libc/src/__support/CPP/bit.h

   1 //===-- Freestanding version of bit_cast  -----------------------*- 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 #ifndef LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H
  10 #define LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H
  11
  12 #include "src/__support/CPP/type_traits.h"
  13 #include "src/__support/macros/attributes.h"
  14 #include "src/__support/macros/config.h" // LIBC_HAS_BUILTIN
  15 #include "src/__support/macros/sanitizer.h"
  16
  17 namespace LIBC_NAMESPACE::cpp {
  18
  19 #if LIBC_HAS_BUILTIN(__builtin_bit_cast)
  20 #define LLVM_LIBC_HAS_BUILTIN_BIT_CAST
  21 #endif
  22
  23 #if LIBC_HAS_BUILTIN(__builtin_memcpy_inline)
  24 #define LLVM_LIBC_HAS_BUILTIN_MEMCPY_INLINE
  25 #endif
  26
  27 // This function guarantees the bitcast to be optimized away by the compiler for
  28 // GCC >= 8 and Clang >= 6.
  29 template <class To, class From>
  30 LIBC_INLINE constexpr To bit_cast(const From &from) {
  31   static_assert(sizeof(To) == sizeof(From), "To and From must be of same size");
  32   static_assert(cpp::is_trivially_copyable<To>::value &&
  33                     cpp::is_trivially_copyable<From>::value,
  34                 "Cannot bit-cast instances of non-trivially copyable classes.");
  35   MSAN_UNPOISON(&from, sizeof(From));
  36 #if defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
  37   return __builtin_bit_cast(To, from);
  38 #else
  39   static_assert(cpp::is_trivially_constructible<To>::value,
  40                 "This implementation additionally requires destination type to "
  41                 "be trivially constructible");
  42   To to;
  43   char *dst = reinterpret_cast<char *>(&to);
  44   const char *src = reinterpret_cast<const char *>(&from);
  45 #if defined(LLVM_LIBC_HAS_BUILTIN_MEMCPY_INLINE)
  46   __builtin_memcpy_inline(dst, src, sizeof(To));
  47 #else
  48   for (unsigned i = 0; i < sizeof(To); ++i)
  49     dst[i] = src[i];
  50 #endif // defined(LLVM_LIBC_HAS_BUILTIN_MEMCPY_INLINE)
  51   return to;
  52 #endif // defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
  53 }
  54
  55 template <class To, class From>
  56 LIBC_INLINE constexpr To bit_or_static_cast(const From &from) {
  57   if constexpr (sizeof(To) == sizeof(From)) {
  58     return bit_cast<To>(from);
  59   } else {
  60     return static_cast<To>(from);
  61   }
  62 }
  63
  64 } // namespace LIBC_NAMESPACE::cpp
  65
  66 #endif // LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H