Combine %_SubString and %_StringCharAt.

src/runtime.cc

 // Copyright 2012 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 3529 matching lines @@
   } else {
     CONVERT_DOUBLE_ARG_CHECKED(from_number, 1);
     CONVERT_DOUBLE_ARG_CHECKED(to_number, 2);
     start = FastD2IChecked(from_number);
     end = FastD2IChecked(to_number);
   }
   RUNTIME_ASSERT(end >= start);
   RUNTIME_ASSERT(start >= 0);
   RUNTIME_ASSERT(end <= value->length());
   isolate->counters()->sub_string_runtime()->Increment();
+  if (end - start == 1) {
+     return isolate->heap()->LookupSingleCharacterStringFromCode(
+         value->Get(start));
+  }
   return value->SubString(start, end);
 }
 
 
 RUNTIME_FUNCTION(MaybeObject*, Runtime_StringMatch) {
   ASSERT_EQ(3, args.length());
 
   CONVERT_ARG_HANDLE_CHECKED(String, subject, 0);
   CONVERT_ARG_HANDLE_CHECKED(JSRegExp, regexp, 1);
   CONVERT_ARG_HANDLE_CHECKED(JSArray, regexp_info, 2);
@@ skipping 9968 matching lines @@
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags,
                                        "Runtime::PerformGC");
   }
 }
 
 
 } }  // namespace v8::internal