Efficient C Programming: Boolean Expressions (Part 1)

Introduction: This chapter will help readers write efficient C code on ARM processors. Some of the techniques involved in this chapter are not only applicable to ARM processors, but also to other RISC processors. (EEFOCUS)

14.4 Boolean Expressions 14.4.1 Range Checks

Boolean expressions are often used to check whether a value is within a specific range. For example, in a graphics window handler, Boolean expressions are often used to determine whether a point on the screen is within the range of the currently active window.

The following program uses a structure to define point coordinates and calculate the current position of the coordinates.

boolPointInRect1(Pointp,Rectangle*r)

{return(px>=r->xmin&&p.xxmax&&

py>=r->ymin&&p.yymax);

}

The above function is compiled into the following instruction sequence.

PointInRect1

LDRa4,[a3,#0]

CMPa1,a4

BLT|L000034.J5.PointInRect1|

LDRa4,[a3,#4]

CMPa4,a1

BLE|L000034.J5.PointInRect1|

LDRa1,[a3,#8]

CMPa2,a1

BLT|L000034.J5.PointInRect1|

LDRa1,[a3,#&c]!

CMPa2,a1

MOVLTa1,#1

MOVLTpc,lr

|L000034.J5.PointInRect1|

MOVa1,#0

MOVpc,lr

But the above code is not the most concise. The compiler's processing of Boolean expressions of the form (x>=min&&x

boolPointInRect2(Pointp,Rectangle*r)

{return((unsigned)(px-r->xmin)xmax&&

(unsigned)(py-r->ymin)ymax);

}

The compiled assembly instruction sequence is as follows.

PointInRect2

LDRa4,[a3,#0]

SUBa1,a1,a4

LDRa4,[a3,#4]

CMPa1,a4

LDRCCa1,[a3,#8]

SUBCCa1,a2,a1

LDRCCa2,[a3,#&c]!

CMPCCa1,a2

MOVCSa1,#0

MOVCCa1,#1

MOVpc,lr