Page 134 - ARM 64 Bit Assembly Language

120 Chapter 5

Listing 5.9 Pre-test loop in C.
.
1 . .
2 while (x >= y) // perform loop test
3 {
.
4 . . // body of loop
5 } // done
.
6 . .

Listing 5.10 Pre-test loop in AArch64 assembly.
.
1 . .
2 loop: cmp x0, x1 // perform loop test
3 blt done // exit loop if x0 < x1
.
4 . . // body of loop
5 b loop // repeat loop
6 done:
.
7 . .

Listing 5.11 Post-test loop in C.
.
1 . .
2 do {
.
3 . . // body of loop
4 } while (x >= y); // perform loop test
.
5 . .

to false, then execution branches to the first instruction following the loop body. All structured
programming languages have a pre-test loop construct. For example, in C, the pre-test loop is
called a while loop. In assembly, a pre-test loop is constructed very similarly to an if-then
statement. The only difference is that it includes an additional branch instruction at the end of
the sequence of instructions that form the body. Listing 5.10 shows a pre-test loop in AArch64
assembly.

5.3.2 Post-test loop

In a post-test loop, the test is performed after the loop body is executed. If the test evaluates
to true, then execution branches to the first instruction in the loop body. Otherwise, execution
continues sequentially. Most structured programming languages have a post-test loop con-
struct. For example, in C, the post-test loop is called a do-while loop. Listing 5.11 shows a