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.

The advantages and disadvantages of loop unrolling can be illustrated using the two sample routines shown in Table 5. 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.

int countbit1(unsigned int n)
{
    int bits = 0;
    while (n != 0)
    {
        if (n & 1) bits++;
        n >>= 1;
    }
    return bits;
}

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;
}

Table 6 shows the corresponding disassembly of the machine code produced by the compiler for each of the sample implementations of Table 5, where the C code for each implementation has been compiled using the option -O2.

countbit1 PROC
    MOV      r1, #0
    B        |L1.20|
|L1.8|
    TST      r0, #1
    ADDNE    r1, r1, #1
    LSR      r0, r0, #1
|L1.20|
    CMP      r0, #0
    BNE      |L1.8|
    MOV      r0, r1
    BX       lr
ENDP

countbit2 PROC
    MOV      r1, r0
    MOV      r0, #0
    B        |L1.48|
|L1.12|
    TST      r1, #1
    ADDNE    r0, r0, #1
    TST      r1, #2
    ADDNE    r0, r0, #1
    TST      r1, #4
    ADDNE    r0, r0, #1
    TST      r1, #8
    ADDNE    r0, r0, #1
    LSR      r1, r1, #4
|L1.48|
    CMP      r1, #0
    BNE      |L1.12|
    BX       lr
ENDP

On the ARM9, checking a single bit takes six cycles in the disassembly of the bit-counting loop shown in the leftmost column. The code size is only nine instructions. The unrolled version of the bit-counting loop checks four bits at a time per loop iteration, taking on average only three cycles per bit. However, the cost is the larger code size of fifteen instructions.