Make ?? a compile-time constant operator.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 26f2ad7adb095eb5f22a4e1473c56684e15d4ad5..4df590bfcf42f38e8056d849d672cb839084346e 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -10552,6 +10552,21 @@ AstNode* Parser::OptimizeBinaryOpNode(intptr_t op_pos,
   }
   if (binary_op == Token::kIFNULL) {
     // Handle a ?? b.
+    if (lhs->IsPotentiallyConst() && lhs->EvalConstExpr() != NULL) {

[Ivan Posva, 2015/12/03 21:29:04]

() ditto: Please keep the style of the file.

[Lasse Reichstein Nielsen, 2015/12/04 12:11:46]

Done.

+      // If we know at compile time whether the lhs is null
+      // and side-effect free, just reduce it to either the lhs or rhs.
+      // This enables ?? to be compile-time constant.
+      Instance& expr_value = Instance::ZoneHandle(Z);
+      intptr_t lhs_pos = lhs->token_pos();
+      if (!GetCachedConstant(lhs_pos, &expr_value)) {
+        expr_value = EvaluateConstExpr(lhs_pos, lhs).raw();
+        CacheConstantValue(lhs_pos, expr_value);
+      }
+      if (expr_value.IsNull()) return rhs;

[Ivan Posva, 2015/12/03 21:29:04]

ditto

[Lasse Reichstein Nielsen, 2015/12/04 12:11:46]

Done.

+      return new(Z) LiteralNode(lhs_pos, expr_value);
+    }
+    // Otherwise create a temporary variable for the lhs so we can both
+    // check it against null and return it if necessary.
     LetNode* result = new(Z) LetNode(op_pos);
     LocalVariable* left_temp = result->AddInitializer(lhs);
     const intptr_t no_pos = Scanner::kNoSourcePos;