7.3 Flow Control

As in PASM, flow control in PIR is done entirely with conditional and unconditional branches. This may seem simplistic, but remember PIR is a thin overlay on the assembly language of a virtual processor. For the average assembly language, the jump is the fundamental unit of flow control.

Any PASM branch instruction is valid, but IMCC has some high-level constructs of its own. The most basic is the unconditional branch: goto.

.sub _main
    goto L1
    print "never printed"
L1:
    print "after branch\n"
    end
.end

The first print statement never runs because the goto always skips over it to the label L1.

The conditional branches combine if or unless with goto.

.sub _main
    $I0 = 42
    if $I0 goto L1
    print "never printed"
L1: print "after branch\n"
    end
.end

In this example, the goto branches to the label L1 only if the value stored in $I0 is true. The unless statement is quite similar, but branches when the tested value is false. An undefined value, 0, or an empty string are all false values. The if ... goto statement is translated directly to Parrot's if, and unless translates to Parrot's unless.

The comparison operators (<, <=, = =, !=, >, >=) combine with if ... goto. These branch when the comparison is true:

.sub _main
    $I0 = 42
    $I1 = 43
    if $I0 < $I1 goto L1
    print "never printed"
L1:
    print "after branch\n"
    end
.end

This example compares $I0 to $I1 and branches to the label L1 if $I0 is less than $I1. The if $I0 < $I1 goto L1 statement translates directly to the PASM lt branch operation.

The rest of the comparison operators are summarized at the end of this chapter.

PIR has no special loop constructs. A combination of conditional and unconditional branches handle iteration:

.sub _main
    $I0 = 1               # product
    $I1 = 5               # counter

REDO:                     # start of loop
    $I0 = $I0 * $I1
    dec $I1
    if $I1 > 0 goto REDO  # end of loop

    print $I0
    print "\n"
    end
.end

This example calculates the factorial 5!. Each time through the loop it multiplies $I0 by the current value of the counter $I1, decrements the counter, and then branches to the start of the loop. The loop ends when $I1 counts down to 0 and the if doesn't branch to REDO. This is a do while-style loop with the condition test at the end, so the code always runs the first time through.

For a while-style loop with the condition test at the start, use a conditional branch together with an unconditional branch:

.sub _main
    $I0 = 1        # product
    $I1 = 5        # counter

REDO:                     # start of loop
    if $I1 <= 0 goto LAST  
    $I0 = $I0 * $I1
    dec $I1
    goto REDO
LAST:                     # end of loop

    print $I0
    print "\n"
    end
.end

This example tests the counter $I1 at the start of the loop. At the end of the loop, it unconditionally branches back to the start of the loop and tests the condition again. The loop ends when the counter $I1 reaches 0 and the if branches to the LAST label. If the counter isn't a positive number before the loop, the loop never executes.

Any high-level flow control construct can be built from conditional and unconditional branches.