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..26dd38d4c520c72822ebe4e90b4a7bbfaaf46dd3 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -2303,7 +2303,6 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ testq(kScratchRegister, kScratchRegister);
   __ j(zero, &runtime);
 
-
   // Check that the first argument is a JSRegExp object.
   __ movq(rax, Operand(rsp, kJSRegExpOffset));
   __ JumpIfSmi(rax, &runtime);
@@ -2386,28 +2385,42 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   STATIC_ASSERT((kStringTag | kSeqStringTag | kTwoByteStringTag) == 0);
   __ j(zero, &seq_two_byte_string, Label::kNear);
   // Any other flat string must be a flat ascii string.
-  __ testb(rbx, Immediate(kIsNotStringMask | kStringRepresentationMask));
+  __ andb(rbx, Immediate(kIsNotStringMask | kStringRepresentationMask));
   __ j(zero, &seq_ascii_string, Label::kNear);
 
-  // Check for flat cons string.
+  // Check for flat cons string or truncated sliced string.
   // A flat cons string is a cons string where the second part is the empty
   // string. In that case the subject string is just the first part of the cons
   // string. Also in this case the first part of the cons string is known to be
   // a sequential string or an external string.
-  STATIC_ASSERT(kExternalStringTag !=0);
-  STATIC_ASSERT((kConsStringTag & kExternalStringTag) == 0);
-  __ testb(rbx, Immediate(kIsNotStringMask | kExternalStringTag));
-  __ j(not_zero, &runtime);
-  // String is a cons string.
+  // A truncated sliced string has the offset 0 and the same length as the
+  // parent string.
+  Label cons_string, encoding;
+  __ cmpq(rbx, Immediate(kConsStringTag));
+  __ j(equal, &cons_string, Label::kNear);
+  __ cmpq(rbx, Immediate(kSlicedStringTag));
+  // If subject is not a sliced string, it can only be a non-string or an
+  // external string.
+  __ j(not_equal, &runtime);
+  // String is sliced, check whether it is truncated.
+  __ movq(rbx, FieldOperand(rdi, SlicedString::kLengthOffset));
+  __ movq(rdi, FieldOperand(rdi, SlicedString::kParentOffset));
+  // rbx: length of sliced string.
+  // rdi: parent string.
+  __ cmpq(rbx, FieldOperand(rdi, SlicedString::kLengthOffset));
+  __ j(not_equal, &runtime);
+  __ jmp(&encoding, Label::kNear);
+  // String is a cons string, check whether it is flat.
+  __ bind(&cons_string);
   __ CompareRoot(FieldOperand(rdi, ConsString::kSecondOffset),
                  Heap::kEmptyStringRootIndex);
   __ j(not_equal, &runtime);
   __ movq(rdi, FieldOperand(rdi, ConsString::kFirstOffset));
+  // rdi: first part of cons string or parent of sliced string.
+  // rbx: map of first part of cons string or map of parent of sliced string.
+  // Is first part of cons or parent of slice a flat two byte string?
+  __ bind(&encoding);
   __ movq(rbx, FieldOperand(rdi, HeapObject::kMapOffset));
-  // String is a cons string with empty second part.
-  // rdi: first part of cons string.
-  // rbx: map of first part of cons string.
-  // Is first part a flat two byte string?
   __ testb(FieldOperand(rbx, Map::kInstanceTypeOffset),
            Immediate(kStringRepresentationMask | kStringEncodingMask));
   STATIC_ASSERT((kSeqStringTag | kTwoByteStringTag) == 0);
@@ -3780,6 +3793,7 @@ 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 +3822,11 @@ 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 +3844,14 @@ 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 +4162,8 @@ 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 +4556,9 @@ 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