Improve keyed loads on strings by using a new stub.

Instead of going through a runtime function for keyed loads
on strings we invoke a separate specialized stub that
assumes string as receiver type and the key to be a number.

The stub calls a JS builtin function to return the corresponding one-character string.


Review URL: http://codereview.chromium.org/521041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3556 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/builtins.cc

@@ -544,6 +544,11 @@ static void Generate_KeyedLoadIC_Generic(MacroAssembler* masm) {
 }
 
 
+static void Generate_KeyedLoadIC_String(MacroAssembler* masm) {
+  KeyedLoadIC::GenerateString(masm);
+}
+
+
 static void Generate_KeyedLoadIC_ExternalByteArray(MacroAssembler* masm) {
   KeyedLoadIC::GenerateExternalArray(masm, kExternalByteArray);
 }

src/builtins.h

@@ -74,6 +74,7 @@ namespace internal {
   V(KeyedLoadIC_Initialize,     KEYED_LOAD_IC, UNINITIALIZED)             \
   V(KeyedLoadIC_PreMonomorphic, KEYED_LOAD_IC, PREMONOMORPHIC)            \
   V(KeyedLoadIC_Generic,        KEYED_LOAD_IC, MEGAMORPHIC)               \
+  V(KeyedLoadIC_String,         KEYED_LOAD_IC, MEGAMORPHIC)               \
   V(KeyedLoadIC_ExternalByteArray,          KEYED_LOAD_IC, MEGAMORPHIC)   \
   V(KeyedLoadIC_ExternalUnsignedByteArray,  KEYED_LOAD_IC, MEGAMORPHIC)   \
   V(KeyedLoadIC_ExternalShortArray,         KEYED_LOAD_IC, MEGAMORPHIC)   \
@@ -147,7 +148,8 @@ namespace internal {
   V(STRING_ADD_LEFT, 1)                  \
   V(STRING_ADD_RIGHT, 1)                 \
   V(APPLY_PREPARE, 1)                    \
-  V(APPLY_OVERFLOW, 1)
+  V(APPLY_OVERFLOW, 1)                   \
+  V(STRING_CHAR_AT, 1)
 
 
 class ObjectVisitor;

src/ia32/ic-ia32.cc

@@ -391,6 +391,48 @@ void KeyedLoadIC::GenerateGeneric(MacroAssembler* masm) {
 }
 
 
+void KeyedLoadIC::GenerateString(MacroAssembler* masm) {
+  // ----------- S t a t e -------------
+  //  -- esp[0] : return address
+  //  -- esp[4] : key
+  //  -- esp[8] : receiver
+  // -----------------------------------
+  Label miss, index_ok;
+
+  // Pop return address.
+  // Performing the load early is better in the common case.
+  __ pop(eax);
+
+  __ mov(ebx, Operand(esp, 1 * kPointerSize));
+  __ test(ebx, Immediate(kSmiTagMask));
+  __ j(zero, &miss);
+  __ mov(ecx, FieldOperand(ebx, HeapObject::kMapOffset));
+  __ movzx_b(ecx, FieldOperand(ecx, Map::kInstanceTypeOffset));
+  __ test(ecx, Immediate(kIsNotStringMask));
+  __ j(not_zero, &miss);
+
+  // Check if key is a smi or a heap number.
+  __ mov(edx, Operand(esp, 0));
+  __ test(edx, Immediate(kSmiTagMask));
+  __ j(zero, &index_ok);
+  __ mov(ecx, FieldOperand(ebx, HeapObject::kMapOffset));
+  __ cmp(ecx, Factory::heap_number_map());
+  __ j(not_equal, &miss);
+
+  __ bind(&index_ok);
+  // Duplicate receiver and key since they are expected on the stack after
+  // the KeyedLoadIC call.
+  __ push(ebx);  // receiver
+  __ push(edx);  // key
+  __ push(eax);  // return address
+  __ InvokeBuiltin(Builtins::STRING_CHAR_AT, JUMP_FUNCTION);
+
+  __ bind(&miss);
+  __ push(eax);
+  GenerateMiss(masm);
+}
+
+
 void KeyedLoadIC::GenerateExternalArray(MacroAssembler* masm,
                                         ExternalArrayType array_type) {
   // ----------- S t a t e -------------

src/ic.cc

@@ -874,7 +874,9 @@ Object* KeyedLoadIC::Load(State state,
 
   if (use_ic) {
     Code* stub = generic_stub();
-    if (object->IsJSObject()) {
+    if (object->IsString() && key->IsNumber()) {
+      stub = string_stub();
+    } else if (object->IsJSObject()) {
       Handle<JSObject> receiver = Handle<JSObject>::cast(object);
       if (receiver->HasExternalArrayElements()) {
         stub = external_array_stub(receiver->GetElementsKind());

src/ic.h

@@ -280,6 +280,7 @@ class KeyedLoadIC: public IC {
   static void GenerateInitialize(MacroAssembler* masm);
   static void GeneratePreMonomorphic(MacroAssembler* masm);
   static void GenerateGeneric(MacroAssembler* masm);
+  static void GenerateString(MacroAssembler* masm);
 
   // Generators for external array types. See objects.h.
   // These are similar to the generic IC; they optimize the case of
@@ -313,6 +314,9 @@ class KeyedLoadIC: public IC {
   static Code* pre_monomorphic_stub() {
     return Builtins::builtin(Builtins::KeyedLoadIC_PreMonomorphic);
   }
+  static Code* string_stub() {
+    return Builtins::builtin(Builtins::KeyedLoadIC_String);
+  }
   static Code* external_array_stub(JSObject::ElementsKind elements_kind);
 
   static void Clear(Address address, Code* target);

src/runtime.cc

@@ -1384,6 +1384,17 @@ static Object* CharCodeAt(String* subject, Object* index) {
 }
 
 
+static Object* CharFromCode(Object* char_code) {
+  uint32_t code;
+  if (Array::IndexFromObject(char_code, &code)) {
+    if (code <= 0xffff) {
+      return Heap::LookupSingleCharacterStringFromCode(code);
+    }
+  }
+  return Heap::empty_string();
+}
+
+
 static Object* Runtime_StringCharCodeAt(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 2);
@@ -1394,16 +1405,17 @@ static Object* Runtime_StringCharCodeAt(Arguments args) {
 }
 
 
+static Object* Runtime_StringCharAt(Arguments args) {
+  NoHandleAllocation ha;
+  ASSERT(args.length() == 2);
+  return CharFromCode(Runtime_StringCharCodeAt(args));
+}
+
+
 static Object* Runtime_CharFromCode(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 1);
-  uint32_t code;
-  if (Array::IndexFromObject(args[0], &code)) {
-    if (code <= 0xffff) {
-      return Heap::LookupSingleCharacterStringFromCode(code);
-    }
-  }
-  return Heap::empty_string();
+  return CharFromCode(args[0]);
 }
 
 // Forward declarations.

src/runtime.h

@@ -146,6 +146,7 @@ namespace internal {
   \
   /* Strings */ \
   F(StringCharCodeAt, 2, 1) \
+  F(StringCharAt, 2, 1) \
   F(StringIndexOf, 3, 1) \
   F(StringLastIndexOf, 3, 1) \
   F(StringLocaleCompare, 2, 1) \

src/runtime.js

@@ -477,6 +477,17 @@ function TO_STRING() {
 }
 
 
+// Specialized version of String.charAt. It assumes string as
+// the receiver type and that the index is a number.
+function STRING_CHAR_AT(pos) {
+  var char_code = %_FastCharCodeAt(this, pos);
+  if (!%_IsSmi(char_code)) {
+    return %StringCharAt(this, pos);
+  }
+  return %CharFromCode(char_code);
+}
+
+
 /* -------------------------------------
    - - -   C o n v e r s i o n s   - - -
    -------------------------------------