Script for Laboratory: Designing embedded ASIPs - CES

9 Custom instruction identification
The concept of instruction set extension in processor is a challenging and
highly prospective research area- both in academia and industry. Instruction
set extension in processor design platform mainly depends on detecting cluster
of operations. In our lab we try to find these custom instructions to a limited extend.
There are lots of state-of-the art ways to identify these custom instructions.
None of these ways proved to be that efficient to identify custom instructions
automatically. Currently we are not using any automatic tool to identify
custom instructions. Our main goal in this lab is to use a group of promising
custom instructions that can be extracted manually with human effort. But to
aid the manual effort for finding custom instructions a number of tools are used
for profiling as well as to analyze the Control Flow Graph (CFG) and Data
Flow Graph (DFG). In this tutorial the concept of custom instructions and the
possibility of different custom instructions are described. The use of profiling
tools is also described within the scope of this tutorial.
9.1 What is a custom instruction
The cluster of information, when implemented as a single instruction, should maximize some
metric and finally improve the performance. Such cluster of operations, forming a single
instruction is generally called a custom instruction.
Identification of custom instructions mostly depends on application domains. Analysis of
applications reveals different classes of custom instructions. Generally, four classes of custom
instructions can be found in different application domains. The classes are
• SIMD Instruction
• Zero-Overhead loop Instructions
• Memory Instructions
• Composite Instructions
Figure 9-1 shows a very simple custom instruction from a C-Code segment. Now the very
important question is when and which custom instruction needs to be added as a new
instruction in the template instruction set. We take a very simplistic assumption in our
experiment. The instruction which has been executed most of the time in the application and
basically increases the performance in terms of cycle count without or with a very little
penalty of increased latency and reduces the energy consumption significantly but might have
considerable amount of area increase are chosen as custom instructions. So the bottom line to
choose a new instruction can be summarized as follows considering our lab:
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