[hydrogen] don't bailout assignments to consts

src/hydrogen.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3270 matching lines @@
                variable->mode() == LET);
         return Bailout("reference to uninitialized variable");
       }
       return ast_context()->ReturnValue(value);
     }
 
     case Variable::CONTEXT: {
       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());
       return ast_context()->ReturnInstruction(instr, expr->id());
     }
 
     case Variable::LOOKUP:
       return Bailout("reference to a variable which requires dynamic lookup");
   }
 }
@@ skipping 645 matching lines @@
 
   if (expr->is_compound()) {
     HandleCompoundAssignment(expr);
     return;
   }
 
   if (prop != NULL) {
     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");
+        CHECK_ALIVE(VisitForValue(expr->value()));
+        return ast_context()->ReturnValue(Pop());
       }
-      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->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) {
         return Bailout("non-initializer assignment to const");
       }
       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");
 
     // Handle the assignment.
     switch (var->location()) {
       case Variable::UNALLOCATED:
         CHECK_ALIVE(VisitForValue(expr->value()));
         HandleGlobalVariableAssignment(var,
                                        Top(),
@@ skipping 14 matching lines @@
         // We do not allow the arguments object to occur in a context where it
         // may escape, but assignments to stack-allocated locals are
         // permitted.
         CHECK_ALIVE(VisitForValue(expr->value(), ARGUMENTS_ALLOWED));
         HValue* value = Pop();
         Bind(var, value);
         return ast_context()->ReturnValue(value);
       }
 
       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.
         if (info()->scope()->arguments() != NULL) {
           // Parameters will rewrite to context slots.  We have no direct way
           // to detect that the variable is a parameter.
           int count = info()->scope()->num_parameters();
           for (int i = 0; i < count; ++i) {
             if (var == info()->scope()->parameter(i)) {
               return Bailout("assignment to parameter in arguments object");
             }
           }
         }
 
         CHECK_ALIVE(VisitForValue(expr->value()));
         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());
         }
         return ast_context()->ReturnValue(Pop());
       }
 
       case Variable::LOOKUP:
         return Bailout("assignment to LOOKUP variable");
     }
@@ skipping 3204 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal