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A library for Java to C/CUDA translator based on deep reification
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by Shigeru Chiba


Bytespresso-C is a translator from Java bytecode into C or C-like languages such as CUDA. It has been designed for offloading part of the execution of Java code onto external hardware such as GPU, a PC cluster, and a supercomputer. Since the target application domain is high-performance computing, Bytespresso-C has not been designed for translating a Java program largely exploiting object orientation. The best platform for such a program is the JVM. Bytespresso rather focuses on parallel numerical computing written with arrays but constructed on top of an object-oriented framework.

Bytespresso-C is similar to a JIT compiler. It dynamically translates the bytecode of a specified method in the running program (and the methods directly or indirectly invoked by that method). The C code generated after the translation is compiled by an external compiler and executed in a separate process. JNI (Java Native Interface) is not used. The resulting value is sent back to the Java code through the Unix pipe. Since translation is done at runtime, some optimization techniques are applied to the code generation. They include constant propagation and devirtualization.

Running examples

To compile the source code, include javassist.jar (and JUnit) in the class path. The option -Djdk.internal.lambda.dumpProxyClasses=./bin (if ./bin is a directory included in the class path) must be given to the JVM.

Examples are in src/test/array. is a program of N-body simulation. When calling main in Nbody, it generates C code (bytespresso.c), comiles it, and runs it. The default compiler is cc and it is supposed to generate a.out as executable binary. To change the configuration, see the Javadoc comments in javassist.offload.javatoc.StandaloneDriver.

The examples also include and The former one runs with MPI and the latter one runs with CUDA. See and, which implement embedded DSLs for processing vectors. They launch compilers through javassist.offload.lib.MPIDriver and CudaDriver.

Under src/test/npbench3cg, a Bytespresso version of the CG program in the NAS Parallel benchmark suite is found. For details, see src/test/npbench3cg/

How to translate and run a Java method

The packages the users should look at are:

  • javassist.offload
  • javassist.offload.lib
  • javassist.offload.javatoc

The other packages are for the implementation of the translator.

The following code translates a lambda function passed to into C and execute it:

import javassist.offload.javatoc.DriverException;
import javassist.offload.javatoc.StdDriver;
import javassist.offload.lib.Util;

public class DSL {
    public static void main(String[] args) throws Exception { -> {

    public static void hello(String m) {
        Util.print("Hello ").print(m).println();

    static void run(Runnable code) throws DriverException {
        new StdDriver().invoke(code);

The run method above is used only for giving a good look to the source code. So the following version of main is equivalent to the main method above:

    public static void main(String[] args) throws Exception {
        new StdDriver().invoke(() -> {

StdDriver also provides invoke methods taking an instance of java.util.function.IntSupplier or DoubleSupplier, which returns a resulting value. They execute the given lambda function and return the result.

The code translated into a C program is the body of the function given to the invoke method in StdDriver. The hello method and other methods called directly or indirectly from the function are also translated into the C program. StdDriver collects all but only necessary methods that will be invoked while the function given to invoke is running. They are translated and included in the generated C program.

The program above can be compiled by a normal Java compiler and run on the normal Java VM although the JVM option shown in Section "Java Lambdas" is needed. The Java program running on the JVM dynamically generates a source program in C, compiles it, and executes the compiled binary. Since the generated C program is compiled by an external C compiler, the external compiler (the cc command by default) has to be available. The compilation command can be changed by calling setCompiler method in StdDriver.

If the translated code is run in a distributed environment with MPI, MPIDriver should be used instead of StdDriver. It runs the translated code by mpiexec command, that is, it runs multiple copies of the translated code on different distributed nodes. The translated code is executed without communicating with the JVM where MPIDriver is running. So the return value from the translated code is not sent back from the invoke method in MPIDriver. It is discarded.

MPIDriver provides compileOnly method. If this method is called, it translates a specified method and generates the C code but it does not compile or execute the C code. This option is convenient if the target hardware is a supercomputer, where the executable code must be submitted to the job queue for execution and it is not immediately executed.

How to write a translated Java method

From the viewpoint of the semantics, the Java method specified for the translation is executed in a separate new environment as if it is a remote method and executed through Java RMI (Remote Method Invocation). All the values of the static fields referred to during the execution are copied to the new environment as well as the arguments to the method. The copying is "deep copy" at the translation time. Any references do not point to objects in the original environment. The updates in the new environment are never reflected on the original environment after the execution. Only the return value is copied back to the original environment. Furthermore, the translation assumes that all the classes necessary for the execution in the new environment are statically known. No unknown new class will be loaded by reflection API. So the translator can statically find a "leaf class", which is a class that is not inherited by any subclass. This helps devirtualization. Not only a final class but also a non-final class may be a leaf class.

Currently, the translated Java method must be subject to the following restrictions:

  • Only conservative garbage collection is available if it is explicitly specified (see doGarbageCollection() in javassist.offload.javatoc.StdDriver). Otherwise, all objects must be explicitly deallocated.

  • No exception is thrown. NullPointerException and ArrayIndexOutOfBoundsException are ignored and may cause a segmentation fault. A try-catch statement cannot be used.

  • Standard Java API is not available.

    • Only charAt and length methods are available on a String object. String concatenation by + is not available.
    • System.out.println() or System.err.println() are not available. Use javassist.offload.lib.Util.print().
  • The value of a field annotated with @Final will not change during the execution of the translated method. It can be updated at any time before the translation. This annotation is used for constant propagation during the translation.

Interaction between Java and C

If the method translated into C calls a static method annotated with @Remote, the call is translated into a remote method call. The called method annotated with @Remote is executed by the JVM where the StdDriver is running. If the MPIDriver is used for the translation, the @Remote method is executed by the JVM launched on the local node (every MPI node launches a different instance of the JVM). Since the@Remote method runs on the JVM, it can exploit all the features of Java, including the standard Java API, exceptions, and garbage collection. The @Remote method enables a "callback" from C to Java.

In the current implementation, the types of the parameters and the return value of the @Remote method must be a primitive type, an array type of primitive type, or java.lang.String. The other types are not supported. This restriction is also applied to the return type of the method invoked by the StdDriver or the MPIDriver. The type must be a primitive type and so on.

Using native language constructs available in C

To maximize the execution performance, native languages constructs of C are available from the translated code. For example, a @Native method can be easily implemented.

@Native("fprintf(stdout, \"%d\", (int)v1); return 0;")
public static Util print(int i) {
    return null;

This method takes one integer parameter. If this method is called on the JVM, the body of this method written in Java is executed. On the other hand, if it is called from the C code generated after the translation, the C code given as the argument to @Native is executed. It is translated into the code similar to the following function in C:

struct Util* Util_print(int v1) {
    fprintf(stdout, "%d", (int)v1);
    return 0;

The first parameter to the function is v1, the second one is v2, ... A primitive type in Java is translated into the corresponding primitive type in C.

Another kind of native method is a static method annotated with @Foreign. The function body is not given to such a native method. Bytespresso-C assumes that there is a library function with the same name as the @Foreign method. For example, is a @Foreign method. If it is called, the library function free in C is called. Since garbage collection is not available, all the objects created by new in the translated code must be deallocated by this free method after they become garbage.

Besides native methods, Bytespresso-C provides compile-time reflection or a compile-time metaobject protocol. A metaclass is specified by the @Metaclass annotation attached to the class declaration. For example, the metaclass of javassist.offload.lib.MPI.Request class is NativeClass. The MPI.Request class in Java implements a native type MPI_Request in C. The translation of the code related to MPI.Request is controlled by the NativeClass metaclass. The MPI.Request class is translated into a struct type holding a value of type MPI_Request. A pointer to that value can be obtained by toNativeBody method in javassist.offload.lib.Unsafe.

Another example of class with a non-standard metaclass is Float2Array class in javassist.offload.lib. This class is translated into a two-dimensional float array in C, i.e. float a[][], if the array size is statically determined. Bytespresso-C determines it by data-flow analysis. Note that the use of multi-dimensional array often gives better chance of optimization to a C compiler. If a Float2Array object is constructed on the JVM and passed to the translated code through an argument or a static field, the array size is statically determined and the object is translated into a two-dimensional float array in C. If the object is accessed through a pointer but the data-flow analysis cannot statically determine which object that pointer points to, then the Float2Array object is accessed as a one-dimensional array of float.

ImmutableClass is also a metaclass. A class with this metaclass is an immutable class; An instance of such a class is an immutable object. An immutable class:

  • contains only final fields.
  • does not recursively contain a field of that immutable class.
  • does not extend a superclass except java.lang.Object but may implement an immutable interface.
  • is not extended by a subclass.
  • is not a subtype of java.lang.Object. An immutable object cannot be assigned to a variable of type java.lang.Object.
  • is not an element type of array that is passed between the JVM and the C code.

Because of these properties, an immutable object is passed by value. It is always allocated on stack memory and hence it does not have to be explicitly deallocated by free method in javassist.offload.lib.Util. The use of an immutable object is a key technique to apply object inlining and devirtualization to the translated program. A drawback of immutable objects is that the whole object is copied when they are assigned to a variable.


To be written.

Offload onto a remote machine

The execution of a Java method can be offloaded onto a remote machine. For example, if you want to use a remote machine named playground, first write the following two shell scripts:

scp bytespresso.c playground:
ssh playground "source .profile; cc -O3 bytespresso.c"

ssh playground "source .profile; ./a.out"

Here bytespresso.c is the default name of the generated source file (for CUDA, it is Then start the program using Bytespresso-C with the following VM arguments to the Java command:

-Dc.compiler=./ -Dc.exec=./

These arguments sets Java's system properties.

Java Lambdas

If a Java program is written with lambdas, the following VM arguments has to be passed to the Java command:


Here ./bin is a folder name (or a directory name) included in the class path.

Deep reification

The implementation of Bytespresso-C uses a component named deep reification (this deep-reification component is called Bytespresso. Bytespresso-C is the name of a translator using this component). It is a component for constructing an abstract syntax tree from Java bytecode. It extracts all the methods including not only a given method but also the methods directly or indirectly called by that given method. Then it constructs the abstract syntax trees of those methods. It also collects all the classes that the methods refer to and the objects accessed through static fields.

The component consists of three packages javassist.offload, javassist.offload.reify and javassist.offload.ast. The main class of the component is javassist.offload.reify.Reifier.

The following code is an example:

CtMethod cm = ...
Object[] args = { ... };
Reifier reifier = new Reifier(cm, args);
Reifier.Snapshot image = reifier.snap();

System.out.println(" -- types --");
for (TypeDef t: image.classTable.allTypes())

System.out.println(" -- functions --");
for (JMethod f: image.functionTable)
    if (f != image.function)

See reifyAndPrint method in javassist.offload.reify.Reifier for the complete code. Also see main in javassist.offload.reify.Reifier. CtMethod is a class provided by Javassist bytecode engineering toolkit (see It is similar to java.lang.reflect.Method but enables an access to bytecode.

reifier.snap() extracts a snapshot of the current execution environment that is necessary to invoke cm with arguments args. It is a self-contained minimum part of the environment. It reads the bytecode of the method specified by cm and constructs the abstract syntax tree for the bytecode under the assumption that the actual arguments are args. The returned value image contains not only the abstract syntax tree of cm but also the tress of the other methods directly or indirectly called by cm. It also contains the list of the classes and objects used for the execution of cm.

The abstract syntax tree supports Visitor pattern. All the node classes provide accept method. They also provide methods for accessing and changing child nodes.

Since snap receives the actual arguments passed to cm, it specializes the method bodies by propagating the arguments and also referring to their final fields and static final fields. Primitive type values are ignored for specialization.

As for dynamic method dispatch, snap collects all possible target classes on each call. A method call is represented by a Call node, which has calledFunction method. If there are multiple target classes, calledFunction returns a Dispatcher object. If it is determined that there is only a single target class, for example, by specialization, calledFunction returns a Function object, which represents a single method body.

Extending deep reification

The component for deep reification can be extended. By giving a constructor, a custom class-table object, a custom function-table object, and
a factory object for tree nodes, Reifier is extended to construct customized abstract syntax trees.

Reifier uses a TypeDef object for representing a type. Since TypeDef is an interface, the developers can define a new class implementing TypeDef and a class table for containing the instances of that class. If this class table is given to the constructor of Reifier, it is used when constructing abstract syntax trees. The TypeDef objects included in the tree will be instances of that new class implementing TypeDef.

A factory object given to a constructor of Reifier is an instance of FunctionMetaclass or its subclass. It is a factory of objects representing an abstract syntax tree for method. The developers can give a custom factory object for instantiating an appropriate class. Note that the classes of objects representing an abstract syntax tree for method are also specified by the @Metaclass annotation attached to the method. The factory object is used as the default option when no @Metaclass is attached to the method.

Developers' notes

src/test/javassist/offload/test/ runs tests.

What is a metaclass?

To be written.


Shigeru Chiba, YungYu Zhuang, Maximilian Scherr, "Deeply Reifying Running Code for Constructing a Domain-Specific Language", Proc. of the 13th International Conference on Principles and Practices of Programming on the Java Platform: Virtual Machines, Languages, and Tools (PPPJ'16), Article No. 1, August 2016.

Shigeru Chiba, YungYu Zhuang, Maximilian Scherr, "A Design of Deep Reification", MASS 2016 - Workshop on Modularity Across the System Stack, MODULARITY Companion'16, pp.168-171, March, 2016.

Copyright notices

Copyright (C) 2016- by Shigeru Chiba, All rights reserved.

This software is distributed under the MPL/LGPL/Apache triple license as Javassist is.

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