Accept constant string concatenation and constant string length as constant

runtime/vm/ast.cc

Index: runtime/vm/ast.cc
===================================================================
--- runtime/vm/ast.cc	(revision 43823)
+++ runtime/vm/ast.cc	(working copy)
@@ -324,7 +324,7 @@
   if (left_val == NULL) {
     return NULL;
   }
-  if (!left_val->IsNumber() && !left_val->IsBool()) {
+  if (!left_val->IsNumber() && !left_val->IsBool() && !left_val->IsString()) {
     return NULL;
   }
   const Instance* right_val = this->right()->EvalConstExpr();
@@ -333,6 +333,10 @@
   }
   switch (kind_) {
     case Token::kADD:
+      if (left_val->IsString()) {
+        return right_val->IsString() ? left_val : NULL;
+      }
+      // Fall-through intentional.
     case Token::kSUB:
     case Token::kMUL:
     case Token::kDIV:
@@ -531,6 +535,23 @@
 }
 
 
+bool InstanceGetterNode::IsPotentiallyConst() const {
+  return receiver()->IsPotentiallyConst() &&
+    field_name().Equals(Symbols::Length());
+}
+
+
+const Instance* InstanceGetterNode::EvalConstExpr() const {
+  const Instance* receiver_val = receiver()->EvalConstExpr();
+  if ((receiver_val != NULL) &&
+      receiver_val->IsString() &&
+      field_name().Equals(Symbols::Length())) {

[hausner, 2015/02/17 23:07:03]

Maybe reverse the order of checks? Testing whether the field name is identical
to the length symbol is cheap and rules out all non-const cases right away.
Ditto in the case above.

[regis, 2015/02/18 00:38:08]

Done.

+    return receiver_val;

[hausner, 2015/02/17 23:07:03]

I think you should return a value that represents an integer, not a string.
Otherwise the expression str.length + 1 will not evaluate to a compile time
constant. I think you can just return Integer::Handle() or
Smi::Handle(Smi::New(0)).

I would also add the above case to the test.

[regis, 2015/02/18 00:38:08]

Good catch. length + 1 was not working. It does now.

+  }
+  return NULL;
+}
+
+
 AstNode* LoadIndexedNode::MakeAssignmentNode(AstNode* rhs) {
   return new StoreIndexedNode(token_pos(), array(), index_expr(),
                               rhs, super_class());