Page 135 - ARM 64 Bit Assembly Language

Structured programming 121

Listing 5.12 Post-test loop in AArch64 assembly.
.
1 . .
2 do:
.
3 . . // body of loop
4 cmp x0, x1 // perform loop test
5 bge do // repeat loop if x0 >= x1
.
6 . .

Listing 5.13 Guarded loop in AArch64 assembly.
.
1 . .
2 cmp x0, x1
3 blt loopEnd // if (x0 < x1) goto loopEnd
4 loop:
.
5 . . // body of loop
6 cmp x0, x1
7 bge do // if (x0 >= x1) goto loop
8 loopEnd:
.
9 . .

do-while loop in C, and Listing 5.12 shows a post-test loop in AArch64 assembly. The body
of a post-test loop will always be executed at least once.
A post-test loop is more efficient than a pre-test loop because it will always perform the loop
test one time less than a pre-test loop, and has one fewer branch instruction. When the test
fails, the post-test loop can exit immediately. A pre-test loop must first branch up to the begin-
ning and then branch back down to the end. When the programmer can guarantee that the loop
body must always execute at least once, a post-test loop is obviously the better choice.
Even if the loop body is not guaranteed to execute at least once in all cases, the post-test loop
can provide slightly better performance. In order to take advantage of this efficiency, pre-test
loops can be turned into post-test loops by adding one check before the pre-test loop begins.
This is commonly referred to as a guarded loop. The guarded loop is shown in Listing 5.13.

5.3.3 For loop

Many structured programming languages have a for loop construct, which is a type of count-
ing loop. The for loop is not essential, and is only included as a matter of syntactical conve-
nience. In some cases, a for loop is easier to write and understand than an equivalent pre-test