Fixing issue 1757 (string slices of external strings).

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index b04e1ff59834cbe2261e260dbbe279a6a958d256..3fc4e15e0560bd972527b731670e77e0a126fe81 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -4084,22 +4084,22 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
                  Heap::kEmptyStringRootIndex);
   __ j(not_equal, &call_runtime_);
   // Get the first of the two strings and load its instance type.
-  __ movq(object_, FieldOperand(object_, ConsString::kFirstOffset));
+  __ movq(kScratchRegister, FieldOperand(object_, ConsString::kFirstOffset));
   __ jmp(&assure_seq_string, Label::kNear);
 
   // SlicedString, unpack and add offset.
   __ bind(&sliced_string);
   __ addq(scratch_, FieldOperand(object_, SlicedString::kOffsetOffset));
-  __ movq(object_, FieldOperand(object_, SlicedString::kParentOffset));
+  __ movq(kScratchRegister, FieldOperand(object_, SlicedString::kParentOffset));
 
   __ bind(&assure_seq_string);
-  __ movq(result_, FieldOperand(object_, HeapObject::kMapOffset));
+  __ movq(result_, FieldOperand(kScratchRegister, HeapObject::kMapOffset));
   __ movzxbl(result_, FieldOperand(result_, Map::kInstanceTypeOffset));
   // If the first cons component is also non-flat, then go to runtime.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ testb(result_, Immediate(kStringRepresentationMask));
   __ j(not_zero, &call_runtime_);
-  __ jmp(&flat_string);
+  __ movq(object_, kScratchRegister);
 
   // Check for 1-byte or 2-byte string.
   __ bind(&flat_string);