OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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method. However, the Java Virtual Machine Specification also permits a class file to
contain custom attributes. Furthermore, attributes are structured in a manner that
ensures that all virtual machines can parse custom attributes correctly, even if they
do not recognize the information stored within them.
Each attribute begins with an indication of its name. This is followed by a fourbyte
value specifying the attribute’s total size. The data stored in the attribute is
present immediately after its size. As a result, if a virtual machine encounters a nonstandard
attribute that it does not recognize, the attribute can be ignored by simply
skipping the attribute and continuing to parse the Java class file.
Using additional attributes in this manner can aid in numerous types of optimizations
including the removal of unnecessary array bounds checks and null pointer
checks [91], and method inlining [109]. Other work has also been conducted which
provided information to the JIT compiler to improve low level operations such as
instruction scheduling and register allocation [16, 63]. Further studies have also considered
communicating profile data to the optimization system through attributes
[70]. The goal of this strategy is to provide information about what methods will execute
with greatest frequency so that targeted optimizations can be performed before
any profile data is collected for the current run of the application.
Regardless of the type of optimization being performed, care must be taken to
ensure that the attribute does not contain information derived from additional class
files. If this constraint is not met, invalid information may be used at run time because
different versions of the class file were utilized at compile time and run time. Since the
data contained within non-standard attributes is not validated by the verifier, such
errors will not be detected until an unexpected behaviour occurs unless the extensions
to the virtual machine that read the attribute data validate it before use.
3.2.3 Devirtualization of Method Calls
Java is an object oriented language which provides polymorphism by allowing derived
classes to override methods originally defined in the parent class. Consequently,
many of the methods invoked in Java applications are virtual, requiring that the
actual method invoked be determined at runtime based on the dynamic type of the
receiver object. Performing virtual method dispatch is more expensive than nonvirtual
dispatch. This cost occurs because the precise method to invoke must be
determined at runtime.
Numerous implementations of virtual method dispatch are possible [59]. However,