OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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fstore 0, fstore 1 and fstore 2 are never executed by any of the
benchmarks that were profiled. An additional 4 bytecodes each represent
less than one in a million bytecodes executed. In comparison, if each
of the bytecodes defined by the Java Virtual Machine Specification was
executed with equal frequency it would be expected that each would represent
one out of every 200 bytecodes. Consequently, unless the performance
difference between an infrequently executed specialized bytecode and its
equivalent general purpose form is very large, no change in application
performance will be observed.
While the JIT compiler is able to minimize the impact despecialization has on
application performance, the presence of the specialized bytecode can still slow application
performance in the following ways:
Class Loading / Verification: A minor difference in performance may
exist for those operations that occur before and during the JIT compilation
process. For example, since despecialization increases class file size, it may
require marginally more time to load the class file from disk. Verification
may also take additional time since it is performed before JIT compilation.
However, each of these tasks is only performed once during the lifetime
of the application, accounting for a tiny fraction of most application’s
runtime. Most of the application runtime is spent once these tasks have
completed and consists of executing code generated by the JIT compiler.
JIT Compilation: While despecialization does not impact the quality of
the code generated by JIT compilation in many cases, the process of generating
that code can be slowed slightly by despecialization. This slowing
is a result of the fact that additional argument bytes must be decoded
during the JIT compilation process. However, because JIT compilation
only occurs once, the addition of this minor amount of work has almost
no impact on the overall performance of the application.
Interpreter Performance: Most of the Sun virtual machines tested are
classified as mixed-mode, making use of both an interpreter and a JIT
compiler. The interpreter is used to execute the code initially. Once a
method is determined to execute frequently, it is JIT compiled. Using
this strategy ensures that time is not spent compiling code that is only
executed a small number of times. Figure 5.2 shows the results achieved