ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.6 Loop unrolling 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.
Small loops can be unrolled for higher performance, with the disadvantage of increased code
size. When a loop is unrolled, a loop counter needs to be updated less often and fewer branches
are executed. If the loop iterates only a few times, it can be fully unrolled so that the loop
overhead completely disappears. The compiler unrolls loops automatically at -O3 -Otime.
Otherwise, any unrolling must be done in source code.
Note
Manual unrolling of loops might hinder the automatic re-rolling of loops and other loop
optimizations by the compiler.
The advantages and disadvantages of loop unrolling can be illustrated using the two sample
routines shown in Table 5-3. Both routines efficiently test a single bit by extracting the lowest
bit and counting it, after which the bit is shifted out.
The first implementation uses a loop to count bits. The second routine is the first implementation
unrolled four times, with an optimization applied by combining the four shifts of n into one shift.
Unrolling frequently provides new opportunities for optimization.
Table 5-3 C code for rolled and unrolled bit-counting loops
Bit-counting loop
int countbit1(unsigned int n)
{
int bits = 0;
while (n != 0)
{
if (n & 1) bits++;
n >>= 1;
}
return bits;
}
Unrolled bit-counting loop
int countbit2(unsigned int n)
{
int bits = 0;
while (n != 0)
{
if (n & 1) bits++;
if (n & 2) bits++;
if (n & 4) bits++;
if (n & 8) bits++;
n >>= 4;
}
return bits;
}
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