OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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the stack challenging in some cases. As a result, the less than relationship is replaced
with a greater than relationship by switching the order of the values once they are
already present on the operand stack. This is accomplished using a swap bytecode.
Once the order of the values has been swapped, the greater than relationship can be
used in order to achieve the same result as the original less than relationship. This
is illustrated in Figure 4.6 which shows the sequence of bytecodes before and after
branch despecialization is performed. A similar transformation can be employed in
order to express uses of the less than or equal relationship in terms of the greater
than or equal relationship.
4.1.6 Widening Despecializations
Widening despecializations increase the number of bytes that are used to represent an
integer value. For example, bipush is normally used to generate most constant values
between -128 and 127. The value that is placed on the stack is dictated by a single
operand byte that immediately follows the opcode. The sipush opcode makes use of
two operand bytes in order to facilitate generating any constant value between -32768
and 32767. As a result, replacing occurrences of bipush with sipush is classified as a
widening despecialization because the number of bytes used to represent the constant
has increased, or been widened, from one byte to two bytes.
Despecializing occurrences of sipush by replacing them with either ldc or ldc w
is also an example of a widening despecialization. In this case, the number of operand
bytes that appear in the code stream is not increased. In fact, if the constant pool
contains less than 256 elements, allowing an ldc bytecode to be used for despecialization,
the number of operand bytes in the code stream actually decreases by one.
However, the constant value in the constant pool is expressed using four bytes regardless
of its magnitude. As a result, replacing sipush with either ldc or ldc w is a
widening despecialization because the number of bytes used to express the constant
value that is being loaded has increased from two bytes to four bytes.
It is also possible to despecialize all occurrences of the ldc bytecode. The ldc w
bytecode is used in its place. This is a widening despecialization because ldc uses
a single operand byte in order to specify an offset into the constant pool between 1
and 255 1 . The ldc w bytecode uses two operand bytes in order to specify its offset
into the constant pool. As a result, it can be used to access all of the slots normally
1 The Java Virtual Machine Specification requires that constant pool entry 0 remain unused with
the intent that this location store a pointer to virtual machine implementation specific data. As a
result, ldc will never refer to position zero in the constant pool.