Code Generation

The translation is approached as a particular case of tree decoration. Each node is decorated with a new attribute trans that will held the translation for such node. To compute it, we define a translation transformation t_class for each type of node class in the AST:

    translation = t_num t_var t_op t_neg t_assign t_list t_print;

Some of these transformations are straightforward. The translation of a NUM node is its value:

    t_num: NUM => { $NUM->{tr} = $NUM->{attr} }

The translation of a binary operation node $b is to apply the associated binary operator $op to the registers $x->{reg} and $y->{reg} where the operands were stored and store it in the register $b->{reg} associated with the node:

    t_op:  /TIMES|PLUS|DIV|MINUS/:b($x, $y) => {
        my $op = $Op{ref($b)};
        $b->{tr} = "$b->{reg} = $x->{reg} $op $y->{reg}";
      }

To keep track of the involved variables a hash is used as a rudimentary symbol table:

    { our %s; }
    t_assign: ASSIGN($v, $e) => {
      $s{$v->{attr}} = "num";
      $ASSIGN->{tr} = "$v->{reg} = $e->{reg}"
    }

The former rule says that the translation of an ASSIGN node consists in assigning the contents of the register assigned to the expression subtree to the register assigned to the left hand side.

The translation of the root node (EXPS) consists of concatenating the translations of its children:

{
  sub cat_trans {
    my $t = shift;

    my $tr = "";
    for ($t->children) {
      (ref($_) =~ m{NUM|VAR|TERMINAL})
        or $tr .= cat_trans($_)."\n"
    }
    $tr .= $t->{tr} ;
  }
}

t_list: EXPS(@S)
  => {
    $EXPS->{tr} = "";
    my @tr = map { cat_trans($_) } @S;
    $EXPS->{tr} =
      reduce { "$a\n$b" } @tr if @tr;
  }

The treeregexp @S matches the children of the EXPS node. The associated lexical variable @S contains the references to the nodes that matched.

The method bud⁸of Parse::Eyapp::Node nodes makes a bootom up traversing of the AST applying to the node being visited the only one transformation that matches⁹. After the call

$t->bud(our @translation);

the attribute $t->{trans} contains a translation to PIR for the whole tree.