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[ADT] restrict bit_cast to trivially-constructible To 

Summary: As discussed in r341853 by blaikie, the reinterpret_cast was technically an aliasing violation. Restrict our bit_cast implementation to To which are trivially-constructible (and note the existing restriction to constexpr). Once we move to C++17 we can use a version of bit_cast without these restrictions, or if we care we can SFINAE a different implementation when To isn't trivially-constructible.

Originally landed in r342710 and reverted in r342711 because is_trivially_copyable is only in GCC 5.1 and later.

Reviewers: dblaikie, rsmith

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D52332

llvm-svn: 342739
This commit is contained in:
JF Bastien 2018-09-21 14:31:25 +00:00
parent 2adecff1aa
commit 19262fc596
1 changed files with 14 additions and 11 deletions
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25
llvm/include/llvm/ADT/bit.h
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@ -20,8 +20,19 @@
namespace llvm {
// This implementation of bit_cast is different from the C++17 one in two ways:
// - It isn't constexpr because that requires compiler support.
// - It requires trivially-constructible To, to avoid UB in the implementation.
template <typename To, typename From
, typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
#if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
(defined(__GNUC__) && __GNUC__ >= 5)
, typename = typename std::is_trivially_constructible<To>::type
#elif __has_feature(is_trivially_constructible)
, typename = typename std::enable_if<__is_trivially_constructible(To)>::type
#else
// See comment below.
#endif
#if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
(defined(__GNUC__) && __GNUC__ >= 5)
, typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
@ -38,17 +49,9 @@ template <typename To, typename From
#endif
>
inline To bit_cast(const From &from) noexcept {
alignas(To) unsigned char storage[sizeof(To)];
std::memcpy(&storage, &from, sizeof(To));
#if defined(__GNUC__)
// Before GCC 7.2, GCC thought that this violated strict aliasing.
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstrict-aliasing"
#endif
return reinterpret_cast<To &>(storage);
#if defined(__GNUC__)
#pragma GCC diagnostic pop
#endif
To to;
std::memcpy(&to, &from, sizeof(To));
return to;
}
} // namespace llvm