OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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difference in performance due to despecialization. These results are contrary to what
was initially expected before work in this area was conducted, as it was assumed
that specialized bytecodes existed because they significantly improved application
performance.
When this unexpected behaviour was observed, the architecture of the Java Virtual
Machine was examined. Considering its architecture revealed that there are at
least two reasons why this initially unanticipated behaviour occurred:
Optimization: Modern Java Virtual Machines perform a variety of advanced
optimizations. This includes making use of a just in time compiler
which converts Java bytecodes to architecture specific machine language
instructions at runtime. It is the use of this optimization strategy which
contributes to the minimal impact despecialization has on application performance.
Many of the despecializations described previously are performed by replacing
an instruction that does not require any operands with a general
purpose bytecode that explicitly specifies a value as an operand. This
operand was necessary when the specialized bytecode was employed because
the necessary value was dictated by the definition of the specialized
instruction. When JIT compilation is performed, the general purpose
bytecode is converted to one or more machine language instructions. The
operand bytes are decoded during the compilation process and the values
they represent are used directly. Consequently, it is not necessary
to decode the operand bytes again once JIT compilation has been performed.
Similarly, the constant value dictate by the specialized bytecode
can also be used directly during the JIT compilation process. Consequently,
the JIT compiler is able to generate identical machine code for
both the specialized bytecode and its equivalent general purpose form.
For example, the machine code generated for the specialized instruction
iload 0 is identical to the machine code generated for the general purpose
bytecode iload 0x00. As a result, no difference in execution performance
should be observed once JIT compilation has been performed.
Execution Frequency: Examining the frequency of execution of the
specialized bytecodes reveals the second reason that some specialized bytecodes
have little impact on application performance. The profiling conducted
for this study revealed that the specialized bytecodes dstore 0,