OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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information for each method so that it was not necessary to parse the method every
time it was invoked. A class was introduced to represent the fully qualified class name
as three pointers – one to the string representing the class name, one to the string
representing the method name and one to the string representing the signature. This
reduced the amount of space required to represent each fully qualified method name
to 12 bytes since the pointers pointed to copies of the names maintained within other
Kaffe data structures. Using this technique also meant that comparison between
method names could be performed using pointer comparisons instead of expensive
string operations.
Before each bytecode was executed, a check was performed in order to determine
if that bytecode represented the start of a new multicode block. This was determined
by searching for its program counter value in the branch targets map entry for the
current method. If it was determined that the bytecode started a new multicode block
then the count associate with the current bytecode sequence was incremented and the
vector representing the sequence was cleared. Once the checks associated with the
start of a new block were performed, the bytecode that was about to execute was
appended to the vector representing the current multicode block.
An exit handler was used in order to ensure that the information recorded during
profiling was saved before the application completed. In addition, the exit handler
was responsible for incrementing the count associated with the last multicode block
executed. The output generated was referred to as a compressed multicode block file.
Its format was text based, with each line beginning with a count of the number of
times the block was executed. This was followed by a space separated list of bytecode
names representing the bytecode sequence.
Once the data was collected it was evaluated using an application that implemented
the multicode identification algorithm presented in Chapter 7. This tool
implemented both the total bytecodes replaced and transfer reduction scoring systems.
In addition, it also implemented the timing based scoring system described
in Section 7.8. The scoring system, number of multicodes to identify and maximum
multicode length to consider are all provided to the application as command line parameters.
The output of the application is a rule file in the format required by the
class file mutator.