Example of the use of conditional execution

This example uses two implementations of the Greatest Common Divisor (gcd) algorithm (Euclid). It demonstrates how you can use conditional execution to improve code density and execution speed. The detailed analysis of execution speed only applies to an ARM7™ processor. The code density calculations apply to all ARM processors.

In C the algorithm can be expressed as:

int gcd(int a, int b)
{
    while (a != b)
      {
        if (a > b)
            a = a ‑ b;
        else
            b = b ‑ a;
      }
    return a;
}

You can implement the gcd function with conditional execution of branches only, in the following way:

gcd     CMP      r0, r1
        BEQ      end
        BLT      less
        SUBS     r0, r0, r1  ; could be SUB r0, r0, r1 for ARM
        B        gcd
less
        SUBS     r1, r1, r0  ; could be SUB r1, r1, r0 for ARM
        B        gcd
end

The code is seven instructions long because of the number of branches. Every time a branch is taken, the processor must refill the pipeline and continue from the new location. The other instructions and non‑executed branches use a single cycle each.

By using the conditional execution feature of the ARM instruction set, you can implement the gcd function in only four instructions:

gcd
        CMP      r0, r1
        SUBGT    r0, r0, r1
        SUBLE    r1, r1, r0
        BNE      gcd

In addition to improving code size, this code executes faster in most cases. Table 2.3 and Table 2.4 show the number of cycles used by each implementation for the case where r0 equals 1 and r1 equals 2. In this case, replacing branches with conditional execution of all instructions saves three cycles.

The conditional version of the code executes in the same number of cycles for any case where r0 equals r1. In all other cases, the conditional version of the code executes in fewer cycles.

gcd
        CMP     r0, r1
        ITE     GT
        SUBGT   r0, r0, r1
        SUBLE   r1, r1, r0
        BNE     gcd