Add one-char string table for faster String.charAt to the VM.

runtime/vm/symbols.cc

Index: runtime/vm/symbols.cc
===================================================================
--- runtime/vm/symbols.cc	(revision 14980)
+++ runtime/vm/symbols.cc	(working copy)
@@ -26,8 +26,8 @@
 };
 
 
-const char* Symbols::Name(intptr_t symbol) {
-  ASSERT((symbol > kIllegal) && (symbol < kMaxId));
+const char* Symbols::Name(SymbolId symbol) {
+  ASSERT((symbol > kIllegal) && (symbol < kMaxPredefinedId));
   return names[symbol];
 }
 
@@ -40,12 +40,12 @@
   SetupSymbolTable(isolate);
 
   // Create all predefined symbols.
-  ASSERT((sizeof(names) / sizeof(const char*)) == Symbols::kMaxId);
+  ASSERT((sizeof(names) / sizeof(const char*)) == Symbols::kMaxPredefinedId);
   ObjectStore* object_store = isolate->object_store();
   Array& symbol_table = Array::Handle();
   dart::String& str = String::Handle();
 
-  for (intptr_t i = 1; i < Symbols::kMaxId; i++) {
+  for (intptr_t i = 1; i < Symbols::kMaxPredefinedId; i++) {
     // The symbol_table needs to be reloaded as it might have grown in the
     // previous iteration.
     symbol_table = object_store->symbol_table();
@@ -54,6 +54,11 @@
     predefined_[i] = str.raw();
   }
   Object::RegisterSingletonClassNames();
+
+  for (uint32_t c = 0; c <= kMaxOneByteCharCode; c++) {
+    ASSERT(kMaxPredefinedId + c < kMaxId);
+    predefined_[kMaxPredefinedId + c] = New(&c, 1);
+  }
 }
 
 
@@ -117,7 +122,6 @@
 template<typename T>
 RawString* Symbols::New(const T* characters, intptr_t len) {
   Isolate* isolate = Isolate::Current();
-  ASSERT(isolate != Dart::vm_isolate());
   String& symbol = String::Handle(isolate, String::null());
   Array& symbol_table = Array::Handle(isolate, Array::null());
 
@@ -200,6 +204,14 @@
 }
 
 
+RawString* Symbols::FromCharCode(uint32_t char_code) {
+  if (char_code > kMaxOneByteCharCode) {
+    return New(&char_code, 1);
+  }
+  return predefined_[kNullCharId + char_code];
+}
+
+
 void Symbols::GrowSymbolTable(const Array& symbol_table) {
   // TODO(iposva): Avoid exponential growth.
   intptr_t table_size = symbol_table.Length() - 1;