OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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6.2.5 Fields
Only four bytecodes are included in the Java Virtual Machine Specification for field
access. The getfield and putfield bytecodes provide the ability to read and write
instance fields respectively. Similarly, the getstatic and putstatic bytecodes provide
read and write access for static fields. Each of these bytecodes is able to access
data of any type including array types.
Figures 6.6 and 6.7 show the distribution of the types of the instance fields read
and written. While the distributions are not identical, they show many similarities,
with fields containing integers, object references and doubles being accessed with
greatest frequency. Initially it may appear that integer writes are performed more
frequently than integer reads because Figure 6.6 shows almost 32 percent of the fields
accessed having integer type while Figure 6.7 shows more than 56 percent of the fields
written having integer type. This is not the case. The getfield opcode accounts
for more than 9.5 percent of the opcodes executed by the benchmarks considered
during this study. In comparison, putfield accounts for only 1.3 percent of the
bytecodes executed. Consequently, integer field reads account for about 3 percent of
all bytecodes executed while integer field writes account for about 0.73 percent of all
opcodes executed.
Figures 6.8 and 6.9 show the distribution of the types of the static fields read
and written. While the relative type distributions for getfield and putfield were
similar, getstatic and putstatic show considerably different distributions. In particular,
while arrays of object references and scalar object references were the static
field types read with greatest frequency, the most frequently written static field types
were double precision floating point numbers and integers.
Static fields are accessed with much lower frequency than instance fields. Uses of
getstatic account for 0.36 percent of the bytecodes executed while uses of putstatic
account for less than 0.02 percent of the bytecodes executed. Based on these execution
frequencies it was possible to determine how frequently each specific data type was
accessed by both getstatic and putstatic. This analysis revealed that while static
fields of type object reference array and object reference were read with much greater
frequency than they were written, double, integer and long static fields were read and
written with almost identical frequency. This pattern of behaviour is considerably
different than that observed for getfield and putfield. In that case, every type was
read more frequently than it was written, and in general, the reads far out numbered
the writes.
The utility of the information presented on fields for improving Java application