The check for internalized strings relied on the fact that we had less

src/x64/code-stubs-x64.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 3418 matching lines @@
 
 
 static void BranchIfNotInternalizedString(MacroAssembler* masm,
                                           Label* label,
                                           Register object,
                                           Register scratch) {
   __ JumpIfSmi(object, label);
   __ movq(scratch, FieldOperand(object, HeapObject::kMapOffset));
   __ movzxbq(scratch,
              FieldOperand(scratch, Map::kInstanceTypeOffset));
-  // Ensure that no non-strings have the internalized bit set.
-  STATIC_ASSERT(LAST_TYPE < kNotStringTag + kIsInternalizedMask);
   STATIC_ASSERT(kInternalizedTag != 0);
-  __ testb(scratch, Immediate(kIsInternalizedMask));
-  __ j(zero, label);
+  __ and_(scratch, Immediate(kIsNotStringMask | kIsInternalizedMask));
+  __ cmpb(scratch, Immediate(kInternalizedTag | kStringTag));
+  __ j(not_equal, label);
 }
 
 
 void ICCompareStub::GenerateGeneric(MacroAssembler* masm) {
   Label check_unequal_objects, done;
   Condition cc = GetCondition();
   Factory* factory = masm->isolate()->factory();
 
   Label miss;
   CheckInputType(masm, rdx, left_, &miss);
@@ skipping 2384 matching lines @@
   Label miss;
   Condition cond = masm->CheckEitherSmi(left, right, tmp1);
   __ j(cond, &miss, Label::kNear);
 
   // Check that both operands are internalized strings.
   __ movq(tmp1, FieldOperand(left, HeapObject::kMapOffset));
   __ movq(tmp2, FieldOperand(right, HeapObject::kMapOffset));
   __ movzxbq(tmp1, FieldOperand(tmp1, Map::kInstanceTypeOffset));
   __ movzxbq(tmp2, FieldOperand(tmp2, Map::kInstanceTypeOffset));
   STATIC_ASSERT(kInternalizedTag != 0);
-  __ and_(tmp1, tmp2);
-  __ testb(tmp1, Immediate(kIsInternalizedMask));
-  __ j(zero, &miss, Label::kNear);
+  __ and_(tmp1, Immediate(kIsNotStringMask | kIsInternalizedMask));
+  __ cmpb(tmp1, Immediate(kInternalizedTag | kStringTag));
+  __ j(not_equal, &miss, Label::kNear);
+
+  __ and_(tmp2, Immediate(kIsNotStringMask | kIsInternalizedMask));
+  __ cmpb(tmp2, Immediate(kInternalizedTag | kStringTag));
+  __ j(not_equal, &miss, Label::kNear);
 
   // Internalized strings are compared by identity.
   Label done;
   __ cmpq(left, right);
   // Make sure rax is non-zero. At this point input operands are
   // guaranteed to be non-zero.
   ASSERT(right.is(rax));
   __ j(not_equal, &done, Label::kNear);
   STATIC_ASSERT(EQUAL == 0);
   STATIC_ASSERT(kSmiTag == 0);
@@ skipping 23 matching lines @@
 
   // Check that both operands are unique names. This leaves the instance
   // types loaded in tmp1 and tmp2.
   STATIC_ASSERT(kInternalizedTag != 0);
   __ movq(tmp1, FieldOperand(left, HeapObject::kMapOffset));
   __ movq(tmp2, FieldOperand(right, HeapObject::kMapOffset));
   __ movzxbq(tmp1, FieldOperand(tmp1, Map::kInstanceTypeOffset));
   __ movzxbq(tmp2, FieldOperand(tmp2, Map::kInstanceTypeOffset));
 
   Label succeed1;
-  __ testb(tmp1, Immediate(kIsInternalizedMask));
-  __ j(not_zero, &succeed1, Label::kNear);
   __ cmpb(tmp1, Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
+  __ j(equal, &succeed1, Label::kNear);
+  __ and_(tmp1, Immediate(kIsNotStringMask | kIsInternalizedMask));
+  __ cmpb(tmp1, Immediate(kInternalizedTag | kStringTag));
   __ j(not_equal, &miss, Label::kNear);

[Yang, 2013/06/26 15:00:27]

We could be clever here to save one instruction:

SYMBOL_TYPE is right next to the area reserved for internalized strings.

You could check that SYMBOL_TYPE >= tmp1 >= kInternalizedTag.

Add a static assert, something like
STATIC_ASSERT((SYMBOL_TYPE - 1) & kInternalizedMask == kInternalizedTag);
or
STATIC_ASSERT(FIRST_NONSTRING_TYPE == SYMBOL_TYPE)

You could also put this logic into a function called JumpIfNotUniqueName, and
reuse it later too.

[mvstanton, 2013/06/26 16:09:37]

Done.

   __ bind(&succeed1);
 
   Label succeed2;
-  __ testb(tmp2, Immediate(kIsInternalizedMask));
-  __ j(not_zero, &succeed2, Label::kNear);
   __ cmpb(tmp2, Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
+  __ j(equal, &succeed2, Label::kNear);
+  __ and_(tmp2, Immediate(kIsNotStringMask | kIsInternalizedMask));
+  __ cmpb(tmp2, Immediate(kInternalizedTag | kStringTag));
   __ j(not_equal, &miss, Label::kNear);
   __ bind(&succeed2);
 
   // Unique names are compared by identity.
   Label done;
   __ cmpq(left, right);
   // Make sure rax is non-zero. At this point input operands are
   // guaranteed to be non-zero.
   ASSERT(right.is(rax));
   __ j(not_equal, &done, Label::kNear);
@@ skipping 43 matching lines @@
   __ j(not_equal, &not_same, Label::kNear);
   STATIC_ASSERT(EQUAL == 0);
   STATIC_ASSERT(kSmiTag == 0);
   __ Move(rax, Smi::FromInt(EQUAL));
   __ ret(0);
 
   // Handle not identical strings.
   __ bind(&not_same);
 
   // Check that both strings are internalized strings. If they are, we're done
-  // because we already know they are not identical.
+  // because we already know they are not identical. We also know they are both
+  // strings.
   if (equality) {
     Label do_compare;
     STATIC_ASSERT(kInternalizedTag != 0);
     __ and_(tmp1, tmp2);
     __ testb(tmp1, Immediate(kIsInternalizedMask));
     __ j(zero, &do_compare, Label::kNear);
     // Make sure rax is non-zero. At this point input operands are
     // guaranteed to be non-zero.
     ASSERT(right.is(rax));
     __ ret(0);
@@ skipping 135 matching lines @@
     __ Cmp(entity_name, Handle<Name>(name));
     __ j(equal, miss);
 
     Label good;
     // Check for the hole and skip.
     __ CompareRoot(entity_name, Heap::kTheHoleValueRootIndex);
     __ j(equal, &good, Label::kNear);
 
     // Check if the entry name is not a unique name.
     __ movq(entity_name, FieldOperand(entity_name, HeapObject::kMapOffset));
+    __ cmpb(FieldOperand(entity_name, Map::kInstanceTypeOffset),
+            Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
+    __ j(equal, &good, Label::kNear);
+    __ testb(FieldOperand(entity_name, Map::kInstanceTypeOffset),
+             Immediate(kIsNotStringMask));
+    __ j(not_zero, miss);
     __ testb(FieldOperand(entity_name, Map::kInstanceTypeOffset),
              Immediate(kIsInternalizedMask));
-    __ j(not_zero, &good, Label::kNear);
-    __ cmpb(FieldOperand(entity_name, Map::kInstanceTypeOffset),
-            Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
-    __ j(not_equal, miss);
+    __ j(zero, miss);
 
     __ bind(&good);
   }
 
   NameDictionaryLookupStub stub(properties, r0, r0, NEGATIVE_LOOKUP);
   __ Push(Handle<Object>(name));
   __ push(Immediate(name->Hash()));
   __ CallStub(&stub);
   __ testq(r0, r0);
   __ j(not_zero, miss);
@@ skipping 108 matching lines @@
     __ j(equal, &in_dictionary);
 
     if (i != kTotalProbes - 1 && mode_ == NEGATIVE_LOOKUP) {
       // If we hit a key that is not a unique name during negative
       // lookup we have to bailout as this key might be equal to the
       // key we are looking for.
 
       // Check if the entry name is not a unique name.
       Label cont;
       __ movq(scratch, FieldOperand(scratch, HeapObject::kMapOffset));
+      __ cmpb(FieldOperand(scratch, Map::kInstanceTypeOffset),
+              Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
+      __ j(equal, &cont);
+      __ testb(FieldOperand(scratch, Map::kInstanceTypeOffset),
+               Immediate(kIsNotStringMask));
+      __ j(not_zero, &maybe_in_dictionary);
       __ testb(FieldOperand(scratch, Map::kInstanceTypeOffset),
                Immediate(kIsInternalizedMask));
-      __ j(not_zero, &cont);
-      __ cmpb(FieldOperand(scratch, Map::kInstanceTypeOffset),
-              Immediate(static_cast<uint8_t>(SYMBOL_TYPE)));
-      __ j(not_equal, &maybe_in_dictionary);
+      __ j(zero, &maybe_in_dictionary);

[Yang, 2013/06/26 15:00:27]

JumpIfNotUniqueName that I suggested above would be nice here.

[mvstanton, 2013/06/26 16:09:37]

Done.

       __ bind(&cont);
     }
   }
 
   __ bind(&maybe_in_dictionary);
   // If we are doing negative lookup then probing failure should be
   // treated as a lookup success. For positive lookup probing failure
   // should be treated as lookup failure.
   if (mode_ == POSITIVE_LOOKUP) {
     __ movq(scratch, Immediate(0));
@@ skipping 751 matching lines @@
   __ bind(&fast_elements_case);
   GenerateCase(masm, FAST_ELEMENTS);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64