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 /// \file
10 /// This file implements the C++20 <bit> header.
11 ///
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_ADT_BIT_H
15 #define LLVM_ADT_BIT_H
16 
17 #include "llvm/Support/Compiler.h"
18 #include <cstring>
19 #include <type_traits>
20 
21 namespace llvm {
22 
23 // This implementation of bit_cast is different from the C++17 one in two ways:
24 //  - It isn't constexpr because that requires compiler support.
25 //  - It requires trivially-constructible To, to avoid UB in the implementation.
26 template <
27     typename To, typename From,
28     typename = std::enable_if_t<sizeof(To) == sizeof(From)>
29 #if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
30     (defined(__GNUC__) && __GNUC__ >= 5)
31     ,
32     typename = std::enable_if_t<std::is_trivially_constructible<To>::value>
33 #elif __has_feature(is_trivially_constructible)
34     ,
35     typename = std::enable_if_t<__is_trivially_constructible(To)>
36 #else
37   // See comment below.
38 #endif
39 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
40     (defined(__GNUC__) && __GNUC__ >= 5)
41     ,
42     typename = std::enable_if_t<std::is_trivially_copyable<To>::value>,
43     typename = std::enable_if_t<std::is_trivially_copyable<From>::value>
44 #elif __has_feature(is_trivially_copyable)
45     ,
46     typename = std::enable_if_t<__is_trivially_copyable(To)>,
47     typename = std::enable_if_t<__is_trivially_copyable(From)>
48 #else
49 // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
50 // llvm/Support/type_traits.h's is_trivially_copyable we don't want to
51 // provide a good-enough answer here: developers in that configuration will hit
52 // compilation failures on the bots instead of locally. That's acceptable
53 // because it's very few developers, and only until we move past C++11.
54 #endif
55     >
56 inline To bit_cast(const From &from) noexcept {
57   To to;
58   std::memcpy(&to, &from, sizeof(To));
59   return to;
60 }
61 
62 } // namespace llvm
63 
64 #endif
65