Compiling Java for Embedded Systems

5. Status

As of early July 1997, jc1 was able to compile a simple test program, which calculates the Fibonacci numbers (up to 36), both iteratively and recursively (which is slow!), and prints the results. No manual tinkering was needed with the assembly code generated, which was assembled and linked in with kaffe (a modified pre-0.9.1 snapshot) as the run-time engine. On a SparcStation 5 running Solaris2, it took 16 seconds to execute. In comparison, the same program dynamically compiled by Kaffe's JIT takes 26 seconds, and Sun's JDK 1.1 takes 88 seconds to run the same program.

These numbers are encouraging, but they need some context. Start-up times (for class loading and just-in-time compilation) should in all cases be fairly minor, since the execution time is dominated by recursive calls. In the jc1-compiled case, only the actually test class (calculating Fibonacci plus the main methods that prints the result) is compiled by jc1; all the other classes loaded (including the classes for I/O) are compiled by kaffe's JIT-compiler. This means there would be some slight extra speed up if all the classes were jc1-compiled. Do note that the test-program uses simple C-like features that are easy to compile: integer arithmetic, simple control structures, and direct recursion. The results cannot be directly generalized to larger programs that use object-oriented features more heavily.

The basic structure of the compiler works, but there is still quite a bit of work to do. Many of the byte-codes are not supported yet, and neither is exception handling. Only some very simple Java-specific optimizations are implemented. (Of course, jc1 benefits from the existing language-independent optimizations in gcc.)

The Kaffe VM works on most Java code. We have enhanced it in various ways, and modified the data structures to be simpler and more suitable for being emitted by the compiler.

The Java support in the gdb debugger is partially written, but there is still quite a bit to go before it is user-friendly. We have not started work on our own library implementation or source-code compiler.