[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..0dca258239a0b6c8c22e00de8bada4675a3d9052 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -3288,9 +3288,6 @@ void HGraphBuilder::VisitVariableProxy(VariableProxy* expr) {
       if (variable->mode() == LET || variable->mode() == CONST_HARMONY) {
         return Bailout("reference to harmony declared context slot");
       }
-      if (variable->mode() == CONST) {
-        return Bailout("reference to const context slot");

[fschneider, 2011/12/09 09:37:54]

In order to get rid of this bailout we also additional code for for
HLoadContextSlot to return undefined if we find the hole-value:

1. Load value
2. Check if value is the_hole_value.
3. If no, return value.
4. If yes, return undefined_value.

-      }
       HValue* context = BuildContextChainWalk(variable);
       HLoadContextSlot* instr =
           new(zone()) HLoadContextSlot(context, variable->index());
@@ -3956,20 +3953,18 @@ 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");
+        CHECK_ALIVE(VisitForValue(expr->value()));
+        return ast_context()->ReturnValue(Pop());
       }
-      // 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->IsStackAllocated()) {
-        return Bailout("assignment to let 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));
       }
     } else if (var->mode() == CONST_HARMONY) {
       if (expr->op() != Token::INIT_CONST_HARMONY) {
@@ -3978,6 +3973,10 @@ void HGraphBuilder::VisitAssignment(Assignment* expr) {
       if (!var->IsStackAllocated()) {
         return Bailout("assignment to const context slot");
       }
+    } else if (var->mode() == LET) {
+      if (!var->IsStackAllocated()) {
+        return Bailout("assignment to let context slot");
+      }
     }
 
     if (proxy->IsArguments()) return Bailout("assignment to arguments");
@@ -4012,7 +4011,6 @@ 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 +4029,12 @@ 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()) {

[fschneider, 2011/12/09 09:37:54]

Here I would write

if (var->mode() == CONST)

to handle the CONST case only.

+          instr->ForceIsHoleCheck();
+        }
+
         AddInstruction(instr);
         if (instr->HasObservableSideEffects()) {
           AddSimulate(expr->AssignmentId());