Sharing of descriptor arrays.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 5e306dda57dc5b694281d2240174e522744f4958..ebd54d833ec80c4a003dd2177cb32ee1a529a931 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -2642,22 +2642,28 @@ void FullCodeGenerator::EmitIsStringWrapperSafeForDefaultValueOf(
   __ 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(rcx, rbx);
+  __ cmpq(rcx, Immediate(0));
+  __ j(equal, &done);
+
   __ LoadInstanceDescriptors(rbx, rbx);
-  __ movq(rcx, FieldOperand(rbx, FixedArray::kLengthOffset));
-  // rbx: descriptor array
-  // rcx: length of descriptor array
+  // rbx: descriptor array.
+  // rcx: valid entries in the descriptor array.
   // Calculate the end of the descriptor array.
+  __ imul(rcx, rcx, Immediate(DescriptorArray::kDescriptorSize));
   SmiIndex index = masm_->SmiToIndex(rdx, rcx, kPointerSizeLog2);
   __ lea(rcx,
          Operand(
-             rbx, index.reg, index.scale, FixedArray::kHeaderSize));
+             rbx, index.reg, index.scale, DescriptorArray::kFirstOffset));
   // Calculate location of the first key name.
   __ addq(rbx, 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);
   __ movq(rdx, FieldOperand(rbx, 0));
@@ -2668,10 +2674,11 @@ void FullCodeGenerator::EmitIsStringWrapperSafeForDefaultValueOf(
   __ cmpq(rbx, rcx);
   __ j(not_equal, &loop);
 
+  __ bind(&done);
   // Reload map as register rbx was used as temporary above.
   __ movq(rbx, FieldOperand(rax, 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.
   __ movq(rcx, FieldOperand(rbx, Map::kPrototypeOffset));
   __ testq(rcx, Immediate(kSmiTagMask));