OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

50
...
i − 1
iconst 0 istore 2 iconst 4 istore 3
i i + 1 i + 2 i + 3
...
i + 5
(a) Before Integer Constant Despecialization
...
i − 1
bipush 0x00 istore 2 bipush 0x04 istore 3
i i + 1 i + 2 i + 3 i + 4 i + 5
(b) After Integer Constant Despecialization to bipush
...
i + 6
...
i − 1
sipush 0x00 0x00 istore 2 sipush 0x00 0x04 istore 3
i i + 1 i + 2 i + 3 i + 4 i + 5 i + 6 i + 7
(c) After Integer Constant Despecialization to sipush
...
i + 8
...
i − 1
ldc 0xff istore 2 ldc w 0x01 0x00 istore 3
i i + 1 i + 2 i + 3 i + 4 i + 5 i + 6
(d) After Integer Constant Despecialization to ldc / ldc w
...
i + 7
Figure 4.3: An Example of Integer Constant Despecialization
the operand stack. These operations are at least as expensive as the cost of copying
the single operand byte that represents the constant directly from the code stream.
Figure 4.3a shows a block of code that assigns two constant integer values to
two local variables before integer constant despecialization is performed. The constant
values are generated using the specialized iconst bytecodes. Figure 4.3b
shows the code stream when the specialized iconst bytecodes are replaced with
bipush. The alternative despecializations that could have been utilized are presented
in Figure 4.3c and Figure 4.3d. These figures show the bytecode stream when the
specialized integer constant loading bytecodes are replaced with sipush and ldc or
ldc w respectively. Note that Figure 4.3d assumes that the integer constant value 0