[dotnet] Quick and dirty experimental optimization that demotes lexic…

…al to locals in a few places (so their storage slot goes on the stack and is instantly to hand). Again, optimizer not enabled by default.

dotnet/compiler/NQPOptimizer.pm

@@ -33,6 +33,11 @@ our multi optimizer(PAST::Block $block) {
         $block.control('');
     }
 
+    # If there's no inner blocks, we can demote lexicals to constants.
+    unless get_annotation($block, 'has_nested_blocks') {
+        demote_lexicals_to_locals($block);
+    }
+
     # Remove this block from the block stack.
     @*BLOCK_STACK.shift();
 }
@@ -63,10 +68,13 @@ our multi optimizer(PAST::Op $op) {
 }
 
 our multi optimizer(PAST::Var $var) {
-    # May need to visit the viviself.
+    # May need to visit the viviself as well as children.
     if $var.viviself ~~ PAST::Node {
         optimizer($var.viviself);
     }
+    for @($var) {
+        optimizer($_);
+    }
 }
 
 our multi optimizer(PAST::Val $stmts) {
@@ -91,3 +99,49 @@ sub annotate($node, $key, $value) {
 sub get_annotation($node, $key) {
     $node{'nqp_opt_' ~ $key}
 }
+
+# Demotes lexicals to local variables.
+sub demote_lexicals_to_locals($block) {
+    my %*demoted;
+    my $*counter := 0;
+    for @($block) {
+        demote_lexicals_worker($_);
+    }
+}
+our multi sub demote_lexicals_worker(PAST::Stmts $stmts) {
+    for @($stmts) {
+        demote_lexicals_worker($_);
+    }
+}
+our multi sub demote_lexicals_worker(PAST::Var $var) {
+    if %*demoted{$var.name} {
+        $var.scope('register');
+        $var.name(%*demoted{$var.name});
+    }
+    elsif $var.isdecl && $var.scope eq 'lexical' {
+        # Make sure it's not a context var.
+        my $twigil := pir::substr($var.name, 1, 1);
+        if $twigil ne '*' && $twigil ne '?' {
+            # Good demotion candidate.
+            my $new_name := 'demoted_lexical_' ~ $*counter;
+            $*counter := $*counter + 1;
+            %*demoted{$var.name} := $new_name;
+            $var.name($new_name);
+            $var.scope('register');
+        }
+    }
+    if $var.viviself {
+        demote_lexicals_worker($var.viviself);
+    }
+    for @($var) {
+        demote_lexicals_worker($_);
+    }
+}
+our multi sub demote_lexicals_worker(PAST::Op $op) {
+    for @($op) {
+        demote_lexicals_worker($_);
+    }
+}
+our multi sub demote_lexicals_worker($any) {
+    # Nothing to do for this node type
+}