Make base/numerics abs match runtime libraries

base/numerics/safe_conversions_impl.h

 // Copyright 2014 The Chromium 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 BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 #define BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 
 #include <stdint.h>
 
 #include <limits>
@@ skipping 36 matching lines @@
 // This wrapper undoes the standard integer promotions.
 template <typename T>
 constexpr T BinaryComplement(T x) {
   return static_cast<T>(~x);
 }
 
 // This performs a safe, non-branching absolute value via unsigned overflow.
 template <typename T>
 constexpr T SafeUnsignedAbsImpl(T value, T sign_mask) {
   static_assert(!std::is_signed<T>::value, "Types must be unsigned.");
-  return (value + sign_mask) ^ sign_mask;
+  return (value ^ sign_mask) - sign_mask;
 }
 
 template <typename T,
           typename std::enable_if<std::is_integral<T>::value &&
                                   std::is_signed<T>::value>::type* = nullptr>
 constexpr typename std::make_unsigned<T>::type SafeUnsignedAbs(T value) {
   using UnsignedT = typename std::make_unsigned<T>::type;
   return SafeUnsignedAbsImpl(
       static_cast<UnsignedT>(value),
       // The sign mask is all ones for negative and zero otherwise.
@@ skipping 580 matching lines @@
                    static_cast<BigType>(static_cast<L>(lhs)),
                    static_cast<BigType>(static_cast<R>(rhs)))
              // Let the template functions figure it out for mixed types.
              : C<L, R>::Test(lhs, rhs);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_