JamVM is a new Java Virtual Machine conforming to the JVM specification edition 2 (blue book). It is extremely small - stripped on PowerPC ~110K, Intel 80K. However, unlike other small VMs it supports the full spec, inc. object finalisation and JNI.
A summary of changes since 1.5.3: - GC changes: - Mark phase rewritten to use an explicit fixed-size mark stack, with a moving heap scan pointer. The previous recursive marking could lead to stack overflow when marking complex, deeply-linked structures. If the explicit mark stack overflows, a slower, fallback mechanism is used of scanning the heap. The new mark phase is upto 50% faster than the old recursive marking. - bug fix for unallocated blocks within the heap of >= 1 GB. - JNI changes: - Updated to JNI version 1.6. This adds the function GetObjectRefType(). - Fully implemented JNI weak global references (NewWeakGlobalRef and DeleteWeakGloablRef). Note, these are different to Java Soft/Weak/Phantom References which have been supported since JamVM 1.4.0. - verbose:jni now shows details of the opening of native libraries. If a library fails to open a diagnostic message is shown (if available). This is intended to help debug common library problems. - bug fix for ToReflected[Method|Field] and GetSuperClass. A local reference must be created for the return value. - Platform changes: - Interpreter inlining (aka code-copying JIT) now enabled by default on ARM systems. Testing on Cortex-A8 indicates 82% speed improvement on integer benchmarks, and 55% on floating-point. - If cross-compiling, and interpreter inlining is enabled, runtime relocation checks will be enabled (relocation information is normally generated at compile time; as this can't be done when cross-compiling it must be done at runtime, but it increases the size of the executable by ~30%). - Initial port to Sparc/FreeBSD. My thanks to Per Ola Ingvarsson for architecture dependent definitions. Interpreter inlining (aka code-copying JIT) is currently not supported due to missing code for flushing the instruction/data caches, and branch generation. It also requires libffi, which is enabled by default. - Changes to x86 and x86_64 across all platforms to zero/sign extend return values from native methods whose size is less than an int (i.e. boolean, byte, char and short). This is necessary due to changes in code produced by gcc >= 4.3. - Rare race-condition in thin-locking code on x86 and x86_64 architectures, leading to deadlock. On modern x86 CPUs, an extra memory barrier is required. Seen intermittently while running a thread intensive benchmark using at least 4 cores. - Extensive changes to support 64-bit Big Endian systems. These were previously supported, but support was completely broken with the introduction of the new object layout in JamVM 1.5.2. - Miscellaneous fixes: - Interpreter inlining bug fix: reference to memory after it had been freed - java.lang.reflect.VMField getAnnotation() implemented. This has been missing since the reflection rework in JamVM 1.5.2. - ThreadMXBean: fixes for getThreadInfoForId (VMThreadMXBeanImpl). - Did not correctly report lock or lock owner, when the thread was blocked on an object which was thin-locked by another thread - It was using a constructor which has since been removed - Fixes for NULL handling in findClassFromSignature() and utf8CharLen() - Fixed memory leak in bytecode rewriting. Due to missing parantheses, old bytecode stream was not being freed (for example, this leaks 47K when running "Hello, World") - Bug-fix for theoretical race-condition in thread deletion when notifying joining threads after thread ID has been reused.