Tentative implementation of string slices (hidden under the flag --string-slices).

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 667d78131d4500605fa95ecd336fbb1aeff87911..9bd23b5cc3a4939b2c2b729ed8e29a871c6811a6 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -3148,6 +3148,7 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
 
   Register string = ToRegister(instr->string());
   Register index = no_reg;
+  Register offset = ToRegister(instr->TempAt(0));
   int const_index = -1;
   if (instr->index()->IsConstantOperand()) {
     const_index = ToInteger32(LConstantOperand::cast(instr->index()));
@@ -3170,7 +3171,7 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   DeferredStringCharCodeAt* deferred =
       new DeferredStringCharCodeAt(this, instr);
 
-  Label flat_string, ascii_string, done;
+  Label flat_string, ascii_string, cons_string, two_byte_string, done;
 
   // Fetch the instance type of the receiver into result register.
   __ movq(result, FieldOperand(string, HeapObject::kMapOffset));
@@ -3181,15 +3182,34 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   __ testb(result, Immediate(kStringRepresentationMask));
   __ j(zero, &flat_string, Label::kNear);
 
-  // Handle cons strings and go to deferred code for the rest.
-  __ testb(result, Immediate(kIsConsStringMask));
-  __ j(zero, deferred->entry());
+  // Handle non-flat strings.
+  __ and_(result, Immediate(kStringRepresentationMask));
+  __ cmpb(result, Immediate(kConsStringTag));
+  __ j(equal, &cons_string, Label::kNear);
+  __ cmpb(result, Immediate(kExternalStringTag));
+  __ j(equal, deferred->entry());
+
+  // SlicedString.
+  // Unpack slice, add offset and retrieve the result char.
+  __ movq(offset, FieldOperand(string, SlicedString::kOffsetOffset));
+  __ SmiToInteger32(offset, offset);
+  __ movq(string, FieldOperand(string, SlicedString::kParentOffset));
+  __ movq(result, FieldOperand(string, HeapObject::kMapOffset));
+  __ movzxbl(result, FieldOperand(result, Map::kInstanceTypeOffset));
+  __ addq(string, offset);
+  // Check for ASCII or two-byte string.
+  STATIC_ASSERT(kAsciiStringTag != 0);
+  __ testb(result, Immediate(kStringEncodingMask));
+  __ j(not_zero, &ascii_string, Label::kNear);
+  __ addq(string, offset);
+  __ jmp(&two_byte_string, Label::kNear);
 
   // ConsString.
   // Check whether the right hand side is the empty string (i.e. if
   // this is really a flat string in a cons string). If that is not
   // the case we would rather go to the runtime system now to flatten
   // the string.
+  __ bind(&cons_string);
   __ CompareRoot(FieldOperand(string, ConsString::kSecondOffset),
                  Heap::kEmptyStringRootIndex);
   __ j(not_equal, deferred->entry());
@@ -3210,6 +3230,7 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
 
   // Two-byte string.
   // Load the two-byte character code into the result register.
+  __ bind(&two_byte_string);
   STATIC_ASSERT(kSmiTag == 0 && kSmiTagSize == 1);
   if (instr->index()->IsConstantOperand()) {
     __ movzxwl(result,