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..9f293295255e5a77a25a0ccd1e14b106ec598d73 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);
@@ -2374,6 +2373,11 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ cmpl(rdx, rdi);
   __ j(greater, &runtime);
 
+  // Reset offset for possibly sliced string.  This also serves as indicator
+  // whether the subject string is a sliced string.  0 is not suitable for this
+  // purpose because a slice can start at offset 0 but have a shorter length.
+#define NOT_SLICED -1
+  __ Set(r14, NOT_SLICED);
   // rax: RegExp data (FixedArray)
   // Check the representation and encoding of the subject string.
   Label seq_ascii_string, seq_two_byte_string, check_code;
@@ -2386,28 +2390,43 @@ 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 theSTATIC_ASSERT(String::kMaxLength < 0x7fffffff && 0x7fffffff < NOT_SLICED); same length as the
+  // parent string.
+  Label cons_string, check_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.
+  __ SmiToInteger32(r14, FieldOperand(rdi, SlicedString::kOffsetOffset));
+  __ SmiToInteger32(r15, FieldOperand(rdi, SlicedString::kLengthOffset));
+  __ addq(r15, r14);  // Add offset to length to get input end.
+  __ movq(rdi, FieldOperand(rdi, SlicedString::kParentOffset));
+  // r14: offset of sliced string
+  // r15: length of sliced string + slice offset
+  // rdi: parent string.
+  __ jmp(&check_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(&check_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);
@@ -2504,33 +2523,43 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   // rbx: previous index
   // rcx: encoding of subject string (1 if ascii 0 if two_byte);
   // r11: code
+  // r14: slice offset or NOT_SLICE
+  // r15: slice length + slice offset if r14 == NOTS_SLICE
+
+  // Argument 2: Previous index.
+  __ movq(arg2, rbx);
 
   // Argument 4: End of string data
   // Argument 3: Start of string data
-  Label setup_two_byte, setup_rest;
+  Label setup_two_byte, setup_rest, got_length, length_not_from_slice;
+  // Prepare start and end index of the input.
+  // Load the length from the original sliced string if that is the case.
+  __ cmpq(r14, Immediate(NOT_SLICED));
+  __ j(equal, &length_not_from_slice, Label::kNear);
+  __ addq(rbx, r14);
+  __ jmp(&got_length);
+  __ bind(&length_not_from_slice);
+  __ SmiToInteger32(r15, FieldOperand(rdi, String::kLengthOffset));
+  __ bind(&got_length);
+
+  // rbx: start index of the input
+  // r15: end index of the input
   __ testb(rcx, rcx);  // Last use of rcx as encoding of subject string.
   __ j(zero, &setup_two_byte, Label::kNear);
-  __ SmiToInteger32(rcx, FieldOperand(rdi, String::kLengthOffset));
-  __ lea(arg4, FieldOperand(rdi, rcx, times_1, SeqAsciiString::kHeaderSize));
+  __ lea(arg4, FieldOperand(rdi, r15, times_1, SeqAsciiString::kHeaderSize));
   __ lea(arg3, FieldOperand(rdi, rbx, times_1, SeqAsciiString::kHeaderSize));
   __ jmp(&setup_rest, Label::kNear);
   __ bind(&setup_two_byte);
-  __ SmiToInteger32(rcx, FieldOperand(rdi, String::kLengthOffset));
-  __ lea(arg4, FieldOperand(rdi, rcx, times_2, SeqTwoByteString::kHeaderSize));
+  __ lea(arg4, FieldOperand(rdi, r15, times_2, SeqTwoByteString::kHeaderSize));
   __ lea(arg3, FieldOperand(rdi, rbx, times_2, SeqTwoByteString::kHeaderSize));
-
   __ bind(&setup_rest);
-  // Argument 2: Previous index.
-  __ movq(arg2, rbx);
 
-  // Argument 1: Subject string.
-#ifdef _WIN64
-  __ movq(arg1, rdi);
-#else
-  // Already there in AMD64 calling convention.
-  ASSERT(arg1.is(rdi));
-  USE(arg1);
-#endif
+  // Argument 1: Original subject string.
+  // The original subject is in the previous stack frame. Therefore we have to
+  // use rbp, which points exactly to one pointer size below the previous rsp.
+  // (Because creating a new stack frame pushes the previous rbp onto the stack
+  // and thereby moves up rsp by one kPointerSize.)
+  __ movq(arg1, Operand(rbp, kSubjectOffset + kPointerSize));
 
   // Locate the code entry and call it.
   __ addq(r11, Immediate(Code::kHeaderSize - kHeapObjectTag));
@@ -3780,6 +3809,7 @@ 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 +3838,11 @@ 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 +3860,14 @@ 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 +4178,8 @@ 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 +4572,9 @@ 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