[stubs]: Support 1->2 byte copies in CopyStringCharacters

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 500 matching lines @@
   // |capacity| size respecting both array's elements kinds.
   void CopyFixedArrayElements(
       ElementsKind from_kind, compiler::Node* from_array, ElementsKind to_kind,
       compiler::Node* to_array, compiler::Node* element_count,
       compiler::Node* capacity,
       WriteBarrierMode barrier_mode = UPDATE_WRITE_BARRIER,
       ParameterMode mode = INTEGER_PARAMETERS);
 
   // Copies |character_count| elements from |from_string| to |to_string|
   // starting at the |from_index|'th character. |from_string| and |to_string|
-  // must be either both one-byte strings or both two-byte strings.
+  // can either be one-byte strings or two-byte strings, although if
+  // |from_string| is two-byte, then |to_string| must be two-byte.
   // |from_index|, |to_index| and |character_count| must be either Smis or
   // intptr_ts depending on |mode| s.t. 0 <= |from_index| <= |from_index| +
   // |character_count| <= from_string.length and 0 <= |to_index| <= |to_index| +
   // |character_count| <= to_string.length.
   void CopyStringCharacters(compiler::Node* from_string,
                             compiler::Node* to_string,
                             compiler::Node* from_index,
                             compiler::Node* to_index,
                             compiler::Node* character_count,
-                            String::Encoding encoding, ParameterMode mode);
+                            String::Encoding from_encoding,
+                            String::Encoding to_encoding, ParameterMode mode);
 
   // Loads an element from |array| of |from_kind| elements by given |offset|
   // (NOTE: not index!), does a hole check if |if_hole| is provided and
   // converts the value so that it becomes ready for storing to array of
   // |to_kind| elements.
   compiler::Node* LoadElementAndPrepareForStore(compiler::Node* array,
                                                 compiler::Node* offset,
                                                 ElementsKind from_kind,
                                                 ElementsKind to_kind,
                                                 Label* if_hole);
@@ skipping 474 matching lines @@
       compiler::Node* feedback_vector, compiler::Node* slot,
       compiler::Node* value);
 
   // Create a new AllocationSite and install it into a feedback vector.
   compiler::Node* CreateAllocationSiteInFeedbackVector(
       compiler::Node* feedback_vector, compiler::Node* slot);
 
   enum class IndexAdvanceMode { kPre, kPost };
 
   void BuildFastLoop(
+      const VariableList& var_list, MachineRepresentation index_rep,
+      compiler::Node* start_index, compiler::Node* end_index,
+      std::function<void(CodeStubAssembler* assembler, compiler::Node* index)>
+          body,
+      int increment, IndexAdvanceMode mode = IndexAdvanceMode::kPre);
+
+  void BuildFastLoop(
       MachineRepresentation index_rep, compiler::Node* start_index,
       compiler::Node* end_index,
       std::function<void(CodeStubAssembler* assembler, compiler::Node* index)>
           body,
-      int increment, IndexAdvanceMode mode = IndexAdvanceMode::kPre);
+      int increment, IndexAdvanceMode mode = IndexAdvanceMode::kPre) {
+    BuildFastLoop(VariableList(0, zone()), index_rep, start_index, end_index,
+                  body, increment, mode);
+  }
 
   enum class ForEachDirection { kForward, kReverse };
 
   void BuildFastFixedArrayForEach(
       compiler::Node* fixed_array, ElementsKind kind,
       compiler::Node* first_element_inclusive,
       compiler::Node* last_element_exclusive,
       std::function<void(CodeStubAssembler* assembler,
                          compiler::Node* fixed_array, compiler::Node* offset)>
           body,
@@ skipping 163 matching lines @@
   }
 #else
 #define CSA_SLOW_ASSERT(x)
 #endif
 
 DEFINE_OPERATORS_FOR_FLAGS(CodeStubAssembler::AllocationFlags);
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_CODE_STUB_ASSEMBLER_H_