ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.5 Optimization of loop termination in C code
Loops are a common construct in most programs. Because a significant amount of execution
time is often spent in loops, it is worthwhile paying attention to time-critical loops.
The loop termination condition can cause significant overhead if written without caution. Where
possible:
• use simple termination conditions
• write count-down-to-zero loops
• use counters of type unsigned int
• test for equality against zero.
Following any or all of these guidelines, separately or in combination, is likely to result in better
code.
Table 5-1 shows two sample implementations of a routine to calculate n! that together illustrate
loop termination overhead. The first implementation calculates n! using an incrementing loop,
while the second routine calculates n! using a decrementing loop.
Table 5-1 C code for incrementing and decrementing loops
Incrementing loop
int fact1(int n)
{
int i, fact = 1;
for (i = 1; i