MoarVM: Allow 'chain' op to use child as callee

Modify the 'chain' op to allow the option to use the first child as the
callee. Before the name of the op served as the operator sub and the
children the operands. This modification makes it so that, if there is
no name provided to the chain, child 0 serves as the operator and
children 1 and 2 the operands.

This modification is being made to coincide with a Rakudo development
allowing negated chained ops to continue to work as chained.

See <rakudo/rakudo#1304>.

src/vm/moar/QAST/QASTOperationsMAST.nqp

@@ -628,37 +628,49 @@ QAST::MASTOperations.add_core_op('hash', -> $qastcomp, $op {
 });
 
 # Chaining.
+# TODO: Provide static-optimizations where possible for invocations involving metaops
 my $chain_gen := sub ($qastcomp, $op) {
     # First, we build up the list of nodes in the chain
     my @clist;
     my $cqast := $op;
+
+    # Check if callee sub in name, if not first child is callee, not arg
+    my $arg_idx;
+    my &get_arg_idx := -> $cq { nqp::if( $cq.name, 0, 1 ) };
+
     while nqp::istype($cqast, QAST::Op)
     && ($cqast.op eq 'chain' || $cqast.op eq 'chainstatic') {
         nqp::push(@clist, $cqast);
-        $cqast := $cqast[0];
+        $arg_idx := get_arg_idx($cqast);
+        $cqast := $cqast[$arg_idx];
     }
 
     my @ops;
     my $regalloc := $*REGALLOC;
     my $res_reg  := $regalloc.fresh_register($MVM_reg_obj);
+    my $decont_reg  := $regalloc.fresh_register($MVM_reg_obj);
     my $endlabel := MAST::Label.new();
 
     $cqast := nqp::pop(@clist);
-    my $aqast := $cqast[0];
+    $arg_idx := get_arg_idx($cqast);
+
+    my $aqast := $cqast[$arg_idx];
     my $acomp := $qastcomp.as_mast($aqast, :want($MVM_reg_obj));
     push_ilist(@ops, $acomp);
 
     my $more := 1;
     while $more {
-        my $bqast := $cqast[1];
+        my $bqast := $cqast[$arg_idx + 1];
         my $bcomp := $qastcomp.as_mast($bqast, :want($MVM_reg_obj));
         push_ilist(@ops, $bcomp);
 
         my $callee := $qastcomp.as_mast: :want($MVM_reg_obj),
-            $cqast.op eq 'chainstatic'
-                ?? QAST::VM.new:   :moarop<getlexstatic_o>,
-                   QAST::SVal.new: :value($cqast.name)
-                !! QAST::Var.new:  :name( $cqast.name), :scope<lexical>;
+            !$cqast.name
+                ?? $cqast[0]
+                !! $cqast.op eq 'chainstatic'
+                    ?? QAST::VM.new:   :moarop<getlexstatic_o>,
+                       QAST::SVal.new: :value($cqast.name)
+                    !! QAST::Var.new:  :name( $cqast.name), :scope<lexical>;
 
         push_ilist(@ops, $callee);
         nqp::push(@ops, MAST::Call.new(
@@ -667,13 +679,16 @@ my $chain_gen := sub ($qastcomp, $op) {
             :result($res_reg),
             $acomp.result_reg, $bcomp.result_reg
         ));
+        push_op(@ops, 'decont', $decont_reg, $callee.result_reg);
 
         $regalloc.release_register($callee.result_reg, $MVM_reg_obj);
+        $regalloc.release_register($decont_reg, $MVM_reg_obj);
         $regalloc.release_register($acomp.result_reg, $MVM_reg_obj);
 
         if @clist {
             push_op(@ops, 'unless_o', $res_reg, $endlabel);
             $cqast := nqp::pop(@clist);
+            $arg_idx := get_arg_idx($cqast);
             $acomp := $bcomp;
         }
         else {