Just-In-Time (JIT) compiler is valuable Java related technology that dynamically compiles selected bytecode methods of executing Java applications into native (binary code) methods, thus greatly reducing the overall execution time of such applications.  IBM's JIT compilers provided with various configurations of J9VM are all based on the same advanced  technology adapted for effective use with Java ME CLDC, Java ME CDC, Java SE, and Java EE configurations of J9VM.  Specifically, the "embedded" JIT compiler used in Apogee's JREs does not implement, due to limited memory and CPU resources of targeted devices, the same set of optimizations that are implemented  in the "desktop" JIT compiler, and does not include certain non-essential features that tend to increase the runtime footprint.  However, this embedded JIT still implements all of the "high payoff" optimizations, which means that it typically achieves the 5 to 8 times runtime speed-ups of compiled Java applications compared with interpretive execution of such applications, while having the minimum overhead and small runtime footprint.

IBM JIT compiler technology is highly configurable and tunable for achieving the highest possible speed-ups of Java applications when subjected to JIT compilation. Specifically, it allows:

• Turning on/off the individual optimizations.
• Controlling of the amount of memory allocated for compiled data.
• Instructing the compiler which bytecode methods are to be compiled.
• Instructing the compiler which bytecode methods should not be compiled.
• Instructing the compiler when the compile methods based on the number of invocations.
  
• Tracing and debugging the JIT execution
• Compiling the bytecode methods on the application or separate thread.
  

JIT behavior is controlled via -Xjit[:opt=val] directive, where available options include:

Option	Description
bcount=	number of calls before compiling methods with loops
count=	number of calls before compiling methods with no loops
disableArrayCopyOpts	disable array copy optimiations
disableDirectMemoryOps	disable generation of direct memory instructions
disableDirectToJNI	disable direct calls to JNI
disableFPCodeGen	disable floating point code generation
disableFPEmulation	disable floating point emulation
disableGRA	disable global register allocator
disableInlining	disable inlining
disableLocalCSE	disable local common subexpression elimination
disableLocalVP	disable local value propagation
disableScheduling	disable instruction scheduling
disableTreeSimplification	disable tree simplification
enableCompilationThread	enable compilation on a separate thread
help	display help information
bcLimit=	maximum bytecode size of the method
codetotal=	maximum size of memory (KB) allocated for compiled data
exclude=	do not compile methods beginning with XXX
limit=	only compile methods beginning with XXX
verbose	write compiled method names to stdout