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

src/ia32/codegen-ia32.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 13277 matching lines @@
   // ebx: instance type
   // Calculate length of sub string using the smi values.
   Label result_longer_than_two;
   __ mov(ecx, Operand(esp, 1 * kPointerSize));  // To index.
   __ test(ecx, Immediate(kSmiTagMask));
   __ j(not_zero, &runtime);
   __ mov(edx, Operand(esp, 2 * kPointerSize));  // From index.
   __ test(edx, Immediate(kSmiTagMask));
   __ j(not_zero, &runtime);
   __ sub(ecx, Operand(edx));
+  __ cmp(ecx, FieldOperand(eax, String::kLengthOffset));
+  Label return_eax;
+  __ j(equal, &return_eax);
   // 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.
   __ SmiUntag(ecx);  // Result length is no longer smi.
   __ cmp(ecx, 2);
   __ j(greater, &result_longer_than_two);
   __ j(less, &runtime);
 
   // Sub string of length 2 requested.
   // eax: string
@@ skipping 84 matching lines @@
   ASSERT_EQ(1, kSmiTagSize + kSmiShiftSize);
   __ add(esi, Operand(ebx));
 
   // eax: result string
   // ecx: result length
   // edx: original value of esi
   // edi: first character of result
   // esi: character of sub string start
   StringHelper::GenerateCopyCharactersREP(masm, edi, esi, ecx, ebx, false);
   __ mov(esi, edx);  // Restore esi.
+
+  __ bind(&return_eax);
   __ IncrementCounter(&Counters::sub_string_native, 1);
   __ ret(3 * kPointerSize);
 
   // Just jump to runtime to create the sub string.
   __ bind(&runtime);
   __ TailCallRuntime(Runtime::kSubString, 3, 1);
+
 }
 
 
 void StringCompareStub::GenerateCompareFlatAsciiStrings(MacroAssembler* masm,
                                                         Register left,
                                                         Register right,
                                                         Register scratch1,
                                                         Register scratch2,
                                                         Register scratch3) {
   Label result_not_equal;
@@ skipping 311 matching lines @@
   masm.GetCode(&desc);
   // Call the function from C++.
   return FUNCTION_CAST<MemCopyFunction>(buffer);
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32