Perl 6 Essentials

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.