[libcxx-commits] [libcxx] [libc++] Implement `<bit>` parts of P3793R2 Better shifting (PR #219389)

[A. Jiang via libcxx-commits]

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@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___BIT_SHIFT_H
+#define _LIBCPP___BIT_SHIFT_H
+
+#include <__config>
+#include <__type_traits/integer_traits.h>
+#include <__type_traits/make_unsigned.h>
+#include <limits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Writing two full functions for shl and shr makes it easier for the compiler
+// to optimize the code.
+
+#if _LIBCPP_STD_VER >= 29
+
+template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Shift>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp shl(_Tp __t, _Shift __cnt) noexcept {
+  constexpr auto __n = static_cast<_Shift>(numeric_limits<make_unsigned_t<_Tp>>::digits);
+  if constexpr (is_signed_v<_Shift>) {
+    if (__cnt < 0) {
+      if (__cnt <= -__n)
+        return static_cast<_Tp>(__t < 0 ? -1 : 0);
+      return __t >> -__cnt;
+    }
+  }
+  if (__cnt >= __n)
+    return static_cast<_Tp>(0);
+  return __t << __cnt;
+}
+
+template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Shift>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp shr(_Tp __t, _Shift __cnt) noexcept {
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frederick-vs-ja wrote:

Ditto, `[[nodiscard]]` test coverage.

https://github.com/llvm/llvm-project/pull/219389