AN720 Datasheet, PDF(1/28 Page) Silicon Laboratories – PRECISION32™ OPTIMIZATION CONSIDERATIONS FOR CODE SIZE AND SPEED

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AN720 Datasheet, PDF (1/28 Pages) Silicon Laboratories – PRECISION32™ OPTIMIZATION CONSIDERATIONS FOR CODE SIZE AND SPEED

AN720

PRECISION32â¢ OPTIMIZATION CONSIDERATIONS FOR

CODE SIZE AND SPEED

1. Introduction

The code size and execution speed of a 32-bit MCU project can vary greatly depending on the way the code is

written, the toolchain libraries used, and the compiler and linker options. This document addresses how to

determine what portions of code are taking extra space or time and ways to optimize for space or speed for

different tool chains, including GCC redlib and newlib (Precision32 IDE) and Keil.

2. Key Points

The key topics of this document are:

ï®ï How to determine what portions of the project are taking the most space

ï®ï Ways to benchmark code execution speed

ï®ï Common strategies to reduce code size or improve execution speed

ï®ï Code startup time and ways to reduce it

3. Using CoreMarkâ¢ as a Speed Benchmark

CoreMark is a standard code base that can be ported to various processors to provide a speed benchmark. The

CoreMark software provides a score that rates how fast the core and code is, providing a relative comparison

between various toolchain options and settings. The CoreMark software package cannot be modified except for

device-specific information in the portme files. For modes that do not support printf (nohosting libraries), the

results were calculated using the value of the variable in code. See the CoreMark website for more information on

the test and score reporting requirements (www.coremark.org).

4. Non-Toolchain Considerations

The coding style and technique can have a great effect on the overall size of the project.

4.1. Coding Techniques

There are many ways coding technique can affect code size, including library calls, inline code or data, or code

optimizations made for global variables or pointers.

For more information on writing C code for ARM architectures, see the following resources:

ï®ï EETimes - Energy efficient C code for ARM devices by Chris Shore: http://www.eetimes.com/design/

embedded/4210470/Efficient-C-Code-for-ARM-Devices

ï®ï Compiler Coding Practices - ARM: http://infocenter.arm.com/help/index.jsp?topic=/

com.arm.doc.dui0472c/CJAFJCFG.html

These guidelines will largely apply regardless of the compiler used for the project.

4.2. Number of Function Parameters

Functions with either Keil or GCC can have as many parameters as desired. In general, the first four parameters

are passed to the function efficiently using registers. Any additional parameters beyond four must be moved on or

off the stack, which results in extra code size for each additional parameter and extra time to execute those

instructions. If possible, keeping functions to no more than four parameters can help reduce code size and

execution time.

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