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

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 1a6efcbd61acc781addb3f175e7c5e7f6cd1dd6b..094e158f9a53c7890582b11ad15b388a2020ad04 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -3780,6 +3780,7 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   Label flat_string;
   Label ascii_string;
   Label got_char_code;
+  Label sliced_string;
 
   // If the receiver is a smi trigger the non-string case.
   __ JumpIfSmi(object_, receiver_not_string_);
@@ -3808,8 +3809,11 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   __ j(zero, &flat_string);
 
   // Handle non-flat strings.
-  __ testb(result_, Immediate(kIsConsStringMask));
-  __ j(zero, &call_runtime_);
+  __ and_(result_, Immediate(kStringRepresentationMask));
+  __ cmpb(result_, Immediate(kSlicedStringTag));
+  __ j(equal, &sliced_string);
+  __ cmpb(result_, Immediate(kExternalStringTag));
+  __ j(equal, &call_runtime_);
 
   // ConsString.
   // Check whether the right hand side is the empty string (i.e. if
@@ -3827,6 +3831,14 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   STATIC_ASSERT(kSeqStringTag == 0);
   __ testb(result_, Immediate(kStringRepresentationMask));
   __ j(not_zero, &call_runtime_);
+  __ jmp(&flat_string);
+
+  // SlicedString, unpack and add offset.
+  __ bind(&sliced_string);
+  __ addq(scratch_, FieldOperand(object_, SlicedString::kOffsetOffset));
+  __ movq(object_, FieldOperand(object_, SlicedString::kParentOffset));
+  __ movq(result_, FieldOperand(object_, HeapObject::kMapOffset));
+  __ movzxbl(result_, FieldOperand(result_, Map::kInstanceTypeOffset));
 
   // Check for 1-byte or 2-byte string.
   __ bind(&flat_string);
@@ -4137,6 +4149,8 @@ void StringAddStub::Generate(MacroAssembler* masm) {
   __ and_(rcx, Immediate(kStringRepresentationMask));
   __ cmpl(rcx, Immediate(kExternalStringTag));
   __ j(equal, &string_add_runtime);
+  // We cannot encounter sliced strings here since:
+  STATIC_ASSERT(SlicedString::kMinLength >= String::kMinNonFlatLength);
   // Now check if both strings are ascii strings.
   // rax: first string
   // rbx: length of resulting flat string
@@ -4529,6 +4543,9 @@ void StringHelper::GenerateHashGetHash(MacroAssembler* masm,
 void SubStringStub::Generate(MacroAssembler* masm) {
   Label runtime;
 
+  if (FLAG_string_slices) {
+    __ jmp(&runtime);
+  }
   // Stack frame on entry.
   //  rsp[0]: return address
   //  rsp[8]: to