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

src/ia32/code-stubs-ia32.cc

Index: src/ia32/code-stubs-ia32.cc
diff --git a/src/ia32/code-stubs-ia32.cc b/src/ia32/code-stubs-ia32.cc
index 3a33c9e6f91c53979a41cbfbda043837ddd6f455..0c99a1488f8f664b60e1c64fc0c24ff6fa0eb3f9 100644
--- a/src/ia32/code-stubs-ia32.cc
+++ b/src/ia32/code-stubs-ia32.cc
@@ -4827,6 +4827,7 @@ 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.
   STATIC_ASSERT(kSmiTag == 0);
@@ -4857,8 +4858,11 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   __ j(zero, &flat_string);
 
   // Handle non-flat strings.
-  __ test(result_, Immediate(kIsConsStringMask));
-  __ j(zero, &call_runtime_);
+  __ and_(result_, kStringRepresentationMask);
+  __ cmp(result_, kSlicedStringTag);
+  __ j(equal, &sliced_string);
+  __ cmp(result_, kExternalStringTag);
+  __ j(equal, &call_runtime_);
 
   // ConsString.
   // Check whether the right hand side is the empty string (i.e. if
@@ -4876,6 +4880,14 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   STATIC_ASSERT(kSeqStringTag == 0);
   __ test(result_, Immediate(kStringRepresentationMask));
   __ j(not_zero, &call_runtime_);
+  __ jmp(&flat_string);
+
+  // SlicedString, unpack and add offset.
+  __ bind(&sliced_string);
+  __ add(scratch_, FieldOperand(object_, SlicedString::kOffsetOffset));
+  __ mov(object_, FieldOperand(object_, SlicedString::kParentOffset));
+  __ mov(result_, FieldOperand(object_, HeapObject::kMapOffset));
+  __ movzx_b(result_, FieldOperand(result_, Map::kInstanceTypeOffset));
 
   // Check for 1-byte or 2-byte string.
   __ bind(&flat_string);
@@ -5201,6 +5213,8 @@ void StringAddStub::Generate(MacroAssembler* masm) {
   __ and_(ecx, kStringRepresentationMask);
   __ cmp(ecx, 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.
   // eax: first string
   // ebx: length of resulting flat string as a smi
@@ -5612,6 +5626,9 @@ void StringHelper::GenerateHashGetHash(MacroAssembler* masm,
 void SubStringStub::Generate(MacroAssembler* masm) {
   Label runtime;
 
+  if (FLAG_string_slices) {
+    __ jmp(&runtime);
+  }
   // Stack frame on entry.
   //  esp[0]: return address
   //  esp[4]: to