OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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based on these profile results will also decrease class file size by utilizing one byte to
represent those loads and stores that occur with greatest frequency and two bytes to
represent loads and stores that are performed less frequently.
Researchers and developers creating new intermediate representations and virtual
machines should also consider these profiling results. In particular, these results
indicate that while the Java Virtual Machine Specification provides a symmetric set
of load and store bytecodes, the behavior of Java applications is not symmetric. The
symmetries present that do not reflect the reality of the applications tested include:
• Type Symmetry: The virtual machine specification provides 8 specialized
load / store bytecodes for each of the 5 primitive data types supported by the
virtual machine. However, the profile data collected shows that integer values
are loaded and stored with great frequency while floating point and long integer
variables are rarely used.
• Slot Symmetry: Specialized load and store bytecodes are provided for slots
0 through 3 for each data type. However, Java applications execute a large
number of instance methods making accesses to slot 0 with object reference type
far more common that accesses to this slot for other data types. The utilization
of dload and lload bytecodes is also negatively impacted by this
symmetry. Because doubles and long integers each occupy two adjacent slots
in the local variable array, it is impossible for any method to make use of more
than 2 of the specialized category 2 load bytecodes.
• Load / Store Symmetry: The profile data collected during this study shows
that applications perform many more loads than stores. Consequently, providing
the same number of specialized load and store bytecodes is illogical. Instead,
more specialized bytecodes should be devoted to performing loads than stores
since loads execute with greater frequency.
Based on these results, the author concludes that future intermediate representations
and virtual machines should have an asymmetric instruction set. If specialized bytecodes
are included, the number of load bytecodes should exceed the number of store
bytecodes and those specialized bytecodes should only be provided for those slots and
data types that are expected to be used frequently.
The concept of multicodes was introduced in Chapter 7. A detailed discussion
was presented which outlined the gains that can be achieved by replacing frequently
executed sequences of bytecodes with a single new multicode. A profiling technique