[builtins] String.prototype.slice as a CSA builtin.

src/code-stub-assembler.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CODE_STUB_ASSEMBLER_H_
 #define V8_CODE_STUB_ASSEMBLER_H_
 
 #include <functional>
 
 #include "src/compiler/code-assembler.h"
@@ skipping 787 matching lines @@
   Node* IsHoleyFastElementsKind(Node* elements_kind);
   Node* IsElementsKindGreaterThan(Node* target_kind,
                                   ElementsKind reference_kind);
 
   // String helpers.
   // Load a character from a String (might flatten a ConsString).
   Node* StringCharCodeAt(Node* string, Node* index,
                          ParameterMode parameter_mode = SMI_PARAMETERS);
   // Return the single character string with only {code}.
   Node* StringFromCharCode(Node* code);
+
+  enum class SubStringFlags { NONE, FROM_TO_ARE_BOUNDED };
+
   // Return a new string object which holds a substring containing the range
   // [from,to[ of string.  |from| and |to| are expected to be tagged.
-  Node* SubString(Node* context, Node* string, Node* from, Node* to);
+  // If flags has the value FROM_TO_ARE_BOUNDED then from and to are in
+  // the range [0, string-length)
+  Node* SubString(Node* context, Node* string, Node* from, Node* to,
+                  SubStringFlags flags = SubStringFlags::NONE);
 
   // Return a new string object produced by concatenating |first| with |second|.
   Node* StringAdd(Node* context, Node* first, Node* second,
                   AllocationFlags flags = kNone);
 
   // 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
   // sequential string instance, and may only be used to access the string
   // data. The pointer is GC-safe as long as a reference to the container
@@ skipping 799 matching lines @@
   }
 #else
 #define CSA_SLOW_ASSERT(csa, x) ((void)0)
 #endif
 
 DEFINE_OPERATORS_FOR_FLAGS(CodeStubAssembler::AllocationFlags);
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_CODE_STUB_ASSEMBLER_H_