OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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are never executed by any of the benchmarks. The fifth bytecode, lstore 2, is
executed extremely infrequently, accounting for less than 1 out of every 1 million
bytecodes executed. The presence of three store bytecodes that access slots 0 and 1
in this list is the result of the pass by value semantics used for all method parameters
of primitive type as described previously. The presence of lstore 2 and fstore 2 in
this list is due to the relatively infrequent use of the long and float data types as
described previously.
These results indicate that future virtual machine designers should eliminate both
the symmetry that exists across data types accessed by load and store bytecodes and
the symmetry that exists in the slot numbers between the load and store bytecodes.
Future instruction sets should include more specialized loads than specialized stores
and both loads and stores should be concentrated on those data types executed with
greatest frequency. The results of this study indicate that integers should receive
the greatest attention in this regard with additional specialized bytecodes also being
allocated for object references and doubles. Specialized loads and stores for floats and
longs are generally executed infrequently leading to the recommendation that few, if
any, specialized bytecodes be allocated for handling these data types.
6.2.3 Constant Loading
There are three general mechanisms provided by the Java Virtual Machine that allow
a constant value to be placed on the operand stack. One option is to use a specialized
bytecode that pushes a specific value dictated by the opcode employed. A second
option is to load a value from the constant pool onto the operand stack. Finally, it is
also possible to generated integer constants within a limited range using the bipush
and sipush bytecodes. These bytecodes read one or two operand bytes directly from
the code stream which represent the constant value to be placed on the stack. Table
6.1 lists the 40 most frequently used constant values and the manner in which each
was placed on the stack.
It is interesting to note the six most frequently executed constant loading bytecodes
are specialized. The remaining four bytecodes in the 10 most frequently executed
constant loading bytecodes are general purpose. These include 4 specialized
integer loads and 2 specialized double loads, leaving 3 general purpose integer load
bytecodes and one general purpose long integer load in the top 10. This result indicates
that the choices made for what constant values should be handled by a specialized
bytecode appear to have been better than the corresponding choices made for