Lecture Notes in Computer Science 4917

Code Arrangement of Embedded Java Virtual Machine for NAND Flash Memory 375
of BytecodeFetching and LookupTable is about 1200 bytes (compiled with arm-elfgcc-3.4.3),
which is almost small enough to fit into two or three 512-bytes-page.
Figure 8 showed the size distribution of bytecode handlers. The average size of a
bytecode handler is 131 bytes, and there are 79 handlers smaller than 56 bytes. In the
other words, a 512-bytes-page could gather 4 to 8 bytecode handlers. The intrahandler
execution flow dominates the number of cache misses generated by the
interpreter. This is the reason that our approach tried to rearrange bytecode handlers
within the BytecodeDispatch trunk.
Fig. 6. Distribution of Bytecode Handler Size (compiled with gcc-3.4.3)
4 Analyzing Control Flow
4.1 Indirect Control Flow Graph
Typical approaches derive the code placement of a program from its control flow
graph (CFG). However, the CFG of a VM interpreter is a special case, its CFG is a
flat spanning tree enclosed by a loop. All bytecode handlers are always sibling code
blocks in the aspect of CFG regardless of executed Java applications. Therefore, the
CFG does not provide information to distinguish the temporal order between each
bytecode handler. If someone wants to improve the program locality by observing the
dynamic execution order of program blocks, and CFG is apparently not a good tool to
this end. Therefore, we proposed a concept called “Indirect Control Flow Graph”
(ICFG); it uses the dynamic instruction sequence to construct the dual CFG of the
interpreter.
Consider a simplified virtual machine with 5 bytecode instructions: A, B, C, D,
and E. Besides, take the following short alphabetic sequences as the input to the
simplified virtual machine:
A-B-A-B-C-D-E-C
Each letter in the sequence represents a bytecode instruction. In Figure 7, the graph
connected with the solid lines is the CFG of the simplified interpreter. Yet, this CFG
cannot convey whether handler B will be called after handler A is executed.
Therefore, we construct the ICFG by using the dashed directed lines to connect the
bytecode handlers in the order of the input sequence. Actually, the Figure 8 expresses
the ICFG in a readable way.