[string] Refactor direct string conversions

src/code-stub-assembler.h

Index: src/code-stub-assembler.h
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index 538d548af89dcdb07d4eeaf044b8c99a751866ea..5139f402fe3009b5c06d73e55242dc861e5bb699 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -692,6 +692,8 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   Node* IsSpecialReceiverInstanceType(Node* instance_type);
   Node* IsStringInstanceType(Node* instance_type);
   Node* IsOneByteStringInstanceType(Node* instance_type);
+  Node* IsExternalStringInstanceType(Node* instance_type);
+  Node* IsShortExternalStringInstanceType(Node* instance_type);
   Node* IsSequentialStringInstanceType(Node* instance_type);
   Node* IsString(Node* object);
   Node* IsJSObject(Node* object);
@@ -738,14 +740,6 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   Node* StringAdd(Node* context, Node* first, Node* second,
                   AllocationFlags flags = kNone);
 
-  // Tries to unpack |string| into a pseudo-sequential string. For instance,
-  // In addition to the work done by TryDerefExternalString and
-  // MaybeDerefIndirectString, this method can also unpack sliced strings into
-  // a (string, offset) pair. The same GC restrictions on the returned string
-  // value apply as for TryDerefExternalString.
-  void TryUnpackString(Variable* var_string, Variable* var_offset,
-                       Variable* var_instance_type, Label* if_bailout);
-
   // Unpack the external string, returning a pointer that (offset-wise) looks
   // like a sequential string.
   // Note that this pointer is not tagged and does not point to a real
@@ -1399,6 +1393,42 @@ class CodeStubArguments {
   Node* fp_;
 };
 
+class ToDirectStringAssembler : public CodeStubAssembler {
+ private:
+  enum StringPointerKind { PTR_TO_DATA, PTR_TO_STRING };
+
+ public:
+  explicit ToDirectStringAssembler(compiler::CodeAssemblerState* state,
+                                   Node* string);
+
+  // Converts flat cons, thin, and sliced strings and returns the direct
+  // string. The result can be either a sequential or external string.
+  Node* TryToDirect(Label* if_bailout);
+
+  // Returns a pointer to the beginning of the string data.
+  Node* PointerToData(Label* if_bailout) {
+    return TryToSequential(PTR_TO_DATA, if_bailout);
+  }
+
+  // Returns a pointer that, offset-wise, looks like a String.
+  Node* PointerToString(Label* if_bailout) {
+    return TryToSequential(PTR_TO_STRING, if_bailout);
+  }
+
+  Node* string() { return var_string_.value(); }
+  Node* instance_type() { return var_instance_type_.value(); }
+  Node* offset() { return var_offset_.value(); }
+  Node* is_external() { return var_is_external_.value(); }
+
+ private:
+  Node* TryToSequential(StringPointerKind ptr_kind, Label* if_bailout);
+
+  Variable var_string_;
+  Variable var_instance_type_;
+  Variable var_offset_;
+  Variable var_is_external_;
+};
+
 #ifdef DEBUG
 #define CSA_ASSERT(csa, x) \
   (csa)->Assert([&] { return (x); }, #x, __FILE__, __LINE__)