OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

56
accessed with ldc. This is a widening despecialization because the number of bytes
used to store the offset into the constant pool has increased from one byte to two
bytes.
The final two bytecodes that are despecialized through widening are jsr and goto.
Each of these bytecodes requires two operand bytes which specify the displacement
of the branch within the code stream. The bytecodes jsr w and goto w perform
corresponding tasks. However, they express the displacement using four bytes. Consequently
this is a widening despecialization because the number of bytes used to
express the displacement increases from two bytes to four bytes.
4.1.7 Other Despecializations
Our final category of despecializations covers nine despecializations that have not yet
been implemented. The first three of these can be considered as a third form of branch
despecialization. This form of despecialization introduces a goto bytecode which
performs an unconditional branch. In addition, both the original branch bytecode and
its branch target are modified as shown in Figure 4.7. Making these modifications
allows those branch bytecodes that test for equality – if icmpeq, if acmpeq and
ifnull – to be despecialized by expressing their functionality in terms of inequality.
It is also possible to replace three typecast bytecodes, i2c, i2b and i2s, with
semantically equivalent sequences of two bytecodes. However it remains to be seen if
these typecast bytecode despecializations cause difficulties with the verification process
that ensures the correctness and security of Java class files loaded by the virtual
machine. The typecast bytecodes and their semantically equivalent despecialized
forms are listed as part of Table 4.4.
The iinc bytecode permits the value of a local variable to be adjusted by a small
constant integer value in an efficient manner. In its basic form, it permits any of
the first 256 local variables that have integer type to be adjusted by a value between
-128 and 127 using a single bytecode. It also supports a wide format that allows
any local variable with integer type to be adjusted by a value between -32768 and
32767. When despecialization is performed each occurrence of iinc is replaced by
the bytecode sequence iload ldc w iadd istore. Note that ldc w is used in
this sequence under the assumption that the despecializations described previously
are also being performed. If this is not the case, any of the iconst , bipush
or sipush bytecodes could be used as appropriate for the value being applied to the
local variable.