[ARM] Add Neon shift instructions vshl, vshr.

src/utils.h

 // Copyright 2012 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_UTILS_H_
 #define V8_UTILS_H_
 
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
@@ skipping 119 matching lines @@
     nibble += 8;
     x >>= 8;
   }
   if (x & 0xf0) {
     nibble += 4;
     x >>= 4;
   }
   return nibble + msb4[x];
 }
 
-
-// The C++ standard leaves the semantics of '>>' undefined for
-// negative signed operands. Most implementations do the right thing,
-// though.
-inline int ArithmeticShiftRight(int x, int s) {
-  return x >> s;
+template <typename T>
+static T ArithmeticShiftRight(T x, int shift) {
+  DCHECK_LE(0, shift);
+  if (x < 0) {
+    // Right shift of signed values is implementation defined. Simulate a
+    // true arithmetic right shift by adding leading sign bits.
+    using UnsignedT = typename std::make_unsigned<T>::type;
+    UnsignedT mask = ~(static_cast<UnsignedT>(~0) >> shift);
+    return (static_cast<UnsignedT>(x) >> shift) | mask;
+  } else {
+    return x >> shift;
+  }
 }
 
-
 template <typename T>
 int Compare(const T& a, const T& b) {
   if (a == b)
     return 0;
   else if (a < b)
     return -1;
   else
     return 1;
 }
 
@@ skipping 1540 matching lines @@
  private:
   T value_;
   ThreadedListZoneEntry<T>* next_;
   DISALLOW_COPY_AND_ASSIGN(ThreadedListZoneEntry);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_UTILS_H_