Make ?? a compile-time constant operator.

runtime/vm/parser.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/parser.h"
 #include "vm/flags.h"
 
 #ifndef DART_PRECOMPILED
 
 #include "lib/invocation_mirror.h"
@@ skipping 10534 matching lines @@
           BinaryOpNode* old = lhs->AsBinaryOpNode();
           BinaryOpWithMask32Node* binop = new(Z) BinaryOpWithMask32Node(
               old->token_pos(), old->kind(), old->left(), old->right(), val);
           return binop;
         }
       }
     }
   }
   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;
     LiteralNode* null_operand =
         new(Z) LiteralNode(no_pos, Instance::ZoneHandle(Z));
     LoadLocalNode* load_left_temp = new(Z) LoadLocalNode(no_pos, left_temp);
     ComparisonNode* null_compare =
         new(Z) ComparisonNode(no_pos,
                               Token::kNE_STRICT,
                               load_left_temp,
@@ skipping 3856 matching lines @@
     const ArgumentListNode& function_args,
     const LocalVariable* temp_for_last_arg,
     bool is_super_invocation) {
   UNREACHABLE();
   return NULL;
 }
 
 }  // namespace dart
 
 #endif  // DART_PRECOMPILED