OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

53
...
iload 3
iflt
...
i − 1
i i + 1
i + 2
(a) Before Despecialization
...
iload 3
iconst 0 if icmplt
...
i − 1
i i + 1 i + 2
i + 3
(b) After Despecializing if Bytecodes
...
iload 0x03 bipush 0x00 if icmplt
...
i − 1
i i + 1 i + 2 i + 3 i + 4
i + 5
(c) After Despecializing Load, Integer Constant and if Bytecodes
Figure 4.5: An Example of Branch Despecialization
branch despecializations have been performed in 4.5b. Finally, Figure 4.5c shows the
statement after both integer constant despecialization and branch despecialization
have been performed.
Performing this despecialization increases the size of the class file by one byte for
each static despecialization performed, with further size increases occurring if integer
constant despecialization is also performed. This despecialization also introduces an
extra element onto the operand stack. As a result, all methods in a class file that
had branch despecialization performed on them had their max stack value increased
by one in order to ensure that there was sufficient space on the runtime stack for the
additional zero value. Note that it is not necessary to increase the max stack value
by one for each static occurrence of a branch bytecode despecialized in this manner
because the extra zero value added to the stack by iconst 0 is consumed immediately
by the if icmp bytecode, making it impossible for this transformation to
increase the maximum stack height reached within the method by more than one.
A second type of branch despecialization was also considered. It reduced the number
of if icmp bytecodes from the six defined by the Java Virtual Machine
Specification to four. The need for two conditions can be eliminated by swapping the