OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

52
...
i − 1
fconst 1 fstore 1 dconst 4 dstore 2
i i + 1 i + 2 i + 3
...
i + 5
(a) Before Non-Integer Constant Despecialization
...
i − 1
ldc 0x20 fstore 1 ldc2 w 0x00 0x1e dstore 2
i i + 1 i + 2 i + 3 i + 4 i + 5 i + 6
...
i + 7
(b) After Non-Integer Constant Despecialization to ldc / ldc w
Figure 4.4: An Example of Non-Integer Constant Despecialization
4.1.5 Branch Despecialization
Two different series of integer comparison conditional branch bytecodes are included
as part of the Java Virtual Machine Specification. One set is used to compare an
integer value to zero. The branches in this set are named ifeq, ifne, ifle, iflt,
ifgt, and ifge. The last two letters indicate what relationship is being tested. For
example, ifeq indicates that the branch is testing for equality between the integers.
The symbol will be used to represent any one of the two letter abbreviations
that appear at the end of the bytecode names. Consequently, an arbitrary branch
bytecode in this set will be referred to as if.
The second set of integer comparison bytecodes provided by the Java Virtual
Machine Specification is used to compare two arbitrary integer values. Both of the
required values are popped from the stack, tested using the appropriate relational
operator and then a branch is performed if the condition being tested is true. The
bytecodes in this set follow the naming pattern if icmp.
Given these two sets of branch bytecodes, it is possible to despecialize the first
set by expressing their functionality using the second set. This is accomplished by
explicitly placing the zero value being tested on the stack. Initially this value is
generated with an iconst 0 bytecode. When both integer constant despecialization
and branch despecialization are performed, the branches are despecialized first so
that all iconst 0 bytecodes generated during that branch despecialization process
are subsequently despecialized into bipush bytecodes.
Figure 4.5a shows the block of code representing a statement of the form if (a
< 0) ... before any despecialization has been performed. It is shown again once