ARM VERSION 1.2 manual Conditional branches only

Writing ARM and Thumb Assembly Language

Because of the number of branches, the code is seven instructions long. 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
SUBLT	r1, r1, r0
BNE	gcd

In addition to improving code size, this code executes faster in most cases. Table 2-2 and Table 2-3 on page 2-24 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.

			Table 2-2 Conditional branches only
r0: a	r1: b	Instruction	Cycles (ARM7)
1	2	CMP r0, r1	1
1	2	BEQ end	1 (not executed)
1	2	BLT less	3
1	2	SUB r1, r1, r0	1
1	2	B gcd	3
1	1	CMP r0, r1	1
1	1	BEQ end	3
			Total = 13

ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-23