[hydrogen] don't bailout assignments to consts

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 519b4b84ba47b52bcc1a70257d1ed2ae140fddd4..94f260ee88e6272b186f8be6b34716b8e011b27e 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -3956,27 +3956,25 @@ void HGraphBuilder::VisitAssignment(Assignment* expr) {
     HandlePropertyAssignment(expr);
   } else if (proxy != NULL) {
     Variable* var = proxy->var();
-    if (var->mode() == CONST) {
-      if (expr->op() != Token::INIT_CONST) {
-        return Bailout("non-initializer assignment to const");
-      }
-      if (!var->IsStackAllocated()) {
-        return Bailout("assignment to const context slot");
-      }
-      // We insert a use of the old value to detect unsupported uses of const
-      // variables (e.g. initialization inside a loop).
-      HValue* old_value = environment()->Lookup(var);
-      AddInstruction(new HUseConst(old_value));
-    } else if (var->mode() == LET) {
+    if (var->mode() == LET) {
       if (!var->IsStackAllocated()) {
         return Bailout("assignment to let context slot");
       }
-    } else if (var->mode() == CONST_HARMONY) {
-      if (expr->op() != Token::INIT_CONST_HARMONY) {
-        return Bailout("non-initializer assignment to const");
+    }
+
+    if (var->is_const_mode()) {

[fschneider, 2011/12/08 13:01:30]

CONST_HARMONY has different semantics from CONST. There is another change under
review to deal with harmony-const (http://codereview.chromium.org/8820015/), so
you should leave it out in this change and only handle plain CONST.

+      // Changing value of const variable only possible at initialization
+      if (expr->op() != Token::INIT_CONST &&
+          expr->op() != Token::INIT_CONST_HARMONY) {
+        CHECK_ALIVE(VisitForValue(expr->value()));
+        return ast_context()->ReturnValue(Pop());
       }
-      if (!var->IsStackAllocated()) {
-        return Bailout("assignment to const context slot");
+
+      if (var->IsStackAllocated()) {
+        // We insert a use of the old value to detect unsupported uses of const
+        // variables (e.g. initialization inside a loop).
+        HValue* old_value = environment()->Lookup(var);
+        AddInstruction(new HUseConst(old_value));
       }
     }
 
@@ -4012,7 +4010,6 @@ void HGraphBuilder::VisitAssignment(Assignment* expr) {
       }
 
       case Variable::CONTEXT: {
-        ASSERT(var->mode() != CONST);
         // Bail out if we try to mutate a parameter value in a function using
         // the arguments object.  We do not (yet) correctly handle the
         // arguments property of the function.
@@ -4031,6 +4028,12 @@ void HGraphBuilder::VisitAssignment(Assignment* expr) {
         HValue* context = BuildContextChainWalk(var);
         HStoreContextSlot* instr =
             new(zone()) HStoreContextSlot(context, var->index(), Top());
+
+        // Ensure that const variable's value will be set only once
+        if (var->is_const_mode()) {
+          instr->ForceIsHoleCheck();
+        }
+
         AddInstruction(instr);
         if (instr->HasObservableSideEffects()) {
           AddSimulate(expr->AssignmentId());