Flatten cons string for single character substrings.

For substrings of non-flat cons strings, we bail out
to runtime. For single character substrings, we forget
to flatten it. This causes successive bailouts.

R=bmeurer@chromium.org
BUG=323041
LOG=Y

Review URL: https://codereview.chromium.org/88173002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18081 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -3968,7 +3968,12 @@ MaybeObject* Heap::AllocateSubString(String* buffer,
   int length = end - start;
   if (length <= 0) {
     return empty_string();
-  } else if (length == 1) {
+  }
+
+  // Make an attempt to flatten the buffer to reduce access time.
+  buffer = buffer->TryFlattenGetString();
+
+  if (length == 1) {
     return LookupSingleCharacterStringFromCode(buffer->Get(start));
   } else if (length == 2) {
     // Optimization for 2-byte strings often used as keys in a decompression
@@ -3979,9 +3984,6 @@ MaybeObject* Heap::AllocateSubString(String* buffer,
     return MakeOrFindTwoCharacterString(this, c1, c2);
   }
 
-  // Make an attempt to flatten the buffer to reduce access time.
-  buffer = buffer->TryFlattenGetString();
-
   if (!FLAG_string_slices ||
       !buffer->IsFlat() ||
       length < SlicedString::kMinLength ||

src/runtime.cc

@@ -4463,10 +4463,6 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_SubString) {
   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);
 }