OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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constant pool due to a previous despecialization of bipush and the value resides at
an index less than 256. When these constraints are met, replacing bipush with ldc
does not increase the size of the code stream because both bipush and ldc require
exactly two bytes: one for the opcode and one for the operand.
A similar situation can also occur for sipush. In this instance, it is actually
possible that performing despecialization will decrease the size of the class file. A one
byte decrease will occur when the value being loaded already exists in the constant
pool at an index less than 256. This occurs because despecializing sipush to ldc
reduces the number of operand bytes that are required in the code stream from 2
bytes to 1 byte. In other instances, despecializing sipush will have no impact on
class file size. This occurs when the value resides in the constant pool at an index
greater than 255. Consequently, sipush must be despecialized to ldc w which also
requires two operand bytes.
In rare cases, despecialization will not increase class file size when bytecodes other
than bipush and sipush are despecialized. This can occur when the despecializations
are only performed on bytecodes that occur ahead of a lookupswitch or tableswitch
bytecode that includes one or more pad bytes. When the despecialization is performed,
the number of pad bytes is reduced in order to ensure that the alignment
rules for the lookupswitch or tableswitch are still maintained, resulting in no
change in class file size. However, in other cases, performing despecialization can
require the introduction of pad bytes that were not required previously, potentially
increasing the size of the class file by 3 bytes beyond the number of additional bytes
introduced at the location within the code stream that is being despecialized.
The impact despecialization had on class file size was measured. Complete despecialization
was performed on all of the classes in the libraries provided by each of the
virtual machines. Complete despecialization was also performed on all of the class
files in the SPEC JVM98 Benchmark Suite except one. The file install.class was
omitted from consideration because it includes a large non-standard attribute that
can be used to install the benchmark suite. Including this file would have resulted in
smaller class file size increase values.
A summary of the class file size increase values is presented in Table 4.10. It shows
the change in class file size observed for the classes that make up the SPEC JVM98
Benchmarks and the class files that make up the class libraries associated with each
of the virtual machines. The first three columns of data report the average class
file size observed before and after despecialization was performed, and the percentage
change this difference represents. The final three columns of data examine the impact