Sharing of descriptor arrays.

src/ia32/full-codegen-ia32.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 2651 matching lines @@
             1 << Map::kStringWrapperSafeForDefaultValueOf);
   __ j(not_zero, if_true);
 
   // Check for fast case object. Return false for slow case objects.
   __ mov(ecx, FieldOperand(eax, JSObject::kPropertiesOffset));
   __ mov(ecx, FieldOperand(ecx, HeapObject::kMapOffset));
   __ cmp(ecx, FACTORY->hash_table_map());
   __ j(equal, if_false);
 
   // Look for valueOf symbol in the descriptor array, and indicate false if
-  // found. The type is not checked, so if it is a transition it is a false
-  // negative.
+  // found. Since we omit an enumeration index check, if it is added via a
+  // transition that shares its descriptor array, this is a false positive.
+  Label entry, loop, done;
+
+  // Skip loop if no descriptors are valid.
+  __ NumberOfOwnDescriptors(ecx, ebx);
+  __ cmp(ecx, 0);
+  __ j(equal, &done);
+
   __ LoadInstanceDescriptors(ebx, ebx);
-  __ mov(ecx, FieldOperand(ebx, FixedArray::kLengthOffset));
-  // ebx: descriptor array
-  // ecx: length of descriptor array
+  // ebx: descriptor array.
+  // ecx: valid entries in the descriptor array.
   // Calculate the end of the descriptor array.
   STATIC_ASSERT(kSmiTag == 0);
   STATIC_ASSERT(kSmiTagSize == 1);
   STATIC_ASSERT(kPointerSize == 4);
-  __ lea(ecx, Operand(ebx, ecx, times_2, FixedArray::kHeaderSize));
+  __ imul(ecx, ecx, DescriptorArray::kDescriptorSize);
+  __ lea(ecx, Operand(ebx, ecx, times_2, DescriptorArray::kFirstOffset));
   // Calculate location of the first key name.
   __ add(ebx, Immediate(DescriptorArray::kFirstOffset));
   // Loop through all the keys in the descriptor array. If one of these is the
   // symbol valueOf the result is false.
-  Label entry, loop;
   __ jmp(&entry);
   __ bind(&loop);
   __ mov(edx, FieldOperand(ebx, 0));
   __ cmp(edx, FACTORY->value_of_symbol());
   __ j(equal, if_false);
   __ add(ebx, Immediate(DescriptorArray::kDescriptorSize * kPointerSize));
   __ bind(&entry);
   __ cmp(ebx, ecx);
   __ j(not_equal, &loop);
 
+  __ bind(&done);
+
   // Reload map as register ebx was used as temporary above.
   __ mov(ebx, FieldOperand(eax, HeapObject::kMapOffset));
 
-  // If a valueOf property is not found on the object check that it's
+  // If a valueOf property is not found on the object check that its
   // prototype is the un-modified String prototype. If not result is false.
   __ mov(ecx, FieldOperand(ebx, Map::kPrototypeOffset));
   __ JumpIfSmi(ecx, if_false);
   __ mov(ecx, FieldOperand(ecx, HeapObject::kMapOffset));
   __ mov(edx, Operand(esi, Context::SlotOffset(Context::GLOBAL_OBJECT_INDEX)));
   __ mov(edx,
          FieldOperand(edx, GlobalObject::kNativeContextOffset));
   __ cmp(ecx,
          ContextOperand(edx,
                         Context::STRING_FUNCTION_PROTOTYPE_MAP_INDEX));
@@ skipping 1842 matching lines @@
   *stack_depth = 0;
   *context_length = 0;
   return previous_;
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32