Porting r10221 to x64 (avoid bailing out to runtime for short substrings).

src/ia32/code-stubs-ia32.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 6163 matching lines @@
 
   __ bind(&seq_or_external_string);
   // Sequential or external string.  Just move string to the expected register.
   __ mov(edi, eax);
 
   __ bind(&underlying_unpacked);
 
   if (FLAG_string_slices) {
     Label copy_routine;
     // edi: underlying subject string
-    // ebx: instance type of original subject string
+    // ebx: instance type of underlying subject string
     // edx: adjusted start index (smi)
     // ecx: length (smi)
     __ cmp(ecx, Immediate(Smi::FromInt(SlicedString::kMinLength)));
     // Short slice.  Copy instead of slicing.
     __ j(less, &copy_routine);
     // 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
     // string's encoding is wrong because we always have to recheck encoding of
     // the newly created string's parent anyways due to externalized strings.
@@ skipping 12 matching lines @@
     __ mov(FieldOperand(eax, SlicedString::kParentOffset), edi);
     __ mov(FieldOperand(eax, SlicedString::kHashFieldOffset),
            Immediate(String::kEmptyHashField));
     __ IncrementCounter(counters->sub_string_native(), 1);
     __ ret(3 * kPointerSize);
 
     __ bind(&copy_routine);
   }
 
   // edi: underlying subject string
-  // ebx: instance type of original subject string
+  // ebx: instance type of underlying subject string
   // edx: adjusted start index (smi)
   // ecx: length (smi)
   // The subject string can only be external or sequential string of either
   // encoding at this point.
   Label two_byte_sequential, runtime_drop_two, sequential_string;
   STATIC_ASSERT(kExternalStringTag != 0);
   STATIC_ASSERT(kSeqStringTag == 0);
   __ test_b(ebx, kExternalStringTag);
   __ j(zero, &sequential_string);
 
   // Handle external string.
-  Label ascii_external, done;
   // Rule out short external strings.
   STATIC_CHECK(kShortExternalStringTag != 0);
   __ test_b(ebx, kShortExternalStringMask);
   __ j(not_zero, &runtime);
   __ mov(edi, FieldOperand(edi, ExternalString::kResourceDataOffset));
   // Move the pointer so that offset-wise, it looks like a sequential string.
   STATIC_ASSERT(SeqTwoByteString::kHeaderSize == SeqAsciiString::kHeaderSize);
   __ sub(edi, Immediate(SeqTwoByteString::kHeaderSize - kHeapObjectTag));
 
   __ bind(&sequential_string);
@@ skipping 1070 matching lines @@
                                  false);
   __ pop(edx);
   __ ret(0);
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32