ARM VERSION 1.2 manual Using conditional execution in ARM state

Writing ARM and Thumb Assembly Language

2.5.3Using conditional execution in ARM state

You can use conditional execution of ARM instructions to reduce the number of branch instructions in your code. This improves code density.

Branch instructions are also expensive in processor cycles. On ARM processors without branch prediction hardware, it typically takes three processor cycles to refill the processor pipeline each time a branch is taken.

Some ARM processors, for example ARM10™ and StrongARM®, have branch prediction hardware. In systems using these processors, the pipeline only needs to be flushed and refilled when there is a misprediction.

2.5.4Example of the use of conditional execution

This example uses two implementations of Euclid’s Greatest Common Divisor (gcd) algorithm. 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) do

{

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
	SUB	r0, r0, r1
	B	gcd
less
	SUB	r1, r1, r0
	B	gcd
end

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