include/llvm/ADT/bit.h

   1 //===-- llvm/ADT/bit.h - C++20 <bit> ----------------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the C++20 <bit> header.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #ifndef LLVM_ADT_BIT_H
  14 #define LLVM_ADT_BIT_H
  15
  16 #include "llvm/Support/Compiler.h"
  17 #include <cstring>
  18 #include <type_traits>
  19
  20 namespace llvm {
  21
  22 // This implementation of bit_cast is different from the C++17 one in two ways:
  23 //  - It isn't constexpr because that requires compiler support.
  24 //  - It requires trivially-constructible To, to avoid UB in the implementation.
  25 template <typename To, typename From
  26           , typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
  27 #if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
  28     (defined(__GNUC__) && __GNUC__ >= 5)
  29           , typename = typename std::is_trivially_constructible<To>::type
  30 #elif __has_feature(is_trivially_constructible)
  31           , typename = typename std::enable_if<__is_trivially_constructible(To)>::type
  32 #else
  33   // See comment below.
  34 #endif
  35 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
  36     (defined(__GNUC__) && __GNUC__ >= 5)
  37           , typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
  38           , typename = typename std::enable_if<std::is_trivially_copyable<From>::value>::type
  39 #elif __has_feature(is_trivially_copyable)
  40           , typename = typename std::enable_if<__is_trivially_copyable(To)>::type
  41           , typename = typename std::enable_if<__is_trivially_copyable(From)>::type
  42 #else
  43 // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
  44 // llvm/Support/type_traits.h's is_trivially_copyable we don't want to
  45 // provide a good-enough answer here: developers in that configuration will hit
  46 // compilation failures on the bots instead of locally. That's acceptable
  47 // because it's very few developers, and only until we move past C++11.
  48 #endif
  49 >
  50 inline To bit_cast(const From &from) noexcept {
  51   To to;
  52   std::memcpy(&to, &from, sizeof(To));
  53   return to;
  54 }
  55
  56 } // namespace llvm
  57
  58 #endif