Computers as Components, 2nd Edition

244 CHAPTER 5 Program Design and Analysis

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y

z

x

w

d

e

c

b

a

Compare the ARM assembly code for the two code fragments. We have written both

assuming that we have only four free registers. In the before version, we do not have to write

out any values, but we must read a and b twice. The after version allows us to retain all values

in registers as long as we need them.

Before version After version

LDR r0,a LDR r0,a

LDR r1,b LDR r1,b

ADD r2,r0,r1 ADD r2,r1,r0

STRr2,w;w=a+b STRr2,w;w=a+b

LDRr r0,c SUB r2,r0,r1

LDR r1,d STR r2,z;z=a–b

ADD r2,r0,r1 LDR r0,c

STRr2,x;x=c+d LDRr1,d

LDR r1,e ADD r2,r1,r0

ADD r0,r1,r2 STR r2,x;x=c+d

STRr0,y;y=x+e LDRr1,e

LDR r0,a ; reload