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     /// CDS support
 75     static native void checkArchivable(Class<?> type);
 76 
 77     /** Represents a context to track nmethod dependencies on CallSite instance target. */
 78     static class CallSiteContext implements Runnable {
 79         //@Injected JVM_nmethodBucket* vmdependencies;
 80         //@Injected jlong last_cleanup;
 81 
 82         static CallSiteContext make(CallSite cs) {
 83             final CallSiteContext newContext = new CallSiteContext();
 84             // CallSite instance is tracked by a Cleanable which clears native
 85             // structures allocated for CallSite context. Though the CallSite can
 86             // become unreachable, its Context is retained by the Cleanable instance
 87             // (which is referenced from Cleaner instance which is referenced from
 88             // CleanerFactory class) until cleanup is performed.
 89             CleanerFactory.cleaner().register(cs, newContext);
 90             return newContext;
 91         }
 92 
 93         @Override
 94         public void run() {
 95             MethodHandleNatives.clearCallSiteContext(this);
 96         }
 97     }
 98 
 99     /** Invalidate all recorded nmethods. */
100     private static native void clearCallSiteContext(CallSiteContext context);
101 
102     private static native void registerNatives();
103     static {
104         registerNatives();
105     }
106 
107     /**
108      * Compile-time constants go here. This collection exists not only for
109      * reference from clients, but also for ensuring the VM and JDK agree on the
110      * values of these constants (see {@link #verifyConstants()}).
111      */
112     static class Constants {
113         Constants() { } // static only
114 
115         static final int
116             MN_IS_METHOD           = 0x00010000, // method (not constructor)
117             MN_IS_CONSTRUCTOR      = 0x00020000, // constructor
118             MN_IS_FIELD            = 0x00040000, // field
119             MN_IS_TYPE             = 0x00080000, // nested type
120             MN_CALLER_SENSITIVE    = 0x00100000, // @CallerSensitive annotation detected
121             MN_TRUSTED_FINAL       = 0x00200000, // trusted final field
122             MN_HIDDEN_MEMBER       = 0x00400000, // members defined in a hidden class or with @Hidden
123             MN_REFERENCE_KIND_SHIFT = 24, // refKind
124             MN_REFERENCE_KIND_MASK = 0x0F000000 >> MN_REFERENCE_KIND_SHIFT;
125 
126         /**
127          * Constant pool reference-kind codes, as used by CONSTANT_MethodHandle CP entries.
128          */
129         static final byte
130             REF_NONE                    = 0,  // null value
131             REF_getField                = 1,
132             REF_getStatic               = 2,
133             REF_putField                = 3,
134             REF_putStatic               = 4,
135             REF_invokeVirtual           = 5,
136             REF_invokeStatic            = 6,
137             REF_invokeSpecial           = 7,
138             REF_newInvokeSpecial        = 8,
139             REF_invokeInterface         = 9,
140             REF_LIMIT                  = 10;
141 
142         /**
143          * Flags for Lookup.ClassOptions
144          */
145         static final int
146             NESTMATE_CLASS            = 0x00000001,
147             HIDDEN_CLASS              = 0x00000002,
148             STRONG_LOADER_LINK        = 0x00000004,
149             ACCESS_VM_ANNOTATIONS     = 0x00000008;
150 
151         /**
152          * Lookup modes
153          */
154         static final int
155             LM_MODULE        = Lookup.MODULE,
156             LM_UNCONDITIONAL = Lookup.UNCONDITIONAL,
157             LM_TRUSTED       = -1;
158 
159     }
160 
161     static boolean refKindIsValid(int refKind) {
162         return (refKind > REF_NONE && refKind < REF_LIMIT);
163     }
164     static boolean refKindIsField(byte refKind) {
165         assert(refKindIsValid(refKind));
166         return (refKind <= REF_putStatic);
167     }
168     static boolean refKindIsGetter(byte refKind) {
169         assert(refKindIsValid(refKind));
170         return (refKind <= REF_getStatic);
171     }
172     static boolean refKindIsSetter(byte refKind) {
173         return refKindIsField(refKind) && !refKindIsGetter(refKind);
174     }
175     static boolean refKindIsMethod(byte refKind) {
176         return !refKindIsField(refKind) && (refKind != REF_newInvokeSpecial);
177     }
178     static boolean refKindIsConstructor(byte refKind) {
179         return (refKind == REF_newInvokeSpecial);
180     }
181     static boolean refKindHasReceiver(byte refKind) {
182         assert(refKindIsValid(refKind));
183         return (refKind & 1) != 0;
184     }
185     static boolean refKindIsStatic(byte refKind) {
186         return !refKindHasReceiver(refKind) && (refKind != REF_newInvokeSpecial);
187     }
188     static boolean refKindDoesDispatch(byte refKind) {
189         assert(refKindIsValid(refKind));
190         return (refKind == REF_invokeVirtual ||
191                 refKind == REF_invokeInterface);
192     }
193     static {
194         final int HR_MASK = ((1 << REF_getField) |
195                              (1 << REF_putField) |
196                              (1 << REF_invokeVirtual) |
197                              (1 << REF_invokeSpecial) |
198                              (1 << REF_invokeInterface)
199                             );
200         for (byte refKind = REF_NONE+1; refKind < REF_LIMIT; refKind++) {
201             assert(refKindHasReceiver(refKind) == (((1<<refKind) & HR_MASK) != 0)) : refKind;
202         }
203     }
204     static String refKindName(byte refKind) {
205         assert(refKindIsValid(refKind));
206         return switch (refKind) {
207             case REF_getField         -> "getField";
208             case REF_getStatic        -> "getStatic";
209             case REF_putField         -> "putField";
210             case REF_putStatic        -> "putStatic";
211             case REF_invokeVirtual    -> "invokeVirtual";
212             case REF_invokeStatic     -> "invokeStatic";
213             case REF_invokeSpecial    -> "invokeSpecial";
214             case REF_newInvokeSpecial -> "newInvokeSpecial";
215             case REF_invokeInterface  -> "invokeInterface";
216             default -> "REF_???";
217         };
218     }
219 
220     private static native int getNamedCon(int which, Object[] name);
221     static boolean verifyConstants() {
222         Object[] box = { null };
223         for (int i = 0; ; i++) {
224             box[0] = null;
225             int vmval = getNamedCon(i, box);
226             if (box[0] == null)  break;
227             String name = (String) box[0];
228             try {
229                 Field con = Constants.class.getDeclaredField(name);
230                 int jval = con.getInt(null);
231                 if (jval == vmval)  continue;
232                 String err = (name+": JVM has "+vmval+" while Java has "+jval);
233                 if (name.equals("CONV_OP_LIMIT")) {
234                     System.err.println("warning: "+err);
235                     continue;
236                 }
237                 throw new InternalError(err);
238             } catch (NoSuchFieldException | IllegalAccessException ex) {
239                 String err = (name+": JVM has "+vmval+" which Java does not define");
240                 // ignore exotic ops the JVM cares about; we just won't issue them
241                 //System.err.println("warning: "+err);
242                 continue;
243             }
244         }
245         return true;
246     }
247     static {
248         VM.setJavaLangInvokeInited();
249         assert(verifyConstants());
250     }
251 
252     // Up-calls from the JVM.
253     // These must NOT be public.
254 
255     /**
256      * The JVM is linking an invokedynamic instruction.  Create a reified call site for it.
257      */
258     static MemberName linkCallSite(Object callerObj,
259                                    Object bootstrapMethodObj,
260                                    Object nameObj, Object typeObj,
261                                    Object staticArguments,
262                                    Object[] appendixResult) {
263         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
264         Class<?> caller = (Class<?>)callerObj;
265         String name = nameObj.toString().intern();
266         MethodType type = (MethodType)typeObj;
267         if (!TRACE_METHOD_LINKAGE)
268             return linkCallSiteImpl(caller, bootstrapMethod, name, type,
269                                     staticArguments, appendixResult);
270         return linkCallSiteTracing(caller, bootstrapMethod, name, type,
271                                    staticArguments, appendixResult);
272     }
273     static MemberName linkCallSiteImpl(Class<?> caller,
274                                        MethodHandle bootstrapMethod,
275                                        String name, MethodType type,
276                                        Object staticArguments,
277                                        Object[] appendixResult) {
278         CallSite callSite = CallSite.makeSite(bootstrapMethod,
279                                               name,
280                                               type,
281                                               staticArguments,
282                                               caller);
283         if (TRACE_METHOD_LINKAGE) {
284             MethodHandle target = callSite.getTarget();
285             System.out.println("linkCallSite target class => " + target.getClass().getName());
286             System.out.println("linkCallSite target => " + target.debugString(0));
287         }
288 
289         if (callSite instanceof ConstantCallSite) {
290             appendixResult[0] = callSite.dynamicInvoker();
291             return Invokers.linkToTargetMethod(type);
292         } else {
293             appendixResult[0] = callSite;
294             return Invokers.linkToCallSiteMethod(type);
295         }
296     }
297     // Tracing logic:
298     static MemberName linkCallSiteTracing(Class<?> caller,
299                                           MethodHandle bootstrapMethod,
300                                           String name, MethodType type,
301                                           Object staticArguments,
302                                           Object[] appendixResult) {
303         Object bsmReference = bootstrapMethod.internalMemberName();
304         if (bsmReference == null)  bsmReference = bootstrapMethod;
305         String staticArglist = staticArglistForTrace(staticArguments);
306         System.out.println("linkCallSite "+getCallerInfo(caller)+" "+
307                            bsmReference+" "+
308                            name+type+"/"+staticArglist);
309         try {
310             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
311                                               staticArguments, appendixResult);
312             System.out.println("linkCallSite linkage => "+res+" + "+appendixResult[0]);
313             return res;
314         } catch (Throwable ex) {
315             ex.printStackTrace(); // print now in case exception is swallowed
316             System.out.println("linkCallSite => throw "+ex);
317             throw ex;
318         }
319     }
320 
321     /**
322      * Return a human-readable description of the caller. Something like
323      * "java.base/java.security.Security.<clinit>(Security.java:82)"
324      */
325     private static String getCallerInfo(Class<?> caller) {
326         for (StackTraceElement e : Thread.currentThread().getStackTrace()) {
327             if (e.getClassName().equals(caller.getName())) {
328                 return e.toString();
329             }
330         }
331         // fallback if the caller is somehow missing from the stack.
332         return caller.getName();
333     }
334 
335     // this implements the upcall from the JVM, MethodHandleNatives.linkDynamicConstant:
336     static Object linkDynamicConstant(Object callerObj,
337                                       Object bootstrapMethodObj,
338                                       Object nameObj, Object typeObj,
339                                       Object staticArguments) {
340         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
341         Class<?> caller = (Class<?>)callerObj;
342         String name = nameObj.toString().intern();
343         Class<?> type = (Class<?>)typeObj;
344         if (!TRACE_METHOD_LINKAGE)
345             return linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
346         return linkDynamicConstantTracing(caller, bootstrapMethod, name, type, staticArguments);
347     }
348 
349     static Object linkDynamicConstantImpl(Class<?> caller,
350                                           MethodHandle bootstrapMethod,
351                                           String name, Class<?> type,
352                                           Object staticArguments) {
353         return ConstantBootstraps.makeConstant(bootstrapMethod, name, type, staticArguments, caller);
354     }
355 
356     private static String staticArglistForTrace(Object staticArguments) {
357         if (staticArguments instanceof Object[] array)
358             return "BSA="+java.util.Arrays.asList(array);
359         if (staticArguments instanceof int[] array)
360             return "BSA@"+java.util.Arrays.toString(array);
361         if (staticArguments == null)
362             return "BSA0=null";
363         return "BSA1="+staticArguments;
364     }
365 
366     // Tracing logic:
367     static Object linkDynamicConstantTracing(Class<?> caller,
368                                              MethodHandle bootstrapMethod,
369                                              String name, Class<?> type,
370                                              Object staticArguments) {
371         Object bsmReference = bootstrapMethod.internalMemberName();
372         if (bsmReference == null)  bsmReference = bootstrapMethod;
373         String staticArglist = staticArglistForTrace(staticArguments);
374         System.out.println("linkDynamicConstant "+caller.getName()+" "+
375                            bsmReference+" "+
376                            name+type+"/"+staticArglist);
377         try {
378             Object res = linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
379             System.out.println("linkDynamicConstantImpl => "+res);
380             return res;
381         } catch (Throwable ex) {
382             ex.printStackTrace(); // print now in case exception is swallowed
383             System.out.println("linkDynamicConstant => throw "+ex);
384             throw ex;
385         }
386     }
387 
388     /** The JVM is requesting pull-mode bootstrap when it provides
389      *  a tuple of the form int[]{ argc, vmindex }.
390      *  The BSM is expected to call back to the JVM using the caller
391      *  class and vmindex to resolve the static arguments.
392      */
393     static boolean staticArgumentsPulled(Object staticArguments) {
394         return staticArguments instanceof int[];
395     }
396 
397     /** A BSM runs in pull-mode if and only if its sole arguments
398      * are (Lookup, BootstrapCallInfo), or can be converted pairwise
399      * to those types, and it is not of variable arity.
400      * Excluding error cases, we can just test that the arity is a constant 2.
401      *
402      * NOTE: This method currently returns false, since pulling is not currently
403      * exposed to a BSM. When pull mode is supported the method block will be
404      * replaced with currently commented out code.
405      */
406     static boolean isPullModeBSM(MethodHandle bsm) {
407         return false;
408 //        return bsm.type().parameterCount() == 2 && !bsm.isVarargsCollector();
409     }
410 
411     /**
412      * The JVM wants a pointer to a MethodType.  Oblige it by finding or creating one.
413      */
414     static MethodType findMethodHandleType(Class<?> rtype, Class<?>[] ptypes) {
415         return MethodType.methodType(rtype, ptypes, true);
416     }
417 
418     /**
419      * The JVM wants to link a call site that requires a dynamic type check.
420      * Name is a type-checking invoker, invokeExact or invoke.
421      * Return a JVM method (MemberName) to handle the invoking.
422      * The method assumes the following arguments on the stack:
423      * 0: the method handle being invoked
424      * 1-N: the arguments to the method handle invocation
425      * N+1: an optional, implicitly added argument (typically the given MethodType)
426      * <p>
427      * The nominal method at such a call site is an instance of
428      * a signature-polymorphic method (see @PolymorphicSignature).
429      * Such method instances are user-visible entities which are
430      * "split" from the generic placeholder method in {@code MethodHandle}.
431      * (Note that the placeholder method is not identical with any of
432      * its instances.  If invoked reflectively, is guaranteed to throw an
433      * {@code UnsupportedOperationException}.)
434      * If the signature-polymorphic method instance is ever reified,
435      * it appears as a "copy" of the original placeholder
436      * (a native final member of {@code MethodHandle}) except
437      * that its type descriptor has shape required by the instance,
438      * and the method instance is <em>not</em> varargs.
439      * The method instance is also marked synthetic, since the
440      * method (by definition) does not appear in Java source code.
441      * <p>
442      * The JVM is allowed to reify this method as instance metadata.
443      * For example, {@code invokeBasic} is always reified.
444      * But the JVM may instead call {@code linkMethod}.
445      * If the result is an * ordered pair of a {@code (method, appendix)},
446      * the method gets all the arguments (0..N inclusive)
447      * plus the appendix (N+1), and uses the appendix to complete the call.
448      * In this way, one reusable method (called a "linker method")
449      * can perform the function of any number of polymorphic instance
450      * methods.
451      * <p>
452      * Linker methods are allowed to be weakly typed, with any or
453      * all references rewritten to {@code Object} and any primitives
454      * (except {@code long}/{@code float}/{@code double})
455      * rewritten to {@code int}.
456      * A linker method is trusted to return a strongly typed result,
457      * according to the specific method type descriptor of the
458      * signature-polymorphic instance it is emulating.
459      * This can involve (as necessary) a dynamic check using
460      * data extracted from the appendix argument.
461      * <p>
462      * The JVM does not inspect the appendix, other than to pass
463      * it verbatim to the linker method at every call.
464      * This means that the JDK runtime has wide latitude
465      * for choosing the shape of each linker method and its
466      * corresponding appendix.
467      * Linker methods should be generated from {@code LambdaForm}s
468      * so that they do not become visible on stack traces.
469      * <p>
470      * The {@code linkMethod} call is free to omit the appendix
471      * (returning null) and instead emulate the required function
472      * completely in the linker method.
473      * As a corner case, if N==255, no appendix is possible.
474      * In this case, the method returned must be custom-generated to
475      * perform any needed type checking.
476      * <p>
477      * If the JVM does not reify a method at a call site, but instead
478      * calls {@code linkMethod}, the corresponding call represented
479      * in the bytecodes may mention a valid method which is not
480      * representable with a {@code MemberName}.
481      * Therefore, use cases for {@code linkMethod} tend to correspond to
482      * special cases in reflective code such as {@code findVirtual}
483      * or {@code revealDirect}.
484      */
485     static MemberName linkMethod(Class<?> callerClass, int refKind,
486                                  Class<?> defc, String name, Object type,
487                                  Object[] appendixResult) {
488         if (!TRACE_METHOD_LINKAGE)
489             return linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
490         return linkMethodTracing(callerClass, refKind, defc, name, type, appendixResult);
491     }
492     static MemberName linkMethodImpl(Class<?> callerClass, int refKind,
493                                      Class<?> defc, String name, Object type,
494                                      Object[] appendixResult) {
495         try {
496             if (refKind == REF_invokeVirtual) {
497                 if (defc == MethodHandle.class) {
498                     return Invokers.methodHandleInvokeLinkerMethod(
499                             name, fixMethodType(callerClass, type), appendixResult);
500                 } else if (defc == VarHandle.class) {
501                     return varHandleOperationLinkerMethod(
502                             name, fixMethodType(callerClass, type), appendixResult);
503                 }
504             }
505         } catch (Error e) {
506             // Pass through an Error, including say StackOverflowError or
507             // OutOfMemoryError
508             throw e;
509         } catch (Throwable ex) {
510             // Wrap anything else in LinkageError
511             throw new LinkageError(ex.getMessage(), ex);
512         }
513         throw new LinkageError("no such method "+defc.getName()+"."+name+type);
514     }
515     private static MethodType fixMethodType(Class<?> callerClass, Object type) {
516         if (type instanceof MethodType mt)
517             return mt;
518         else
519             return MethodType.fromDescriptor((String)type, callerClass.getClassLoader());
520     }
521     // Tracing logic:
522     static MemberName linkMethodTracing(Class<?> callerClass, int refKind,
523                                         Class<?> defc, String name, Object type,
524                                         Object[] appendixResult) {
525         System.out.println("linkMethod "+defc.getName()+"."+
526                            name+type+"/"+Integer.toHexString(refKind));
527         try {
528             MemberName res = linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
529             System.out.println("linkMethod => "+res+" + "+appendixResult[0]);
530             return res;
531         } catch (Throwable ex) {
532             System.out.println("linkMethod => throw "+ex);
533             throw ex;
534         }
535     }
536 
537     /**
538      * Obtain the method to link to the VarHandle operation.
539      * This method is located here and not in Invokers to avoid
540      * initializing that and other classes early on in VM bootup.
541      */
542     private static MemberName varHandleOperationLinkerMethod(String name,
543                                                              MethodType mtype,
544                                                              Object[] appendixResult) {
545         // Get the signature method type
546         final MethodType sigType = mtype.basicType();
547 
548         // Get the access kind from the method name
549         VarHandle.AccessMode ak;
550         try {
551             ak = VarHandle.AccessMode.valueFromMethodName(name);
552         } catch (IllegalArgumentException e) {
553             throw MethodHandleStatics.newInternalError(e);
554         }
555 
556         // Create the appendix descriptor constant
557         VarHandle.AccessDescriptor ad = new VarHandle.AccessDescriptor(mtype, ak.at.ordinal(), ak.ordinal());
558         appendixResult[0] = ad;
559 
560         if (MethodHandleStatics.VAR_HANDLE_GUARDS) {
561             // If not polymorphic in the return type, such as the compareAndSet
562             // methods that return boolean
563             Class<?> guardReturnType = sigType.returnType();
564             if (ak.at.isMonomorphicInReturnType) {
565                 if (ak.at.returnType != mtype.returnType()) {
566                     // The caller contains a different return type than that
567                     // defined by the method
568                     throw newNoSuchMethodErrorOnVarHandle(name, mtype);
569                 }
570                 // Adjust the return type of the signature method type
571                 guardReturnType = ak.at.returnType;
572             }
573 
574             // Get the guard method type for linking
575             final Class<?>[] guardParams = new Class<?>[sigType.parameterCount() + 2];
576             // VarHandle at start
577             guardParams[0] = VarHandle.class;
578             for (int i = 0; i < sigType.parameterCount(); i++) {
579                 guardParams[i + 1] = sigType.parameterType(i);
580             }
581             // Access descriptor at end
582             guardParams[guardParams.length - 1] = VarHandle.AccessDescriptor.class;
583             MethodType guardType = MethodType.methodType(guardReturnType, guardParams, true);
584 
585             MemberName linker = new MemberName(
586                     VarHandleGuards.class, getVarHandleGuardMethodName(guardType),
587                     guardType, REF_invokeStatic);
588 
589             linker = MemberName.getFactory().resolveOrNull(REF_invokeStatic, linker,
590                                                            VarHandleGuards.class, LM_TRUSTED);
591             if (linker != null) {
592                 return linker;
593             }
594             // Fall back to lambda form linkage if guard method is not available
595             // TODO Optionally log fallback ?
596         }
597         return Invokers.varHandleInvokeLinkerMethod(mtype);
598     }
599     static String getVarHandleGuardMethodName(MethodType guardType) {
600         String prefix = "guard_";
601         StringBuilder sb = new StringBuilder(prefix.length() + guardType.parameterCount());
602 
603         sb.append(prefix);
604         for (int i = 1; i < guardType.parameterCount() - 1; i++) {
605             Class<?> pt = guardType.parameterType(i);
606             sb.append(getCharType(pt));
607         }
608         sb.append('_').append(getCharType(guardType.returnType()));
609         return sb.toString();
610     }
611     static char getCharType(Class<?> pt) {
612         return Wrapper.forBasicType(pt).basicTypeChar();
613     }
614     static NoSuchMethodError newNoSuchMethodErrorOnVarHandle(String name, MethodType mtype) {
615         return new NoSuchMethodError("VarHandle." + name + mtype);
616     }
617 
618     /**
619      * The JVM is resolving a CONSTANT_MethodHandle CP entry.  And it wants our help.
620      * It will make an up-call to this method.  (Do not change the name or signature.)
621      * The type argument is a Class for field requests and a MethodType for non-fields.
622      * <p>
623      * Recent versions of the JVM may also pass a resolved MemberName for the type.
624      * In that case, the name is ignored and may be null.
625      */
626     static MethodHandle linkMethodHandleConstant(Class<?> callerClass, int refKind,
627                                                  Class<?> defc, String name, Object type) {
628         try {
629             Lookup lookup = IMPL_LOOKUP.in(callerClass);
630             assert(refKindIsValid(refKind));
631             return lookup.linkMethodHandleConstant((byte) refKind, defc, name, type);
632         } catch (ReflectiveOperationException ex) {
633             throw mapLookupExceptionToError(ex);
634         }
635     }
636 
637     /**
638      * Map a reflective exception to a linkage error.
639      */
640     static LinkageError mapLookupExceptionToError(ReflectiveOperationException ex) {
641         LinkageError err;
642         if (ex instanceof IllegalAccessException) {
643             Throwable cause = ex.getCause();
644             if (cause instanceof AbstractMethodError ame) {
645                 return ame;
646             } else {
647                 err = new IllegalAccessError(ex.getMessage());
648             }
649         } else if (ex instanceof NoSuchMethodException) {
650             err = new NoSuchMethodError(ex.getMessage());
651         } else if (ex instanceof NoSuchFieldException) {
652             err = new NoSuchFieldError(ex.getMessage());
653         } else {
654             err = new IncompatibleClassChangeError();
655         }
656         return initCauseFrom(err, ex);
657     }
658 
659     /**
660      * Use best possible cause for err.initCause(), substituting the
661      * cause for err itself if the cause has the same (or better) type.
662      */
663     static <E extends Error> E initCauseFrom(E err, Exception ex) {
664         Throwable th = ex.getCause();
665         @SuppressWarnings("unchecked")
666         final Class<E> Eclass = (Class<E>) err.getClass();
667         if (Eclass.isInstance(th))
668            return Eclass.cast(th);
669         err.initCause(th == null ? ex : th);
670         return err;
671     }
672 
673     /**
674      * Is this method a caller-sensitive method?
675      * I.e., does it call Reflection.getCallerClass or a similar method
676      * to ask about the identity of its caller?
677      */
678     static boolean isCallerSensitive(MemberName mem) {
679         if (!mem.isInvocable())  return false;  // fields are not caller sensitive
680 
681         return mem.isCallerSensitive() || canBeCalledVirtual(mem);
682     }
683 
684     static boolean canBeCalledVirtual(MemberName mem) {
685         assert(mem.isInvocable());
686         return mem.getName().equals("getContextClassLoader") && canBeCalledVirtual(mem, java.lang.Thread.class);
687     }
688 
689     static boolean canBeCalledVirtual(MemberName symbolicRef, Class<?> definingClass) {
690         Class<?> symbolicRefClass = symbolicRef.getDeclaringClass();
691         if (symbolicRefClass == definingClass)  return true;
692         if (symbolicRef.isStatic() || symbolicRef.isPrivate())  return false;
693         return (definingClass.isAssignableFrom(symbolicRefClass) ||  // Msym overrides Mdef
694                 symbolicRefClass.isInterface());                     // Mdef implements Msym
695     }
696 }