OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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to access local variables rather than method parameters. In almost all cases, these
are programmer declared variables. While these local variables could be temporaries
introduced during compilation, such temporaries are uncommon in Java because the
operand stack is normally used to hold such values.
The four general purpose bytecodes that appear among the 10 most executed
load bytecodes are iload 0x08 at position 4, iload 0x05 at position 6, iload 0x0a
at position 8 and iload 0x04 at position 10. These bytecodes are frequently executed
because they all typically represent programmer declared variables rather than
method parameters. The iload 0x04 bytecode will only be used to access a method
parameter in the 4 percent of methods invoked that have 5 or more parameters. The
remaining general purpose bytecodes are used to access method parameters in an
even smaller proportion of the methods invoked. As a result, it is known that these
bytecodes are being used to access programmer declared local variables most of the
time.
Continuing this analysis further reveals that the majority of the bytecodes that
appear in the top 40 load instructions are typically being used to access programmer
declared local variables. This has occurred because programmers generally write
code that accesses the method parameters a small number of times. Local variables
are then use to complete the computation. Generally, the number of local variable
references required to complete the computation is much larger than the number of
references used to access the parameter values, resulting in the behaviour described
here. The exceptions to this observation are the aload 0, aload 1 and iload 1
bytecodes noted already and the dload 0 and dload 1 bytecodes which appear among
the most executed load bytecodes in positions 23 and 25 respectively.
Further analysis of the dload 0 and dload 1 bytecodes reveals that they are primarily
contributed by a small number of benchmarks. Of these, the Java Grande
Forum Series benchmark contributes by far the most, with over 9 percent of its bytecodes
executed being dload 0. This benchmark also executes dload 1 frequently,
with this bytecode representing nearly 7 percent of the instructions executed by the
benchmark.
An informal analysis revealed that most of the dload 0 bytecodes executed were
used for parameter loading in static mathematical methods such as Math.abs, Math.-
pow and Double.isInfinite. These method invocations were the result of the fact
that the Series benchmark performed a 1000 segment trapezoidal integration of a
function many times. The frequent use of dload 1 is also for parameter access. In
this case, it is primarily the result of the need to repeatedly compute the value of the