Add special case of taking a substring that is the entire string to stub.

src/x64/codegen-x64.cc

 // Copyright 2010 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 11556 matching lines @@
 
   // rax: string
   // rbx: instance type
   // Calculate length of sub string using the smi values.
   Label result_longer_than_two;
   __ movq(rcx, Operand(rsp, kToOffset));
   __ movq(rdx, Operand(rsp, kFromOffset));
   __ JumpIfNotBothPositiveSmi(rcx, rdx, &runtime);
 
   __ SmiSub(rcx, rcx, rdx, NULL);  // Overflow doesn't happen.
-  __ j(negative, &runtime);
+  __ cmpq(FieldOperand(rax, String::kLengthOffset), rcx);
+  Label return_rax;
+  __ j(equal, &return_rax);
   // Special handling of sub-strings of length 1 and 2. One character strings
   // are handled in the runtime system (looked up in the single character
   // cache). Two character strings are looked for in the symbol cache.
   __ SmiToInteger32(rcx, rcx);
   __ cmpl(rcx, Immediate(2));
   __ j(greater, &result_longer_than_two);
   __ j(less, &runtime);
 
   // Sub string of length 2 requested.
   // rax: string
@@ skipping 82 matching lines @@
                         SeqAsciiString::kHeaderSize - kHeapObjectTag));
   }
 
   // rax: result string
   // rcx: result length
   // rdx: original value of rsi
   // rdi: first character of result
   // rsi: character of sub string start
   StringHelper::GenerateCopyCharactersREP(masm, rdi, rsi, rcx, false);
   __ movq(rsi, rdx);  // Restore esi.
+
+  __ bind(&return_rax);
   __ IncrementCounter(&Counters::sub_string_native, 1);
   __ ret(kArgumentsSize);
 
   // Just jump to runtime to create the sub string.
   __ bind(&runtime);
   __ TailCallRuntime(Runtime::kSubString, 3, 1);
 }
 
 
 void StringCompareStub::GenerateCompareFlatAsciiStrings(MacroAssembler* masm,
@@ skipping 212 matching lines @@
 }
 
 #endif
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64