Always generate write_barrier for VTABLE.

Reason for it next:
1. Aggregate PMC in Gen0.
2. We invoke write_barrier(PMC). Which is ignored.
3. VTABLE_foo() allocate new PMC.
4. Aggregate PMC moved into Gen1.
5. Content of PMC gets updated without triggering write barrier.

To avoid it we store result of original VTABLE and invoke write_barrier
after.

lib/Parrot/Pmc2c/MethodEmitter.pm

@@ -39,8 +39,14 @@ sub generate_body {
 
     my $body = $self->body;
 
-    if ( $self->is_vtable ) {
+    if ( $self->is_vtable || $self->name =~ '_orig') {
+        # UGLY HACK to rewrite original body of write-barriered vtable
+        my $orig_name = $self->name;
+        my $n = $self->name;
+        $n =~ s/_orig$//;
+        $self->name($n);
         $self->rewrite_vtable_method($pmc);
+        $self->name($orig_name);
     }
     else {
         $self->rewrite_nci_method($pmc);

lib/Parrot/Pmc2c/PMCEmitter.pm

@@ -278,6 +278,48 @@ sub pre_method_gen {
     1;
 }
 
+sub post_method_gen {
+    my ($self) = @_;
+
+    # vtables
+    foreach my $method ( @{ $self->vtable->methods } ) {
+        my $name = $method->name;
+        next if $name eq 'class_init';
+        next unless $self->implements_vtable($name);
+        # Skip non-updating methods
+        next unless $self->{has_method}{$name}
+                    && $self->vtable_method_does_write($name);
+
+        $method  = $self->get_method($name);
+
+        #warn "Rewriting " . $self->name . "." . $name;
+        $self->add_method(
+            $method->clone({
+                name => $name.'_orig',
+                type => "ORIG",
+            })
+        );
+
+        # Rewrite body with write barrier.
+        my $body;
+        my $need_result = $method->return_type && $method->return_type !~ 'void';
+
+        $body .= $method->return_type . " result;\nresult = " if $need_result;
+
+        # Get parameters.      strip type from param
+        my $parameters = join ', ',
+                         'INTERP', 'SELF', map { /\s*\*?(\S+)$/; $1 }
+                         split (/,/, $method->parameters);
+        $body .= $method->full_method_name($self->name) . "_orig($parameters);\n";
+
+        $body .= "PARROT_GC_WRITE_BARRIER(interp, _self);\n";
+        $body .= "return result;" if $need_result;
+
+        $method->body(Parrot::Pmc2c::Emitter->text($body) );
+    }
+
+}
+
 =item C<gen_methods()>
 
 Returns the C code for the pmc methods.

lib/Parrot/Pmc2c/Parser.pm

@@ -89,6 +89,7 @@ sub parse_pmc {
     $pmc->add_method($class_init) if $class_init;
     $pmc->vtable( $pmc2cMain->read_dump("vtable.pmc") );
     $pmc->pre_method_gen();
+    $pmc->post_method_gen();
 
     return $pmc;
 }