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

runtime/vm/symbols.h

Index: runtime/vm/symbols.h
===================================================================
--- runtime/vm/symbols.h	(revision 14902)
+++ runtime/vm/symbols.h	(working copy)
@@ -146,16 +146,18 @@
 // without having to maintain copies in each isolate.
 class Symbols : public AllStatic {
  public:
+  static const uint32_t kMaxOneByteCharCode = 0x7F;

[Ivan Posva, 2012/11/15 09:07:10]

There is no reason to limit this to the maximum one byte char code. How about
just calling it kMaxCharCode and setting it to 255 to get all Latin-1 characters
out of the way.

[Florian Schneider, 2012/11/15 21:54:42]

For now I only want to deal with OneByteStrings in the VM isolate. We can extend
it to more characters if needed.

+
   // List of strings that are pre created in the vm isolate.
-  enum {
+  enum SymbolId {
     kIllegal = 0,
 
 #define DEFINE_SYMBOL_INDEX(symbol, literal)                                   \
     k##symbol,
 PREDEFINED_SYMBOLS_LIST(DEFINE_SYMBOL_INDEX)
 #undef DEFINE_SYMBOL_INDEX
-
-    kMaxId,
+    kMaxPredefinedId,

[Ivan Posva, 2012/11/15 09:07:10]

This causes us to have a hole in the used strings. How about this or something
similar?
#undef DEFINE_SYMBOL_INDEX
    kNullCharId,
    // Further ids for single char code symbols.
    kMaxId = kNullCharId + kMaxCharCode + 1,
    kMaxPredefinedId = kNullCharId,
  };

[Florian Schneider, 2012/11/15 21:54:42]

Done. kNullCharId is an alias for kMaxPredefinedId.

+    kMaxId = kMaxPredefinedId + kMaxOneByteCharCode + 1,
   };
 
   // Access methods for symbols stored in the vm isolate.
@@ -183,8 +185,10 @@
                         intptr_t length);
 
   // Returns char* of predefined symbol.
-  static const char* Name(intptr_t symbol);
+  static const char* Name(SymbolId symbol);
 
+  static RawString* FromCharCode(uint32_t char_code);
+
  private:
   enum {
     kInitialVMIsolateSymtabSize = ((Symbols::kMaxId + 15) & -16),