Change generated code for substring not to bail out to runtime system.

src/x64/code-stubs-x64.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4894 matching lines @@
     // If coming from the make_two_character_string path, the string
     // is too short to be sliced anyways.
     STATIC_ASSERT(2 < SlicedString::kMinLength);
     __ jmp(&copy_routine);
     __ bind(&result_longer_than_two);
 
     // rax: string
     // rbx: instance type
     // rcx: sub string length
     // rdx: from index (smi)
-    Label allocate_slice, sliced_string, seq_string;
+    Label allocate_slice, sliced_string, seq_or_external_string;
     __ cmpq(rcx, Immediate(SlicedString::kMinLength));
     // Short slice.  Copy instead of slicing.
     __ j(less, &copy_routine);
-    STATIC_ASSERT(kSeqStringTag == 0);
-    __ testb(rbx, Immediate(kStringRepresentationMask));
-    __ j(zero, &seq_string, Label::kNear);
     STATIC_ASSERT(kIsIndirectStringMask == (kSlicedStringTag & kConsStringTag));
     STATIC_ASSERT(kIsIndirectStringMask != 0);
     __ testb(rbx, Immediate(kIsIndirectStringMask));
-    // External string.  Jump to runtime.
-    __ j(zero, &runtime);
+    __ j(zero, &seq_or_external_string, Label::kNear);
 
     __ testb(rbx, Immediate(kSlicedNotConsMask));
     __ j(not_zero, &sliced_string, Label::kNear);
     // Cons string.  Check whether it is flat, then fetch first part.
     __ CompareRoot(FieldOperand(rax, ConsString::kSecondOffset),
                    Heap::kEmptyStringRootIndex);
     __ j(not_equal, &runtime);
     __ movq(rdi, FieldOperand(rax, ConsString::kFirstOffset));
     __ jmp(&allocate_slice, Label::kNear);
 
     __ bind(&sliced_string);
     // Sliced string.  Fetch parent and correct start index by offset.
     __ addq(rdx, FieldOperand(rax, SlicedString::kOffsetOffset));
     __ movq(rdi, FieldOperand(rax, SlicedString::kParentOffset));
     __ jmp(&allocate_slice, Label::kNear);
 
-    __ bind(&seq_string);
-    // Sequential string.  Just move string to the right register.
+    __ bind(&seq_or_external_string);
+    // Sequential or external string.  Just move string to the correct register.
     __ movq(rdi, rax);
 
     __ bind(&allocate_slice);
     // edi: underlying subject string
     // ebx: instance type of original subject string
     // edx: offset
     // ecx: length
     // Allocate new sliced string.  At this point we do not reload the instance
     // type including the string encoding because we simply rely on the info
     // provided by the original string.  It does not matter if the original
@@ skipping 1072 matching lines @@
                                  xmm0,
                                  &slow_elements);
   __ ret(0);
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64