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

[Nikolas Klauser via libcxx-commits]

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@@ -0,0 +1,281 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++29
+
+// template <class T, class S>
+//   constexpr T shl(T x, S s) noexcept;
+
+// Constraints: Each of T and S is a signed or unsigned integer type.
+
+#include <bit>
+#include <cassert>
+#include <limits>
+#include <type_traits>
+#include <concepts>
+
+#include "test_macros.h"
+#include "type_algorithms.h"
+
+struct A {};
+enum E1 : unsigned char { rEd };
+enum class E2 : unsigned char { red };
+
+template <class T>
+concept can_shl_value = requires(T value) { std::shl(value, 1); };
+
+static_assert(can_shl_value<int>, "Concept is broken. We should be able to shift int.");
+static_assert(!can_shl_value<bool>);
+static_assert(!can_shl_value<char>);
+static_assert(!can_shl_value<wchar_t>);
+static_assert(!can_shl_value<char8_t>);
+static_assert(!can_shl_value<char16_t>);
+static_assert(!can_shl_value<char32_t>);
+static_assert(!can_shl_value<A>);
+static_assert(!can_shl_value<A*>);
+static_assert(!can_shl_value<E1>);
+static_assert(!can_shl_value<E2>);
+
+template <class T>
+concept can_shl_shift = requires(T shift) { std::shl(1, shift); };
+
+static_assert(can_shl_shift<int>, "Concept is broken. We should be able to shift by int.");
+static_assert(!can_shl_shift<bool>);
+static_assert(!can_shl_shift<char>);
+static_assert(!can_shl_shift<wchar_t>);
+static_assert(!can_shl_shift<char8_t>);
+static_assert(!can_shl_shift<char16_t>);
+static_assert(!can_shl_shift<char32_t>);
+static_assert(!can_shl_shift<A>);
+static_assert(!can_shl_shift<A*>);
+static_assert(!can_shl_shift<E1>);
+static_assert(!can_shl_shift<E2>);
+
+template <std::signed_integral T>
+constexpr bool test() {
+  using U               = std::make_unsigned_t<T>;
+  constexpr int width   = std::numeric_limits<U>::digits;
+  constexpr U highbit_u = U(1) << (width - 1);
+
+  ASSERT_SAME_TYPE(decltype(std::shl(T(), 0)), T);
+  ASSERT_NOEXCEPT(std::shl(T(), 0));
+
+  assert(std::shl(T(1), 0) == T(1));
+  assert(std::shl(T(1), 0u) == T(1));
+  assert(std::shl(T(1), 1) == T(2));
+  assert(std::shl(T(1), 1u) == T(2));
+  assert(std::shl(T(1), 2) == T(4));
+  assert(std::shl(T(1), 2u) == T(4));
+  assert(std::shl(T(1), 3) == T(8));
+  assert(std::shl(T(1), 3u) == T(8));
+  assert(std::shl(T(1), 4) == T(16));
+  assert(std::shl(T(1), 4u) == T(16));
+  assert(std::shl(T(1), 5) == T(32));
+  assert(std::shl(T(1), 5u) == T(32));
+  assert(std::shl(T(1), 6) == T(64));
+  assert(std::shl(T(1), 6u) == T(64));
+  assert(std::shl(T(1), 7) == T(128));
+  assert(std::shl(T(1), 7u) == T(128));
+
+  // Overlong shifts return 0
+  assert(std::shl(T(1), width) == T(0));
+  assert(std::shl(T(1), static_cast<U>(width)) == T(0));
+  assert(std::shl(T(1), width + 1) == T(0));
+  assert(std::shl(T(1), static_cast<U>(width + 1)) == T(0));
+  assert(std::shl(T(1), width + 100) == T(0));
+  assert(std::shl(T(1), static_cast<U>(width + 100)) == T(0));
+  assert(std::shl(T(1), std::numeric_limits<int>::max()) == T(0));
+  assert(std::shl(T(1), static_cast<U>(std::numeric_limits<int>::max())) == T(0));
+
+  assert(std::shl(T(-1), width) == T(0));
+  assert(std::shl(T(-1), static_cast<U>(width)) == T(0));
+  assert(std::shl(T(-1), width + 5) == T(0));
+  assert(std::shl(T(-1), static_cast<U>(width + 5)) == T(0));
+
+  // Negative shift amounts shift right arithmetically (bidirectional)
+  T highbit = static_cast<T>(highbit_u);
+  assert(std::shl(highbit, -1) == (highbit >> 1));
+  assert(std::shl(highbit, -2) == (highbit >> 2));
+  assert(std::shl(highbit, -3) == (highbit >> 3));
+  assert(std::shl(highbit, -4) == (highbit >> 4));
+  assert(std::shl(highbit, -5) == (highbit >> 5));
+  assert(std::shl(highbit, -6) == (highbit >> 6));
+  assert(std::shl(highbit, -7) == (highbit >> 7));
+  assert(std::shl(highbit, -(width - 1)) == (highbit >> (width - 1)));
+
+  // Negative overlong shifts: sign-extended for negative values, 0 otherwise
+  assert(std::shl(T(-1), -width) == T(-1));
+  assert(std::shl(T(-1), -(width + 1)) == T(-1));
+  assert(std::shl(T(-1), std::numeric_limits<int>::min()) == T(-1));
+  assert(std::shl(T(42), -width) == T(0));
+  assert(std::shl(T(42), -(width + 1)) == T(0));
+
+  if constexpr (width == 128) {
+    assert(std::shl(T(1), 64) == U(1) << 64);
+    assert(std::shl(T(1), 64u) == U(1) << 64);
+    assert(std::shl(T(1), 127) == U(1) << 127);
+    assert(std::shl(T(1), 127u) == U(1) << 127);
+    assert(std::shl(T(1), 128) == T(0));
+    assert(std::shl(T(1), 128u) == T(0));
+    assert(std::shl(T(1), 200) == T(0));
+    assert(std::shl(T(1), 200u) == T(0));
+
+    assert(std::shl(highbit, -64) == (highbit >> 64));
+    assert(std::shl(highbit, -127) == (highbit >> 127));
+    assert(std::shl(highbit, -128) == T(-1));
+    assert(std::shl(highbit, -200) == T(-1));
+  } else if constexpr (width == 256) {
+    assert(std::shl(T(1), 64) == U(1) << 64);
+    assert(std::shl(T(1), 64u) == U(1) << 64);
+    assert(std::shl(T(1), 127) == U(1) << 127);
+    assert(std::shl(T(1), 127u) == U(1) << 127);
+    assert(std::shl(T(1), 128) == U(1) << 128);
+    assert(std::shl(T(1), 128u) == U(1) << 128);
+    assert(std::shl(T(1), 200) == U(1) << 200);
+    assert(std::shl(T(1), 200u) == U(1) << 200);
+    assert(std::shl(T(1), 255) == U(1) << 255);
+    assert(std::shl(T(1), 255u) == U(1) << 255);
+    assert(std::shl(T(1), 256) == T(0));
+    assert(std::shl(T(1), 256u) == T(0));
+    assert(std::shl(T(1), 300) == T(0));
+    assert(std::shl(T(1), 300u) == T(0));
+
+    assert(std::shl(highbit, -64) == (highbit >> 64));
+    assert(std::shl(highbit, -127) == (highbit >> 127));
+    assert(std::shl(highbit, -128) == (highbit >> 128));
+    assert(std::shl(highbit, -200) == (highbit >> 200));
+    assert(std::shl(highbit, -255) == T(-1));
+    assert(std::shl(highbit, -256) == T(-1));
+  }
+
+  return true;
+}
+
+template <std::unsigned_integral T>
+constexpr bool test() {
----------------
philnik777 wrote:

```suggestion
template <class T>
constexpr bool test_unsigned() {
```

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