Avoid handle dereference during graph optimization.

src/hydrogen-instructions.cc

 // Copyright 2012 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 324 matching lines @@
     case kJSArray: return "array";
     case kJSObject: return "object";
     case kUninitialized: return "uninitialized";
   }
   UNREACHABLE();
   return "Unreachable code";
 }
 
 
 HType HType::TypeFromValue(Handle<Object> value) {
+  // Handle dereferencing is safe here: an object's type as checked below
+  // never changes.
+  AllowHandleDereference allow_handle_deref;
+
   HType result = HType::Tagged();
   if (value->IsSmi()) {
     result = HType::Smi();
   } else if (value->IsHeapNumber()) {
     result = HType::HeapNumber();
   } else if (value->IsString()) {
     result = HType::String();
   } else if (value->IsBoolean()) {
     result = HType::Boolean();
   } else if (value->IsJSObject()) {
@@ skipping 757 matching lines @@
   return this;
 }
 
 
 HValue* HCheckInstanceType::Canonicalize() {
   if (check_ == IS_STRING &&
       !value()->type().IsUninitialized() &&
       value()->type().IsString()) {
     return NULL;
   }
-  if (check_ == IS_SYMBOL &&
-      value()->IsConstant() &&
-      HConstant::cast(value())->handle()->IsSymbol()) {
-    return NULL;
+
+  if (check_ == IS_SYMBOL && value()->IsConstant()) {
+    // Dereferencing is safe here: a symbol cannot become a non-symbol.
+    AllowHandleDereference allow_handle_deref;
+    if (HConstant::cast(value())->handle()->IsSymbol()) return NULL;
   }
   return this;
 }
 
 
 void HCheckInstanceType::GetCheckInterval(InstanceType* first,
                                           InstanceType* last) {
   ASSERT(is_interval_check());
   switch (check_) {
     case IS_SPEC_OBJECT:
@@ skipping 412 matching lines @@
 static bool IsInteger32(double value) {
   double roundtrip_value = static_cast<double>(static_cast<int32_t>(value));
   return BitCast<int64_t>(roundtrip_value) == BitCast<int64_t>(value);
 }
 
 
 HConstant::HConstant(Handle<Object> handle, Representation r)
     : handle_(handle),
       has_int32_value_(false),
       has_double_value_(false) {
+  // Dereferencing here is safe: the value of a number object does not change.
+  AllowHandleDereference allow_handle_deref;
   SetFlag(kUseGVN);
   if (handle_->IsNumber()) {
     double n = handle_->Number();
     has_int32_value_ = IsInteger32(n);
     int32_value_ = DoubleToInt32(n);
     double_value_ = n;
     has_double_value_ = true;
   }
   if (r.IsNone()) {
     if (has_int32_value_) {
@@ skipping 58 matching lines @@
 
 
 bool HConstant::ToBoolean() {
   // Converts the constant's boolean value according to
   // ECMAScript section 9.2 ToBoolean conversion.
   if (HasInteger32Value()) return Integer32Value() != 0;
   if (HasDoubleValue()) {
     double v = DoubleValue();
     return v != 0 && !isnan(v);
   }
-  Handle<Object> literal = handle();
-  if (literal->IsTrue()) return true;
-  if (literal->IsFalse()) return false;
-  if (literal->IsUndefined()) return false;
-  if (literal->IsNull()) return false;
-  if (literal->IsString() && String::cast(*literal)->length() == 0) {
+  // Dereferencing is safe: singletons do not change and strings are
+  // immutable.
+  AllowHandleDereference allow_handle_deref;
+  if (handle_->IsTrue()) return true;
+  if (handle_->IsFalse()) return false;
+  if (handle_->IsUndefined()) return false;
+  if (handle_->IsNull()) return false;
+  if (handle_->IsString() && String::cast(*handle_)->length() == 0) {
     return false;
   }
   return true;
 }
 
 void HConstant::PrintDataTo(StringStream* stream) {
   if (has_int32_value_) {
     stream->Add("%d ", int32_value_);
   } else if (has_double_value_) {
     stream->Add("%f ", FmtElm(double_value_));
@@ skipping 1153 matching lines @@
 
 
 void HCheckFunction::Verify() {
   HInstruction::Verify();
   ASSERT(HasNoUses());
 }
 
 #endif
 
 } }  // namespace v8::internal