Some string optimizations:

src/runtime.cc

 // Copyright 2006-2009 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 2258 matching lines @@
                              schar pattern_char,
                              int start_index) {
   for (int i = start_index, n = string.length(); i < n; i++) {
     if (pattern_char == string[i]) {
       return i;
     }
   }
   return -1;
 }
 
+
+template <typename schar>
+static int SingleCharLastIndexOf(Vector<const schar> string,
+                                 schar pattern_char,
+                                 int start_index) {
+  for (int i = start_index; i >= 0; i--) {
+    if (pattern_char == string[i]) {
+      return i;
+    }
+  }
+  return -1;
+}
+
+
 // Trivial string search for shorter strings.
 // On return, if "complete" is set to true, the return value is the
 // final result of searching for the patter in the subject.
 // If "complete" is set to false, the return value is the index where
 // further checking should start, i.e., it's guaranteed that the pattern
 // does not occur at a position prior to the returned index.
 template <typename pchar, typename schar>
 static int SimpleIndexOf(Vector<const schar> subject,
                          Vector<const pchar> pattern,
                          int idx,
@@ skipping 163 matching lines @@
   RUNTIME_ASSERT(start_index <= static_cast<uint32_t>(sub->length()));
   int position = Runtime::StringMatch(sub, pat, start_index);
   return Smi::FromInt(position);
 }
 
 
 static Object* Runtime_StringLastIndexOf(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 3);
 
-  CONVERT_CHECKED(String, sub, args[0]);
-  CONVERT_CHECKED(String, pat, args[1]);
+  CONVERT_ARG_CHECKED(String, sub, 0);
+  CONVERT_ARG_CHECKED(String, pat, 1);
   Object* index = args[2];
 
-  sub->TryFlattenIfNotFlat();
-  pat->TryFlattenIfNotFlat();
-
   uint32_t start_index;
   if (!Array::IndexFromObject(index, &start_index)) return Smi::FromInt(-1);
 
+  if (!sub->IsFlat()) {
+    FlattenString(sub);
+  }
+
   uint32_t pattern_length = pat->length();
   uint32_t sub_length = sub->length();
 
   if (start_index + pattern_length > sub_length) {
     start_index = sub_length - pattern_length;
   }
 
+  if (pattern_length == 1) {
+    AssertNoAllocation no_heap_allocation;  // ensure vectors stay valid
+    if (sub->IsAsciiRepresentation()) {
+      uc16 pchar = pat->Get(0);
+      if (pchar > String::kMaxAsciiCharCode) {
+        return Smi::FromInt(-1);
+      }
+      return Smi::FromInt(SingleCharLastIndexOf(sub->ToAsciiVector(),
+                                                static_cast<char>(pat->Get(0)),
+                                                start_index));
+    } else {
+      return Smi::FromInt(SingleCharLastIndexOf(sub->ToUC16Vector(),
+                                                pat->Get(0),
+                                                start_index));
+    }
+  }
+
+  pat->TryFlattenIfNotFlat();
+
   for (int i = start_index; i >= 0; i--) {
     bool found = true;
     for (uint32_t j = 0; j < pattern_length; j++) {
       if (sub->Get(i + j) != pat->Get(j)) {
         found = false;
         break;
       }
     }
     if (found) return Smi::FromInt(i);
   }
@@ skipping 5693 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal