[turbofan] Introduces the BitcastWordToTaggedSigned and BitcastTaggedToWord opcodes.

src/code-stub-assembler.cc

 // 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.
 
 #include "src/code-stub-assembler.h"
 #include "src/code-factory.h"
 #include "src/frames-inl.h"
 #include "src/frames.h"
 #include "src/ic/handler-configuration.h"
 #include "src/ic/stub-cache.h"
@@ skipping 278 matching lines @@
   Bind(&return_x);
   return var_x.value();
 }
 
 Node* CodeStubAssembler::SmiShiftBitsConstant() {
   return IntPtrConstant(kSmiShiftSize + kSmiTagSize);
 }
 
 Node* CodeStubAssembler::SmiFromWord32(Node* value) {
   value = ChangeInt32ToIntPtr(value);
-  return WordShl(value, SmiShiftBitsConstant());
+  return BitcastWordToTaggedSigned(WordShl(value, SmiShiftBitsConstant()));
 }
 
 Node* CodeStubAssembler::SmiTag(Node* value) {
   int32_t constant_value;
   if (ToInt32Constant(value, constant_value) && Smi::IsValid(constant_value)) {
     return SmiConstant(Smi::FromInt(constant_value));
   }
-  return WordShl(value, SmiShiftBitsConstant());
+  return BitcastWordToTaggedSigned(WordShl(value, SmiShiftBitsConstant()));
 }
 
 Node* CodeStubAssembler::SmiUntag(Node* value) {
-  return WordSar(value, SmiShiftBitsConstant());
+  return WordSar(BitcastTaggedToWord(value), SmiShiftBitsConstant());
 }
 
 Node* CodeStubAssembler::SmiToWord32(Node* value) {
-  Node* result = WordSar(value, SmiShiftBitsConstant());
+  Node* result = SmiUntag(value);
   if (Is64()) {
     result = TruncateInt64ToInt32(result);
   }
   return result;
 }
 
 Node* CodeStubAssembler::SmiToFloat64(Node* value) {
   return ChangeInt32ToFloat64(SmiToWord32(value));
 }
 
@@ skipping 5011 matching lines @@
                        Heap::kTheHoleValueRootIndex);
 
   // Store the WeakCell in the feedback vector.
   StoreFixedArrayElement(feedback_vector, slot, cell, UPDATE_WRITE_BARRIER,
                          CodeStubAssembler::SMI_PARAMETERS);
   return cell;
 }
 
 }  // namespace internal
 }  // namespace v8