Branches (While Loop - !0)

Consider the code block:

int x = 0;

while(x <= 10) {
  x++;

}
x += 4;

while(x > 10) {
  x--;
}
x -= 4;  

When we have a loop condition that involves a non-zero argument, we need to re-express that condition in terms of a zero argument. The code block above translates to:

int x = 0;

while(x -10 <= 0) {         // or [ 0 <= 10 - x]
  x++;
}
x += 4;

while(x -10 > 0) {          // or [ 0 > 10 - x]
  x--;
}
x -= 4;  

Converting to assembly gives:

0:   ld $0x0000, r0       # x = 0

# first loop, false conditional at top
2:   ld $0xfffffff6, r1   # r1 = -10
8:   mov r0, r2            
     add r1, r2           # r2 = x - 10
     bgt r2, 0x0012       # if x-10 > 0, x > 10, !(x <= 10) goto +3

     inc r0               # x++
     br 0x0008            # goto -4

12:  add $4, r0           # x += 4

# second loop, true conditional at bottom
14:  ld $0xfffffff6, r1   # r1 = -10
1a:  br 0x0022            # goto +4
1c:  dec r0               # x--;

     mov r0, r2
     add r1, r2           # r2 = x - 10;
22:  bgt r2, 0x001c       # if x-10 > 0, x > 10 goto -3

     add $-4, r0          # x -= 4  

Note: the condition being evaluated in each case is identical, it is just treated differently. In one block, it is used as a condition to exit the loop; in the other, it is used as a condition to continue the loop.

Branches (While Loop - false)

while(x != 0 ) {
  ~~~i
  ~~~i
  ~~~i
}
~~~o

Address	Machine	Assembly	Notes
	assume r0 = x
0:	9005	beq r0, 0x10	branch to exit
2:	~~~i		true case
	~~~i
	~~~i
8:	80FC	br 0x00	loop
10:	~~~o		exited loop

Note 1: 2 branches total, 1 branch taken on critical path.

Note 2: conditional at top.

Branches (While Loop - true)

while( x == 0 ) {
 ~~i
 ~~i
 ~~i
}
~~ o

Naive / Literal

Address	Machine	Assembly	Notes
	assume r0 = x
0:	9002	beq r0, 0x04	conditional
2:	8005	br 0x0c	exit
4:	~~~i		true case
	~~~i
	~~~i
a:	80FB	br 0x00	loop
c:	~~~o		exited loop

Note: 3 branches total, 2 branches in critical path.

Preferred

Address	Machine	Assembly	Notes
	assume r0 = x
0:	8004	br 0x08	prime (need to check condition)
2:	~~~i		true case
	~~~i
	~~~i
8:	90FD	beq r0, 0x02	loop conditional
	~~~o		exited loop

Note 1: 2 branches total, 1 branch in critical path.

Note 2: conditional at bottom.