LambdaForm.java

/*
 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.
 * 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
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.lang.invoke;

import java.lang.annotation.*;
import java.lang.reflect.Method;
import java.util.List;
import java.util.Arrays;
import java.util.HashMap;

import sun.invoke.util.Wrapper;
import java.lang.reflect.Field;

import static java.lang.invoke.LambdaForm.BasicType.*;
import static java.lang.invoke.MethodHandleStatics.*;
import static java.lang.invoke.MethodHandleNatives.Constants.*;

/**
 * The symbolic, non-executable form of a method handle's invocation semantics.
 * It consists of a series of names.
 * The first N (N=arity) names are parameters,
 * while any remaining names are temporary values.
 * Each temporary specifies the application of a function to some arguments.
 * The functions are method handles, while the arguments are mixes of
 * constant values and local names.
 * The result of the lambda is defined as one of the names, often the last one.
 * <p>
 * Here is an approximate grammar:
 * <blockquote><pre>{@code
 * LambdaForm = "(" ArgName* ")=>{" TempName* Result "}"
 * ArgName = "a" N ":" T
 * TempName = "t" N ":" T "=" Function "(" Argument* ");"
 * Function = ConstantValue
 * Argument = NameRef | ConstantValue
 * Result = NameRef | "void"
 * NameRef = "a" N | "t" N
 * N = (any whole number)
 * T = "L" | "I" | "J" | "F" | "D" | "V"
 * }</pre></blockquote>
 * Names are numbered consecutively from left to right starting at zero.
 * (The letters are merely a taste of syntax sugar.)
 * Thus, the first temporary (if any) is always numbered N (where N=arity).
 * Every occurrence of a name reference in an argument list must refer to
 * a name previously defined within the same lambda.
 * A lambda has a void result if and only if its result index is -1.
 * If a temporary has the type "V", it cannot be the subject of a NameRef,
 * even though possesses a number.
 * Note that all reference types are erased to "L", which stands for {@code Object}.
 * All subword types (boolean, byte, short, char) are erased to "I" which is {@code int}.
 * The other types stand for the usual primitive types.
 * <p>
 * Function invocation closely follows the static rules of the Java verifier.
 * Arguments and return values must exactly match when their "Name" types are
 * considered.
 * Conversions are allowed only if they do not change the erased type.
 * <ul>
 * <li>L = Object: casts are used freely to convert into and out of reference types
 * <li>I = int: subword types are forcibly narrowed when passed as arguments (see {@code explicitCastArguments})
 * <li>J = long: no implicit conversions
 * <li>F = float: no implicit conversions
 * <li>D = double: no implicit conversions
 * <li>V = void: a function result may be void if and only if its Name is of type "V"
 * </ul>
 * Although implicit conversions are not allowed, explicit ones can easily be
 * encoded by using temporary expressions which call type-transformed identity functions.
 * <p>
 * Examples:
 * <blockquote><pre>{@code
 * (a0:J)=>{ a0 }
 *     == identity(long)
 * (a0:I)=>{ t1:V = System.out#println(a0); void }
 *     == System.out#println(int)
 * (a0:L)=>{ t1:V = System.out#println(a0); a0 }
 *     == identity, with printing side-effect
 * (a0:L, a1:L)=>{ t2:L = BoundMethodHandle#argument(a0);
 *                 t3:L = BoundMethodHandle#target(a0);
 *                 t4:L = MethodHandle#invoke(t3, t2, a1); t4 }
 *     == general invoker for unary insertArgument combination
 * (a0:L, a1:L)=>{ t2:L = FilterMethodHandle#filter(a0);
 *                 t3:L = MethodHandle#invoke(t2, a1);
 *                 t4:L = FilterMethodHandle#target(a0);
 *                 t5:L = MethodHandle#invoke(t4, t3); t5 }
 *     == general invoker for unary filterArgument combination
 * (a0:L, a1:L)=>{ ...(same as previous example)...
 *                 t5:L = MethodHandle#invoke(t4, t3, a1); t5 }
 *     == general invoker for unary/unary foldArgument combination
 * (a0:L, a1:I)=>{ t2:I = identity(long).asType((int)->long)(a1); t2 }
 *     == invoker for identity method handle which performs i2l
 * (a0:L, a1:L)=>{ t2:L = BoundMethodHandle#argument(a0);
 *                 t3:L = Class#cast(t2,a1); t3 }
 *     == invoker for identity method handle which performs cast
 * }</pre></blockquote>
 * <p>
 * @author John Rose, JSR 292 EG
 */
class LambdaForm {
    final int arity;
    final int result;
    final boolean forceInline;
    final MethodHandle customized;
    @Stable final Name[] names;
    final String debugName;
    MemberName vmentry;   // low-level behavior, or null if not yet prepared
    private boolean isCompiled;

    // Either a LambdaForm cache (managed by LambdaFormEditor) or a link to uncustomized version (for customized LF)
    volatile Object transformCache;

    public static final int VOID_RESULT = -1, LAST_RESULT = -2;

    enum BasicType {
        L_TYPE('L', Object.class, Wrapper.OBJECT),  // all reference types
        I_TYPE('I', int.class,    Wrapper.INT),
        J_TYPE('J', long.class,   Wrapper.LONG),
        F_TYPE('F', float.class,  Wrapper.FLOAT),
        D_TYPE('D', double.class, Wrapper.DOUBLE),  // all primitive types
        V_TYPE('V', void.class,   Wrapper.VOID);    // not valid in all contexts

        static final BasicType[] ALL_TYPES = BasicType.values();
        static final BasicType[] ARG_TYPES = Arrays.copyOf(ALL_TYPES, ALL_TYPES.length-1);

        static final int ARG_TYPE_LIMIT = ARG_TYPES.length;
        static final int TYPE_LIMIT = ALL_TYPES.length;

        private final char btChar;
        private final Class<?> btClass;
        private final Wrapper btWrapper;

        private BasicType(char btChar, Class<?> btClass, Wrapper wrapper) {
            this.btChar = btChar;
            this.btClass = btClass;
            this.btWrapper = wrapper;
        }

        char basicTypeChar() {
            return btChar;
        }
        Class<?> basicTypeClass() {
            return btClass;
        }
        Wrapper basicTypeWrapper() {
            return btWrapper;
        }
        int basicTypeSlots() {
            return btWrapper.stackSlots();
        }

        static BasicType basicType(byte type) {
            return ALL_TYPES[type];
        }
        static BasicType basicType(char type) {
            switch (type) {
                case 'L': return L_TYPE;
                case 'I': return I_TYPE;
                case 'J': return J_TYPE;
                case 'F': return F_TYPE;
                case 'D': return D_TYPE;
                case 'V': return V_TYPE;
                // all subword types are represented as ints
                case 'Z':
                case 'B':
                case 'S':
                case 'C':
                    return I_TYPE;
                default:
                    throw newInternalError("Unknown type char: '"+type+"'");
            }
        }
        static BasicType basicType(Wrapper type) {
            char c = type.basicTypeChar();
            return basicType(c);
        }
        static BasicType basicType(Class<?> type) {
            if (!type.isPrimitive())  return L_TYPE;
            return basicType(Wrapper.forPrimitiveType(type));
        }

        static char basicTypeChar(Class<?> type) {
            return basicType(type).btChar;
        }
        static BasicType[] basicTypes(List<Class<?>> types) {
            BasicType[] btypes = new BasicType[types.size()];
            for (int i = 0; i < btypes.length; i++) {
                btypes[i] = basicType(types.get(i));
            }
            return btypes;
        }
        static BasicType[] basicTypes(String types) {
            BasicType[] btypes = new BasicType[types.length()];
            for (int i = 0; i < btypes.length; i++) {
                btypes[i] = basicType(types.charAt(i));
            }
            return btypes;
        }
        static byte[] basicTypesOrd(BasicType[] btypes) {
            byte[] ords = new byte[btypes.length];
            for (int i = 0; i < btypes.length; i++) {
                ords[i] = (byte)btypes[i].ordinal();
            }
            return ords;
        }
        static boolean isBasicTypeChar(char c) {
            return "LIJFDV".indexOf(c) >= 0;
        }
        static boolean isArgBasicTypeChar(char c) {
            return "LIJFD".indexOf(c) >= 0;
        }

        static { assert(checkBasicType()); }
        private static boolean checkBasicType() {
            for (int i = 0; i < ARG_TYPE_LIMIT; i++) {
                assert ARG_TYPES[i].ordinal() == i;
                assert ARG_TYPES[i] == ALL_TYPES[i];
            }
            for (int i = 0; i < TYPE_LIMIT; i++) {
                assert ALL_TYPES[i].ordinal() == i;
            }
            assert ALL_TYPES[TYPE_LIMIT - 1] == V_TYPE;
            assert !Arrays.asList(ARG_TYPES).contains(V_TYPE);
            return true;
        }
    }

    LambdaForm(String debugName,
               int arity, Name[] names, int result) {
        this(debugName, arity, names, result, /*forceInline=*/true, /*customized=*/null);
    }
    LambdaForm(String debugName,
               int arity, Name[] names, int result, boolean forceInline, MethodHandle customized) {
        assert(namesOK(arity, names));
        this.arity = arity;
        this.result = fixResult(result, names);
        this.names = names.clone();
        this.debugName = fixDebugName(debugName);
        this.forceInline = forceInline;
        this.customized = customized;
        int maxOutArity = normalize();
        if (maxOutArity > MethodType.MAX_MH_INVOKER_ARITY) {
            // Cannot use LF interpreter on very high arity expressions.
            assert(maxOutArity <= MethodType.MAX_JVM_ARITY);
            compileToBytecode();
        }
    }
    LambdaForm(String debugName,
               int arity, Name[] names) {
        this(debugName, arity, names, LAST_RESULT, /*forceInline=*/true, /*customized=*/null);
    }
    LambdaForm(String debugName,
               int arity, Name[] names, boolean forceInline) {
        this(debugName, arity, names, LAST_RESULT, forceInline, /*customized=*/null);
    }
    LambdaForm(String debugName,
               Name[] formals, Name[] temps, Name result) {
        this(debugName,
             formals.length, buildNames(formals, temps, result), LAST_RESULT, /*forceInline=*/true, /*customized=*/null);
    }
    LambdaForm(String debugName,
               Name[] formals, Name[] temps, Name result, boolean forceInline) {
        this(debugName,
             formals.length, buildNames(formals, temps, result), LAST_RESULT, forceInline, /*customized=*/null);
    }

    private static Name[] buildNames(Name[] formals, Name[] temps, Name result) {
        int arity = formals.length;
        int length = arity + temps.length + (result == null ? 0 : 1);
        Name[] names = Arrays.copyOf(formals, length);
        System.arraycopy(temps, 0, names, arity, temps.length);
        if (result != null)
            names[length - 1] = result;
        return names;
    }

    private LambdaForm(String sig) {
        // Make a blank lambda form, which returns a constant zero or null.
        // It is used as a template for managing the invocation of similar forms that are non-empty.
        // Called only from getPreparedForm.
        assert(isValidSignature(sig));
        this.arity = signatureArity(sig);
        this.result = (signatureReturn(sig) == V_TYPE ? -1 : arity);
        this.names = buildEmptyNames(arity, sig);
        this.debugName = "LF.zero";
        this.forceInline = true;
        this.customized = null;
        assert(nameRefsAreLegal());
        assert(isEmpty());
        assert(sig.equals(basicTypeSignature())) : sig + " != " + basicTypeSignature();
    }

    private static Name[] buildEmptyNames(int arity, String basicTypeSignature) {
        assert(isValidSignature(basicTypeSignature));
        int resultPos = arity + 1;  // skip '_'
        if (arity < 0 || basicTypeSignature.length() != resultPos+1)
            throw new IllegalArgumentException("bad arity for "+basicTypeSignature);
        int numRes = (basicType(basicTypeSignature.charAt(resultPos)) == V_TYPE ? 0 : 1);
        Name[] names = arguments(numRes, basicTypeSignature.substring(0, arity));
        for (int i = 0; i < numRes; i++) {
            Name zero = new Name(constantZero(basicType(basicTypeSignature.charAt(resultPos + i))));
            names[arity + i] = zero.newIndex(arity + i);
        }
        return names;
    }

    private static int fixResult(int result, Name[] names) {
        if (result == LAST_RESULT)
            result = names.length - 1;  // might still be void
        if (result >= 0 && names[result].type == V_TYPE)
            result = VOID_RESULT;
        return result;
    }

    private static String fixDebugName(String debugName) {
        if (DEBUG_NAME_COUNTERS != null) {
            int under = debugName.indexOf('_');
            int length = debugName.length();
            if (under < 0)  under = length;
            String debugNameStem = debugName.substring(0, under);
            Integer ctr;
            synchronized (DEBUG_NAME_COUNTERS) {
                ctr = DEBUG_NAME_COUNTERS.get(debugNameStem);
                if (ctr == null)  ctr = 0;
                DEBUG_NAME_COUNTERS.put(debugNameStem, ctr+1);
            }
            StringBuilder buf = new StringBuilder(debugNameStem);
            buf.append('_');
            int leadingZero = buf.length();
            buf.append((int) ctr);
            for (int i = buf.length() - leadingZero; i < 3; i++)
                buf.insert(leadingZero, '0');
            if (under < length) {
                ++under;    // skip "_"
                while (under < length && Character.isDigit(debugName.charAt(under))) {
                    ++under;
                }
                if (under < length && debugName.charAt(under) == '_')  ++under;
                if (under < length)
                    buf.append('_').append(debugName, under, length);
            }
            return buf.toString();
        }
        return debugName;
    }

    private static boolean namesOK(int arity, Name[] names) {
        for (int i = 0; i < names.length; i++) {
            Name n = names[i];
            assert(n != null) : "n is null";
            if (i < arity)
                assert( n.isParam()) : n + " is not param at " + i;
            else
                assert(!n.isParam()) : n + " is param at " + i;
        }
        return true;
    }

    /** Customize LambdaForm for a particular MethodHandle */
    LambdaForm customize(MethodHandle mh) {
        LambdaForm customForm = new LambdaForm(debugName, arity, names, result, forceInline, mh);
        if (COMPILE_THRESHOLD > 0 && isCompiled) {
            // If shared LambdaForm has been compiled, compile customized version as well.
            customForm.compileToBytecode();
        }
        customForm.transformCache = this; // LambdaFormEditor should always use uncustomized form.
        return customForm;
    }

    /** Get uncustomized flavor of the LambdaForm */
    LambdaForm uncustomize() {
        if (customized == null) {
            return this;
        }
        assert(transformCache != null); // Customized LambdaForm should always has a link to uncustomized version.
        LambdaForm uncustomizedForm = (LambdaForm)transformCache;
        if (COMPILE_THRESHOLD > 0 && isCompiled) {
            // If customized LambdaForm has been compiled, compile uncustomized version as well.
            uncustomizedForm.compileToBytecode();
        }
        return uncustomizedForm;
    }

    /** Renumber and/or replace params so that they are interned and canonically numbered.
     *  @return maximum argument list length among the names (since we have to pass over them anyway)
     */
    private int normalize() {
        Name[] oldNames = null;
        int maxOutArity = 0;
        int changesStart = 0;
        for (int i = 0; i < names.length; i++) {
            Name n = names[i];
            if (!n.initIndex(i)) {
                if (oldNames == null) {
                    oldNames = names.clone();
                    changesStart = i;
                }
                names[i] = n.cloneWithIndex(i);
            }
            if (n.arguments != null && maxOutArity < n.arguments.length)
                maxOutArity = n.arguments.length;
        }
        if (oldNames != null) {
            int startFixing = arity;
            if (startFixing <= changesStart)
                startFixing = changesStart+1;
            for (int i = startFixing; i < names.length; i++) {
                Name fixed = names[i].replaceNames(oldNames, names, changesStart, i);
                names[i] = fixed.newIndex(i);
            }
        }
        assert(nameRefsAreLegal());
        int maxInterned = Math.min(arity, INTERNED_ARGUMENT_LIMIT);
        boolean needIntern = false;
        for (int i = 0; i < maxInterned; i++) {
            Name n = names[i], n2 = internArgument(n);
            if (n != n2) {
                names[i] = n2;
                needIntern = true;
            }
        }
        if (needIntern) {
            for (int i = arity; i < names.length; i++) {
                names[i].internArguments();
            }
        }
        assert(nameRefsAreLegal());
        return maxOutArity;
    }

    /**
     * Check that all embedded Name references are localizable to this lambda,
     * and are properly ordered after their corresponding definitions.
     * <p>
     * Note that a Name can be local to multiple lambdas, as long as
     * it possesses the same index in each use site.
     * This allows Name references to be freely reused to construct
     * fresh lambdas, without confusion.
     */
    boolean nameRefsAreLegal() {
        assert(arity >= 0 && arity <= names.length);
        assert(result >= -1 && result < names.length);
        // Do all names possess an index consistent with their local definition order?
        for (int i = 0; i < arity; i++) {
            Name n = names[i];
            assert(n.index() == i) : Arrays.asList(n.index(), i);
            assert(n.isParam());
        }
        // Also, do all local name references
        for (int i = arity; i < names.length; i++) {
            Name n = names[i];
            assert(n.index() == i);
            for (Object arg : n.arguments) {
                if (arg instanceof Name) {
                    Name n2 = (Name) arg;
                    int i2 = n2.index;
                    assert(0 <= i2 && i2 < names.length) : n.debugString() + ": 0 <= i2 && i2 < names.length: 0 <= " + i2 + " < " + names.length;
                    assert(names[i2] == n2) : Arrays.asList("-1-", i, "-2-", n.debugString(), "-3-", i2, "-4-", n2.debugString(), "-5-", names[i2].debugString(), "-6-", this);
                    assert(i2 < i);  // ref must come after def!
                }
            }
        }
        return true;
    }

    /** Invoke this form on the given arguments. */
    // final Object invoke(Object... args) throws Throwable {
    //     // NYI: fit this into the fast path?
    //     return interpretWithArguments(args);
    // }

    /** Report the return type. */
    BasicType returnType() {
        if (result < 0)  return V_TYPE;
        Name n = names[result];
        return n.type;
    }

    /** Report the N-th argument type. */
    BasicType parameterType(int n) {
        return parameter(n).type;
    }

    /** Report the N-th argument name. */
    Name parameter(int n) {
        assert(n < arity);
        Name param = names[n];
        assert(param.isParam());
        return param;
    }

    /** Report the N-th argument type constraint. */
    Object parameterConstraint(int n) {
        return parameter(n).constraint;
    }

    /** Report the arity. */
    int arity() {
        return arity;
    }

    /** Report the number of expressions (non-parameter names). */
    int expressionCount() {
        return names.length - arity;
    }

    /** Return the method type corresponding to my basic type signature. */
    MethodType methodType() {
        return signatureType(basicTypeSignature());
    }
    /** Return ABC_Z, where the ABC are parameter type characters, and Z is the return type character. */
    final String basicTypeSignature() {
        StringBuilder buf = new StringBuilder(arity() + 3);
        for (int i = 0, a = arity(); i < a; i++)
            buf.append(parameterType(i).basicTypeChar());
        return buf.append('_').append(returnType().basicTypeChar()).toString();
    }
    static int signatureArity(String sig) {
        assert(isValidSignature(sig));
        return sig.indexOf('_');
    }
    static BasicType signatureReturn(String sig) {
        return basicType(sig.charAt(signatureArity(sig) + 1));
    }
    static boolean isValidSignature(String sig) {
        int arity = sig.indexOf('_');
        if (arity < 0)  return false;  // must be of the form *_*
        int siglen = sig.length();
        if (siglen != arity + 2)  return false;  // *_X
        for (int i = 0; i < siglen; i++) {
            if (i == arity)  continue;  // skip '_'
            char c = sig.charAt(i);
            if (c == 'V')
                return (i == siglen - 1 && arity == siglen - 2);
            if (!isArgBasicTypeChar(c))  return false; // must be [LIJFD]
        }
        return true;  // [LIJFD]*_[LIJFDV]
    }
    static MethodType signatureType(String sig) {
        Class<?>[] ptypes = new Class<?>[signatureArity(sig)];
        for (int i = 0; i < ptypes.length; i++)
            ptypes[i] = basicType(sig.charAt(i)).btClass;
        Class<?> rtype = signatureReturn(sig).btClass;
        return MethodType.methodType(rtype, ptypes);
    }

    /*
     * Code generation issues:
     *
     * Compiled LFs should be reusable in general.
     * The biggest issue is how to decide when to pull a name into
     * the bytecode, versus loading a reified form from the MH data.
     *
     * For example, an asType wrapper may require execution of a cast
     * after a call to a MH.  The target type of the cast can be placed
     * as a constant in the LF itself.  This will force the cast type
     * to be compiled into the bytecodes and native code for the MH.
     * Or, the target type of the cast can be erased in the LF, and
     * loaded from the MH data.  (Later on, if the MH as a whole is
     * inlined, the data will flow into the inlined instance of the LF,
     * as a constant, and the end result will be an optimal cast.)
     *
     * This erasure of cast types can be done with any use of
     * reference types.  It can also be done with whole method
     * handles.  Erasing a method handle might leave behind
     * LF code that executes correctly for any MH of a given
     * type, and load the required MH from the enclosing MH's data.
     * Or, the erasure might even erase the expected MT.
     *
     * Also, for direct MHs, the MemberName of the target
     * could be erased, and loaded from the containing direct MH.
     * As a simple case, a LF for all int-valued non-static
     * field getters would perform a cast on its input argument
     * (to non-constant base type derived from the MemberName)
     * and load an integer value from the input object
     * (at a non-constant offset also derived from the MemberName).
     * Such MN-erased LFs would be inlinable back to optimized
     * code, whenever a constant enclosing DMH is available
     * to supply a constant MN from its data.
     *
     * The main problem here is to keep LFs reasonably generic,
     * while ensuring that hot spots will inline good instances.
     * "Reasonably generic" means that we don't end up with
     * repeated versions of bytecode or machine code that do
     * not differ in their optimized form.  Repeated versions
     * of machine would have the undesirable overheads of
     * (a) redundant compilation work and (b) extra I$ pressure.
     * To control repeated versions, we need to be ready to
     * erase details from LFs and move them into MH data,
     * whevener those details are not relevant to significant
     * optimization.  "Significant" means optimization of
     * code that is actually hot.
     *
     * Achieving this may require dynamic splitting of MHs, by replacing
     * a generic LF with a more specialized one, on the same MH,
     * if (a) the MH is frequently executed and (b) the MH cannot
     * be inlined into a containing caller, such as an invokedynamic.
     *
     * Compiled LFs that are no longer used should be GC-able.
     * If they contain non-BCP references, they should be properly
     * interlinked with the class loader(s) that their embedded types
     * depend on.  This probably means that reusable compiled LFs
     * will be tabulated (indexed) on relevant class loaders,
     * or else that the tables that cache them will have weak links.
     */

    /**
     * Make this LF directly executable, as part of a MethodHandle.
     * Invariant:  Every MH which is invoked must prepare its LF
     * before invocation.
     * (In principle, the JVM could do this very lazily,
     * as a sort of pre-invocation linkage step.)
     */
    public void prepare() {
        if (COMPILE_THRESHOLD == 0 && !isCompiled) {
            compileToBytecode();
        }
        if (this.vmentry != null) {
            // already prepared (e.g., a primitive DMH invoker form)
            return;
        }
        LambdaForm prep = getPreparedForm(basicTypeSignature());
        this.vmentry = prep.vmentry;
        // TO DO: Maybe add invokeGeneric, invokeWithArguments
    }

    /** Generate optimizable bytecode for this form. */
    MemberName compileToBytecode() {
        if (vmentry != null && isCompiled) {
            return vmentry;  // already compiled somehow
        }
        MethodType invokerType = methodType();
        assert(vmentry == null || vmentry.getMethodType().basicType().equals(invokerType));
        try {
            vmentry = InvokerBytecodeGenerator.generateCustomizedCode(this, invokerType);
            if (TRACE_INTERPRETER)
                traceInterpreter("compileToBytecode", this);
            isCompiled = true;
            return vmentry;
        } catch (Error | Exception ex) {
            throw newInternalError(this.toString(), ex);
        }
    }

    private static void computeInitialPreparedForms() {
        // Find all predefined invokers and associate them with canonical empty lambda forms.
        for (MemberName m : MemberName.getFactory().getMethods(LambdaForm.class, false, null, null, null)) {
            if (!m.isStatic() || !m.isPackage())  continue;
            MethodType mt = m.getMethodType();
            if (mt.parameterCount() > 0 &&
                mt.parameterType(0) == MethodHandle.class &&
                m.getName().startsWith("interpret_")) {
                String sig = basicTypeSignature(mt);
                assert(m.getName().equals("interpret" + sig.substring(sig.indexOf('_'))));
                LambdaForm form = new LambdaForm(sig);
                form.vmentry = m;
                form = mt.form().setCachedLambdaForm(MethodTypeForm.LF_INTERPRET, form);
            }
        }
    }

    // Set this false to disable use of the interpret_L methods defined in this file.
    private static final boolean USE_PREDEFINED_INTERPRET_METHODS = true;

    // The following are predefined exact invokers.  The system must build
    // a separate invoker for each distinct signature.
    static Object interpret_L(MethodHandle mh) throws Throwable {
        Object[] av = {mh};
        String sig = null;
        assert(argumentTypesMatch(sig = "L_L", av));
        Object res = mh.form.interpretWithArguments(av);
        assert(returnTypesMatch(sig, av, res));
        return res;
    }
    static Object interpret_L(MethodHandle mh, Object x1) throws Throwable {
        Object[] av = {mh, x1};
        String sig = null;
        assert(argumentTypesMatch(sig = "LL_L", av));
        Object res = mh.form.interpretWithArguments(av);
        assert(returnTypesMatch(sig, av, res));
        return res;
    }
    static Object interpret_L(MethodHandle mh, Object x1, Object x2) throws Throwable {
        Object[] av = {mh, x1, x2};
        String sig = null;
        assert(argumentTypesMatch(sig = "LLL_L", av));
        Object res = mh.form.interpretWithArguments(av);
        assert(returnTypesMatch(sig, av, res));
        return res;
    }
    private static LambdaForm getPreparedForm(String sig) {
        MethodType mtype = signatureType(sig);
        LambdaForm prep =  mtype.form().cachedLambdaForm(MethodTypeForm.LF_INTERPRET);
        if (prep != null)  return prep;
        assert(isValidSignature(sig));
        prep = new LambdaForm(sig);
        prep.vmentry = InvokerBytecodeGenerator.generateLambdaFormInterpreterEntryPoint(sig);
        return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_INTERPRET, prep);
    }

    // The next few routines are called only from assert expressions
    // They verify that the built-in invokers process the correct raw data types.
    private static boolean argumentTypesMatch(String sig, Object[] av) {
        int arity = signatureArity(sig);
        assert(av.length == arity) : "av.length == arity: av.length=" + av.length + ", arity=" + arity;
        assert(av[0] instanceof MethodHandle) : "av[0] not instace of MethodHandle: " + av[0];
        MethodHandle mh = (MethodHandle) av[0];
        MethodType mt = mh.type();
        assert(mt.parameterCount() == arity-1);
        for (int i = 0; i < av.length; i++) {
            Class<?> pt = (i == 0 ? MethodHandle.class : mt.parameterType(i-1));
            assert(valueMatches(basicType(sig.charAt(i)), pt, av[i]));
        }
        return true;
    }
    private static boolean valueMatches(BasicType tc, Class<?> type, Object x) {
        // The following line is needed because (...)void method handles can use non-void invokers
        if (type == void.class)  tc = V_TYPE;   // can drop any kind of value
        assert tc == basicType(type) : tc + " == basicType(" + type + ")=" + basicType(type);
        switch (tc) {
        case I_TYPE: assert checkInt(type, x)   : "checkInt(" + type + "," + x +")";   break;
        case J_TYPE: assert x instanceof Long   : "instanceof Long: " + x;             break;
        case F_TYPE: assert x instanceof Float  : "instanceof Float: " + x;            break;
        case D_TYPE: assert x instanceof Double : "instanceof Double: " + x;           break;
        case L_TYPE: assert checkRef(type, x)   : "checkRef(" + type + "," + x + ")";  break;
        case V_TYPE: break;  // allow anything here; will be dropped
        default:  assert(false);
        }
        return true;
    }
    private static boolean returnTypesMatch(String sig, Object[] av, Object res) {
        MethodHandle mh = (MethodHandle) av[0];
        return valueMatches(signatureReturn(sig), mh.type().returnType(), res);
    }
    private static boolean checkInt(Class<?> type, Object x) {
        assert(x instanceof Integer);
        if (type == int.class)  return true;
        Wrapper w = Wrapper.forBasicType(type);
        assert(w.isSubwordOrInt());
        Object x1 = Wrapper.INT.wrap(w.wrap(x));
        return x.equals(x1);
    }
    private static boolean checkRef(Class<?> type, Object x) {
        assert(!type.isPrimitive());
        if (x == null)  return true;
        if (type.isInterface())  return true;
        return type.isInstance(x);
    }

    /** If the invocation count hits the threshold we spin bytecodes and call that subsequently. */
    private static final int COMPILE_THRESHOLD;
    static {
        COMPILE_THRESHOLD = Math.max(-1, MethodHandleStatics.COMPILE_THRESHOLD);
    }
    private int invocationCounter = 0;

    @Hidden
    @DontInline
    /** Interpretively invoke this form on the given arguments. */
    Object interpretWithArguments(Object... argumentValues) throws Throwable {
        if (TRACE_INTERPRETER)
            return interpretWithArgumentsTracing(argumentValues);
        checkInvocationCounter();
        assert(arityCheck(argumentValues));
        Object[] values = Arrays.copyOf(argumentValues, names.length);
        for (int i = argumentValues.length; i < values.length; i++) {
            values[i] = interpretName(names[i], values);
        }
        Object rv = (result < 0) ? null : values[result];
        assert(resultCheck(argumentValues, rv));
        return rv;
    }

    @Hidden
    @DontInline
    /** Evaluate a single Name within this form, applying its function to its arguments. */
    Object interpretName(Name name, Object[] values) throws Throwable {
        if (TRACE_INTERPRETER)
            traceInterpreter("| interpretName", name.debugString(), (Object[]) null);
        Object[] arguments = Arrays.copyOf(name.arguments, name.arguments.length, Object[].class);
        for (int i = 0; i < arguments.length; i++) {
            Object a = arguments[i];
            if (a instanceof Name) {
                int i2 = ((Name)a).index();
                assert(names[i2] == a);
                a = values[i2];
                arguments[i] = a;
            }
        }
        return name.function.invokeWithArguments(arguments);
    }

    private void checkInvocationCounter() {
        if (COMPILE_THRESHOLD != 0 &&
            invocationCounter < COMPILE_THRESHOLD) {
            invocationCounter++;  // benign race
            if (invocationCounter >= COMPILE_THRESHOLD) {
                // Replace vmentry with a bytecode version of this LF.
                compileToBytecode();
            }
        }
    }
    Object interpretWithArgumentsTracing(Object... argumentValues) throws Throwable {
        traceInterpreter("[ interpretWithArguments", this, argumentValues);
        if (invocationCounter < COMPILE_THRESHOLD) {
            int ctr = invocationCounter++;  // benign race
            traceInterpreter("| invocationCounter", ctr);
            if (invocationCounter >= COMPILE_THRESHOLD) {
                compileToBytecode();
            }
        }
        Object rval;
        try {
            assert(arityCheck(argumentValues));
            Object[] values = Arrays.copyOf(argumentValues, names.length);
            for (int i = argumentValues.length; i < values.length; i++) {
                values[i] = interpretName(names[i], values);
            }
            rval = (result < 0) ? null : values[result];
        } catch (Throwable ex) {
            traceInterpreter("] throw =>", ex);
            throw ex;
        }
        traceInterpreter("] return =>", rval);
        return rval;
    }

    static void traceInterpreter(String event, Object obj, Object... args) {
        if (TRACE_INTERPRETER) {
            System.out.println("LFI: "+event+" "+(obj != null ? obj : "")+(args != null && args.length != 0 ? Arrays.asList(args) : ""));
        }
    }
    static void traceInterpreter(String event, Object obj) {
        traceInterpreter(event, obj, (Object[])null);
    }
    private boolean arityCheck(Object[] argumentValues) {
        assert(argumentValues.length == arity) : arity+"!="+Arrays.asList(argumentValues)+".length";
        // also check that the leading (receiver) argument is somehow bound to this LF:
        assert(argumentValues[0] instanceof MethodHandle) : "not MH: " + argumentValues[0];
        MethodHandle mh = (MethodHandle) argumentValues[0];
        assert(mh.internalForm() == this);
        // note:  argument #0 could also be an interface wrapper, in the future
        argumentTypesMatch(basicTypeSignature(), argumentValues);
        return true;
    }
    private boolean resultCheck(Object[] argumentValues, Object result) {
        MethodHandle mh = (MethodHandle) argumentValues[0];
        MethodType mt = mh.type();
        assert(valueMatches(returnType(), mt.returnType(), result));
        return true;
    }

    private boolean isEmpty() {
        if (result < 0)
            return (names.length == arity);
        else if (result == arity && names.length == arity + 1)
            return names[arity].isConstantZero();
        else
            return false;
    }

    public String toString() {
        StringBuilder buf = new StringBuilder(debugName+"=Lambda(");
        for (int i = 0; i < names.length; i++) {
            if (i == arity)  buf.append(")=>{");
            Name n = names[i];
            if (i >= arity)  buf.append("\n    ");
            buf.append(n.paramString());
            if (i < arity) {
                if (i+1 < arity)  buf.append(",");
                continue;
            }
            buf.append("=").append(n.exprString());
            buf.append(";");
        }
        if (arity == names.length)  buf.append(")=>{");
        buf.append(result < 0 ? "void" : names[result]).append("}");
        if (TRACE_INTERPRETER) {
            // Extra verbosity:
            buf.append(":").append(basicTypeSignature());
            buf.append("/").append(vmentry);
        }
        return buf.toString();
    }

    @Override
    public boolean equals(Object obj) {
        return obj instanceof LambdaForm && equals((LambdaForm)obj);
    }
    public boolean equals(LambdaForm that) {
        if (this.result != that.result)  return false;
        return Arrays.equals(this.names, that.names);
    }
    public int hashCode() {
        return result + 31 * Arrays.hashCode(names);
    }
    LambdaFormEditor editor() {
        return LambdaFormEditor.lambdaFormEditor(this);
    }

    boolean contains(Name name) {
        int pos = name.index();
        if (pos >= 0) {
            return pos < names.length && name.equals(names[pos]);
        }
        for (int i = arity; i < names.length; i++) {
            if (name.equals(names[i]))
                return true;
        }
        return false;
    }

    LambdaForm addArguments(int pos, BasicType... types) {
        // names array has MH in slot 0; skip it.
        int argpos = pos + 1;
        assert(argpos <= arity);
        int length = names.length;
        int inTypes = types.length;
        Name[] names2 = Arrays.copyOf(names, length + inTypes);
        int arity2 = arity + inTypes;
        int result2 = result;
        if (result2 >= argpos)
            result2 += inTypes;
        // Note:  The LF constructor will rename names2[argpos...].
        // Make space for new arguments (shift temporaries).
        System.arraycopy(names, argpos, names2, argpos + inTypes, length - argpos);
        for (int i = 0; i < inTypes; i++) {
            names2[argpos + i] = new Name(types[i]);
        }
        return new LambdaForm(debugName, arity2, names2, result2);
    }

    LambdaForm addArguments(int pos, List<Class<?>> types) {
        return addArguments(pos, basicTypes(types));
    }

    LambdaForm permuteArguments(int skip, int[] reorder, BasicType[] types) {
        // Note:  When inArg = reorder[outArg], outArg is fed by a copy of inArg.
        // The types are the types of the new (incoming) arguments.
        int length = names.length;
        int inTypes = types.length;
        int outArgs = reorder.length;
        assert(skip+outArgs == arity);
        assert(permutedTypesMatch(reorder, types, names, skip));
        int pos = 0;
        // skip trivial first part of reordering:
        while (pos < outArgs && reorder[pos] == pos)  pos += 1;
        Name[] names2 = new Name[length - outArgs + inTypes];
        System.arraycopy(names, 0, names2, 0, skip+pos);
        // copy the body:
        int bodyLength = length - arity;
        System.arraycopy(names, skip+outArgs, names2, skip+inTypes, bodyLength);
        int arity2 = names2.length - bodyLength;
        int result2 = result;
        if (result2 >= 0) {
            if (result2 < skip+outArgs) {
                // return the corresponding inArg
                result2 = reorder[result2-skip];
            } else {
                result2 = result2 - outArgs + inTypes;
            }
        }
        // rework names in the body:
        for (int j = pos; j < outArgs; j++) {
            Name n = names[skip+j];
            int i = reorder[j];
            // replace names[skip+j] by names2[skip+i]
            Name n2 = names2[skip+i];
            if (n2 == null)
                names2[skip+i] = n2 = new Name(types[i]);
            else
                assert(n2.type == types[i]);
            for (int k = arity2; k < names2.length; k++) {
                names2[k] = names2[k].replaceName(n, n2);
            }
        }
        // some names are unused, but must be filled in
        for (int i = skip+pos; i < arity2; i++) {
            if (names2[i] == null)
                names2[i] = argument(i, types[i - skip]);
        }