OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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3.2 Other Optimization Strategies
Java provides many features related to safety and security. These features, along
with platform independence, object oriented programming structures and dynamic
class loading have a large negative impact on application performance. A variety of
strategies have been developed to help mitigate the performance cost associated with
these features. Many of these strategies are discussed in the subsections below.
3.2.1 Array Bounds Check Removal
The Java Language Specification dictates that a check must be performed to ensure
that every access to an array element is within the bounds of the array. These
checks increase the expense of accessing an array element considerably. In some
applications, the requirement to perform array bounds checking has been shown to
increase application runtime by a factor of two [20] while other studies indicate that
these checks increase runtime by a factor of as much as five [85]. Consequently,
research has been performed which attempts to minimize the number of array bounds
check operations.
Array bounds checks can safely be omitted when it is known conclusively that
the element being accessed is within the bounds of an array. A simple example of
such a case is a for loop where the loop control variable is bounded by a value that
is known to be smaller than the size of the array. In this situation, all array bounds
checks within the loop can be eliminated safely. A variety of different array bounds
removal techniques have been developed [75, 76, 22, 84, 122, 121, 35]. Each of these
techniques has its own strengths and weaknesses.
3.2.2 Providing Extra Data to the VM
Many optimization strategies work with Java class files which contain only the information
included in them by standard versions of javac. However, some optimizations
are eased considerably by including additional information. The Java class file format
provides a mechanism for providing this information in a manner compatible with all
virtual machines through the use of attributes.
Attributes are pieces of information within a Java class file. They can be associated
with the class itself or they can be associated with a specific method or field.
The Java Virtual Machine Specification includes the definition of several standard
attributes such as the code attribute which contains the bytecodes representing a