putStatic        -> "putStatic";
207             case REF_invokeVirtual    -> "invokeVirtual";
208             case REF_invokeStatic     -> "invokeStatic";
209             case REF_invokeSpecial    -> "invokeSpecial";
210             case REF_newInvokeSpecial -> "newInvokeSpecial";
211             case REF_invokeInterface  -> "invokeInterface";
212             default -> "REF_???";
213         };
214     }
215 
216     private static native int getNamedCon(int which, Object[] name);
217     static boolean verifyConstants() {
218         Object[] box = { null };
219         for (int i = 0; ; i++) {
220             box[0] = null;
221             int vmval = getNamedCon(i, box);
222             if (box[0] == null)  break;
223             String name = (String) box[0];
224             try {
225                 Field con = Constants.class.getDeclaredField(name);
226                 int jval = con.getInt(null);
227                 if (jval == vmval)  continue;
228                 String err = (name+": JVM has "+vmval+" while Java has "+jval);
229                 if (name.equals("CONV_OP_LIMIT")) {
230                     System.err.println("warning: "+err);
231                     continue;
232                 }
233                 throw new InternalError(err);
234             } catch (NoSuchFieldException | IllegalAccessException ex) {
235                 String err = (name+": JVM has "+vmval+" which Java does not define");
236                 // ignore exotic ops the JVM cares about; we just won't issue them
237                 //System.err.println("warning: "+err);
238                 continue;
239             }
240         }
241         return true;
242     }
243     static {
244         VM.setJavaLangInvokeInited();
245         assert(verifyConstants());
246     }
247 
248     // Up-calls from the JVM.
249     // These must NOT be public.
250 
251     /**
252      * The JVM is linking an invokedynamic instruction.  Create a reified call site for it.
253      */
254     static MemberName linkCallSite(Object callerObj,
255                                    Object bootstrapMethodObj,
256                                    Object nameObj, Object typeObj,
257                                    Object staticArguments,
258                                    Object[] appendixResult) {
259         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
260         Class<?> caller = (Class<?>)callerObj;
261         String name = nameObj.toString().intern();
262         MethodType type = (MethodType)typeObj;
263         if (!TRACE_METHOD_LINKAGE)
264             return linkCallSiteImpl(caller, bootstrapMethod, name, type,
265                                     staticArguments, appendixResult);
266         return linkCallSiteTracing(caller, bootstrapMethod, name, type,
267                                    staticArguments, appendixResult);
268     }
269     static MemberName linkCallSiteImpl(Class<?> caller,
270                                        MethodHandle bootstrapMethod,
271                                        String name, MethodType type,
272                                        Object staticArguments,
273                                        Object[] appendixResult) {
274         CallSite callSite = CallSite.makeSite(bootstrapMethod,
275                                               name,
276                                               type,
277                                               staticArguments,
278                                               caller);
279         if (TRACE_METHOD_LINKAGE) {
280             MethodHandle target = callSite.getTarget();
281             System.out.println("linkCallSite target class => " + target.getClass().getName());
282             System.out.println("linkCallSite target => " + target.debugString(0));
283         }
284 
285         if (callSite instanceof ConstantCallSite) {
286             appendixResult[0] = callSite.dynamicInvoker();
287             return Invokers.linkToTargetMethod(type);
288         } else {
289             appendixResult[0] = callSite;
290             return Invokers.linkToCallSiteMethod(type);
291         }
292     }
293     // Tracing logic:
294     static MemberName linkCallSiteTracing(Class<?> caller,
295                                           MethodHandle bootstrapMethod,
296                                           String name, MethodType type,
297                                           Object staticArguments,
298                                           Object[] appendixResult) {
299         Object bsmReference = bootstrapMethod.internalMemberName();
300         if (bsmReference == null)  bsmReference = bootstrapMethod;
301         String staticArglist = staticArglistForTrace(staticArguments);
302         System.out.println("linkCallSite "+getCallerInfo(caller)+" "+
303                            bsmReference+" "+
304                            name+type+"/"+staticArglist);
305         try {
306             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
307                                               staticArguments, appendixResult);
308             System.out.println("linkCallSite linkage => "+res+" + "+appendixResult[0]);
309             return res;
310         } catch (Throwable ex) {
311             ex.printStackTrace(); // print now in case exception is swallowed
312             System.out.println("linkCallSite => throw "+ex);
313             throw ex;
314         }
315     }
316 
317     /**
318      * Return a human-readable description of the caller. Something like
319      * "java.base/java.security.Security.<clinit>(Security.java:82)"
320      */
321     private static String getCallerInfo(Class<?> caller) {
322         for (StackTraceElement e : Thread.currentThread().getStackTrace()) {
323             if (e.getClassName().equals(caller.getName())) {
324                 return e.toString();
325             }
326         }
327         // fallback if the caller is somehow missing from the stack.
328         return caller.getName();
329     }
330 
331     // this implements the upcall from the JVM, MethodHandleNatives.linkDynamicConstant:
332     static Object linkDynamicConstant(Object callerObj,
333                                       Object bootstrapMethodObj,
334                                       Object nameObj, Object typeObj,
335                                       Object staticArguments) {
336         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
337         Class<?> caller = (Class<?>)callerObj;
338         String name = nameObj.toString().intern();
339         Class<?> type = (Class<?>)typeObj;
340         if (!TRACE_METHOD_LINKAGE)
341             return linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
342         return linkDynamicConstantTracing(caller, bootstrapMethod, name, type, staticArguments);
343     }
344 
345     static Object linkDynamicConstantImpl(Class<?> caller,
346                                           MethodHandle bootstrapMethod,
347                                           String name, Class<?> type,
348                                           Object staticArguments) {
349         return ConstantBootstraps.makeConstant(bootstrapMethod, name, type, staticArguments, caller);
350     }
351 
352     private static String staticArglistForTrace(Object staticArguments) {
353         if (staticArguments instanceof Object[] array)
354             return "BSA="+java.util.Arrays.asList(array);
355         if (staticArguments instanceof int[] array)
356             return "BSA@"+java.util.Arrays.toString(array);
357         if (staticArguments == null)
358             return "BSA0=null";
359         return "BSA1="+staticArguments;
360     }
361 
362     // Tracing logic:
363     static Object linkDynamicConstantTracing(Class<?> caller,
364                                              MethodHandle bootstrapMethod,
365                                              String name, Class<?> type,
366                                              Object staticArguments) {
367         Object bsmReference = bootstrapMethod.internalMemberName();
368         if (bsmReference == null)  bsmReference = bootstrapMethod;
369         String staticArglist = staticArglistForTrace(staticArguments);
370         System.out.println("linkDynamicConstant "+caller.getName()+" "+
371                            bsmReference+" "+
372                            name+type+"/"+staticArglist);
373         try {
374             Object res = linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
375             System.out.println("linkDynamicConstantImpl => "+res);
376             return res;
377         } catch (Throwable ex) {
378             ex.printStackTrace(); // print now in case exception is swallowed
379             System.out.println("linkDynamicConstant => throw "+ex);
380             throw ex;
381         }
382     }
383 
384     /** The JVM is requesting pull-mode bootstrap when it provides
385      *  a tuple of the form int[]{ argc, vmindex }.
386      *  The BSM is expected to call back to the JVM using the caller
387      *  class and vmindex to resolve the static arguments.
388      */
389     static boolean staticArgumentsPulled(Object staticArguments) {
390         return staticArguments instanceof int[];
391     }
392 
393     /** A BSM runs in pull-mode if and only if its sole arguments
394      * are (Lookup, BootstrapCallInfo), or can be converted pairwise
395      * to those types, and it is not of variable arity.
396      * Excluding error cases, we can just test that the arity is a constant 2.
397      *
398      * NOTE: This method currently returns false, since pulling is not currently
399      * exposed to a BSM. When pull mode is supported the method block will be
400      * replaced with currently commented out code.
401      */
402     static boolean isPullModeBSM(MethodHandle bsm) {
403         return false;
404 //        return bsm.type().parameterCount() == 2 && !bsm.isVarargsCollector();
405     }
406 
407     /**
408      * The JVM wants a pointer to a MethodType.  Oblige it by finding or creating one.
409      */
410     static MethodType findMethodHandleType(Class<?> rtype, Class<?>[] ptypes) {
411         return MethodType.methodType(rtype, ptypes, true);
412     }
413 
414     /**
415      * The JVM wants to link a call site that requires a dynamic type check.
416      * Name is a type-checking invoker, invokeExact or invoke.
417      * Return a JVM method (MemberName) to handle the invoking.
418      * The method assumes the following arguments on the stack:
419      * 0: the method handle being invoked
420      * 1-N: the arguments to the method handle invocation
421      * N+1: an optional, implicitly added argument (typically the given MethodType)
422      * <p>
423      * The nominal method at such a call site is an instance of
424      * a signature-polymorphic method (see @PolymorphicSignature).
425      * Such method instances are user-visible entities which are
426      * "split" from the generic placeholder method in {@code MethodHandle}.
427      * (Note that the placeholder method is not identical with any of
428      * its instances.  If invoked reflectively, is guaranteed to throw an
429      * {@code UnsupportedOperationException}.)
430      * If the signature-polymorphic method instance is ever reified,
431      * it appears as a "copy" of the original placeholder
432      * (a native final member of {@code MethodHandle}) except
433      * that its type descriptor has shape required by the instance,
434      * and the method instance is <em>not</em> varargs.
435      * The method instance is also marked synthetic, since the
436      * method (by definition) does not appear in Java source code.
437      * <p>
438      * The JVM is allowed to reify this method as instance metadata.
439      * For example, {@code invokeBasic} is always reified.
440      * But the JVM may instead call {@code linkMethod}.
441      * If the result is an * ordered pair of a {@code (method, appendix)},
442      * the method gets all the arguments (0..N inclusive)
443      * plus the appendix (N+1), and uses the appendix to complete the call.
444      * In this way, one reusable method (called a "linker method")
445      * can perform the function of any number of polymorphic instance
446      * methods.
447      * <p>
448      * Linker methods are allowed to be weakly typed, with any or
449      * all references rewritten to {@code Object} and any primitives
450      * (except {@code long}/{@code float}/{@code double})
451      * rewritten to {@code int}.
452      * A linker method is trusted to return a strongly typed result,
453      * according to the specific method type descriptor of the
454      * signature-polymorphic instance it is emulating.
455      * This can involve (as necessary) a dynamic check using
456      * data extracted from the appendix argument.
457      * <p>
458      * The JVM does not inspect the appendix, other than to pass
459      * it verbatim to the linker method at every call.
460      * This means that the JDK runtime has wide latitude
461      * for choosing the shape of each linker method and its
462      * corresponding appendix.
463      * Linker methods should be generated from {@code LambdaForm}s
464      * so that they do not become visible on stack traces.
465      * <p>
466      * The {@code linkMethod} call is free to omit the appendix
467      * (returning null) and instead emulate the required function
468      * completely in the linker method.
469      * As a corner case, if N==255, no appendix is possible.
470      * In this case, the method returned must be custom-generated to
471      * perform any needed type checking.
472      * <p>
473      * If the JVM does not reify a method at a call site, but instead
474      * calls {@code linkMethod}, the corresponding call represented
475      * in the bytecodes may mention a valid method which is not
476      * representable with a {@code MemberName}.
477      * Therefore, use cases for {@code linkMethod} tend to correspond to
478      * special cases in reflective code such as {@code findVirtual}
479      * or {@code revealDirect}.
480      */
481     static MemberName linkMethod(Class<?> callerClass, int refKind,
482                                  Class<?> defc, String name, Object type,
483                                  Object[] appendixResult) {
484         if (!TRACE_METHOD_LINKAGE)
485             return linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
486         return linkMethodTracing(callerClass, refKind, defc, name, type, appendixResult);
487     }
488     static MemberName linkMethodImpl(Class<?> callerClass, int refKind,
489                                      Class<?> defc, String name, Object type,
490                                      Object[] appendixResult) {
491         try {
492             if (refKind == REF_invokeVirtual) {
493                 if (defc == MethodHandle.class) {
494                     return Invokers.methodHandleInvokeLinkerMethod(
495                             name, fixMethodType(callerClass, type), appendixResult);
496                 } else if (defc == VarHandle.class) {
497                     return varHandleOperationLinkerMethod(
498                             name, fixMethodType(callerClass, type), appendixResult);
499                 }
500             }
501         } catch (Error e) {
502             // Pass through an Error, including say StackOverflowError or
503             // OutOfMemoryError
504             throw e;
505         } catch (Throwable ex) {
506             // Wrap anything else in LinkageError
507             throw new LinkageError(ex.getMessage(), ex);
508         }
509         throw new LinkageError("no such method "+defc.getName()+"."+name+type);
510     }
511     private static MethodType fixMethodType(Class<?> callerClass, Object type) {
512         if (type instanceof MethodType mt)
513             return mt;
514         else
515             return MethodType.fromDescriptor((String)type, callerClass.getClassLoader());
516     }
517     // Tracing logic:
518     static MemberName linkMethodTracing(Class<?> callerClass, int refKind,
519                                         Class<?> defc, String name, Object type,
520                                         Object[] appendixResult) {
521         System.out.println("linkMethod "+defc.getName()+"."+
522                            name+type+"/"+Integer.toHexString(refKind));
523         try {
524             MemberName res = linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
525             System.out.println("linkMethod => "+res+" + "+appendixResult[0]);
526             return res;
527         } catch (Throwable ex) {
528             System.out.println("linkMethod => throw "+ex);
529             throw ex;
530         }
531     }
532 
533     /**
534      * Obtain the method to link to the VarHandle operation.
535      * This method is located here and not in Invokers to avoid
536      * initializing that and other classes early on in VM bootup.
537      */
538     private static MemberName varHandleOperationLinkerMethod(String name,
539                                                              MethodType mtype,
540                                                              Object[] appendixResult) {
541         // Get the signature method type
542         final MethodType sigType = mtype.basicType();
543 
544         // Get the access kind from the method name
545         VarHandle.AccessMode ak;
546         try {
547             ak = VarHandle.AccessMode.valueFromMethodName(name);
548         } catch (IllegalArgumentException e) {
549             throw MethodHandleStatics.newInternalError(e);
550         }
551 
552         // Create the appendix descriptor constant
553         VarHandle.AccessDescriptor ad = new VarHandle.AccessDescriptor(mtype, ak.at.ordinal(), ak.ordinal());
554         appendixResult[0] = ad;
555 
556         if (MethodHandleStatics.VAR_HANDLE_GUARDS) {
557             // If not polymorphic in the return type, such as the compareAndSet
558             // methods that return boolean
559             Class<?> guardReturnType = sigType.returnType();
560             if (ak.at.isMonomorphicInReturnType) {
561                 if (ak.at.returnType != mtype.returnType()) {
562                     // The caller contains a different return type than that
563                     // defined by the method
564                     throw newNoSuchMethodErrorOnVarHandle(name, mtype);
565                 }
566                 // Adjust the return type of the signature method type
567                 guardReturnType = ak.at.returnType;
568             }
569 
570             // Get the guard method type for linking
571             final Class<?>[] guardParams = new Class<?>[sigType.parameterCount() + 2];
572             // VarHandle at start
573             guardParams[0] = VarHandle.class;
574             for (int i = 0; i < sigType.parameterCount(); i++) {
575                 guardParams[i + 1] = sigType.parameterType(i);
576             }
577             // Access descriptor at end
578             guardParams[guardParams.length - 1] = VarHandle.AccessDescriptor.class;
579             MethodType guardType = MethodType.methodType(guardReturnType, guardParams, true);
580 
581             MemberName linker = new MemberName(
582                     VarHandleGuards.class, getVarHandleGuardMethodName(guardType),
583                     guardType, REF_invokeStatic);
584 
585             linker = MemberName.getFactory().resolveOrNull(REF_invokeStatic, linker,
586                                                            VarHandleGuards.class, LM_TRUSTED);
587             if (linker != null) {
588                 return linker;
589             }
590             // Fall back to lambda form linkage if guard method is not available
591             // TODO Optionally log fallback ?
592         }
593         return Invokers.varHandleInvokeLinkerMethod(mtype);
594     }
595     static String getVarHandleGuardMethodName(MethodType guardType) {
596         String prefix = "guard_";
597         StringBuilder sb = new StringBuilder(prefix.length() + guardType.parameterCount());
598 
599         sb.append(prefix);
600         for (int i = 1; i < guardType.parameterCount() - 1; i++) {
601             Class<?> pt = guardType.parameterType(i);
602             sb.append(getCharType(pt));
603         }
604         sb.append('_').append(getCharType(guardType.returnType()));
605         return sb.toString();
606     }
607     static char getCharType(Class<?> pt) {
608         return Wrapper.forBasicType(pt).basicTypeChar();
609     }
610     static NoSuchMethodError newNoSuchMethodErrorOnVarHandle(String name, MethodType mtype) {
611         return new NoSuchMethodError("VarHandle." + name + mtype);
612     }
613 
614     /**
615      * The JVM is resolving a CONSTANT_MethodHandle CP entry.  And it wants our help.
616      * It will make an up-call to this method.  (Do not change the name or signature.)
617      * The type argument is a Class for field requests and a MethodType for non-fields.
618      * <p>
619      * Recent versions of the JVM may also pass a resolved MemberName for the type.
620      * In that case, the name is ignored and may be null.
621      */
622     static MethodHandle linkMethodHandleConstant(Class<?> callerClass, int refKind,
623                                                  Class<?> defc, String name, Object type) {
624         try {
625             Lookup lookup = IMPL_LOOKUP.in(callerClass);
626             assert(refKindIsValid(refKind));
627             return lookup.linkMethodHandleConstant((byte) refKind, defc, name, type);
628         } catch (ReflectiveOperationException ex) {
629             throw mapLookupExceptionToError(ex);
630         }
631     }
632 
633     /**
634      * Map a reflective exception to a linkage error.
635      */
636     static LinkageError mapLookupExceptionToError(ReflectiveOperationException ex) {
637         LinkageError err;
638         if (ex instanceof IllegalAccessException) {
639             Throwable cause = ex.getCause();
640             if (cause instanceof AbstractMethodError ame) {
641                 return ame;
642             } else {
643                 err = new IllegalAccessError(ex.getMessage());
644             }
645         } else if (ex instanceof NoSuchMethodException) {
646             err = new NoSuchMethodError(ex.getMessage());
647         } else if (ex instanceof NoSuchFieldException) {
648             err = new NoSuchFieldError(ex.getMessage());
649         } else {
650             err = new IncompatibleClassChangeError();
651         }
652         return initCauseFrom(err, ex);
653     }
654 
655     /**
656      * Use best possible cause for err.initCause(), substituting the
657      * cause for err itself if the cause has the same (or better) type.
658      */
659     static <E extends Error> E initCauseFrom(E err, Exception ex) {
660         Throwable th = ex.getCause();
661         @SuppressWarnings("unchecked")
662         final Class<E> Eclass = (Class<E>) err.getClass();
663         if (Eclass.isInstance(th))
664            return Eclass.cast(th);
665         err.initCause(th == null ? ex : th);
666         return err;
667     }
668 
669     /**
670      * Is this method a caller-sensitive method?
671      * I.e., does it call Reflection.getCallerClass or a similar method
672      * to ask about the identity of its caller?
673      */
674     static boolean isCallerSensitive(MemberName mem) {
675         if (!mem.isInvocable())  return false;  // fields are not caller sensitive
676 
677         return mem.isCallerSensitive() || canBeCalledVirtual(mem);
678     }
679 
680     static boolean canBeCalledVirtual(MemberName mem) {
681         assert(mem.isInvocable());
682         return mem.getName().equals("getContextClassLoader") && canBeCalledVirtual(mem, java.lang.Thread.class);
683     }
684 
685     static boolean canBeCalledVirtual(MemberName symbolicRef, Class<?> definingClass) {
686         Class<?> symbolicRefClass = symbolicRef.getDeclaringClass();
687         if (symbolicRefClass == definingClass)  return true;
688         if (symbolicRef.isStatic() || symbolicRef.isPrivate())  return false;
689         return (definingClass.isAssignableFrom(symbolicRefClass) ||  // Msym overrides Mdef
690                 symbolicRefClass.isInterface());                     // Mdef implements Msym
691     }
692 }