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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. Reviewers: dblaikie, rsmith Subscribers: dexonsmith, kristina, llvm-commits Differential Revision: https://reviews.llvm.org/D52332 llvm-svn: 342710
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@ -20,8 +20,12 @@
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namespace llvm {
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namespace llvm {
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// This implementation of bit_cast is different from the C++17 one in two ways:
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// - It isn't constexpr because that requires compiler support.
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// - It requires trivially-constructible To, to avoid UB in the implementation.
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template <typename To, typename From
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template <typename To, typename From
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, typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
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, typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
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, typename = typename std::is_trivially_constructible<To>::type
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#if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
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#if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
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(defined(__GNUC__) && __GNUC__ >= 5)
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(defined(__GNUC__) && __GNUC__ >= 5)
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, typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
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, typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
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@ -38,17 +42,9 @@ template <typename To, typename From
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#endif
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#endif
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>
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>
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inline To bit_cast(const From &from) noexcept {
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inline To bit_cast(const From &from) noexcept {
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alignas(To) unsigned char storage[sizeof(To)];
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To to;
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std::memcpy(&storage, &from, sizeof(To));
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std::memcpy(&to, &from, sizeof(To));
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#if defined(__GNUC__)
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return to;
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// Before GCC 7.2, GCC thought that this violated strict aliasing.
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#pragma GCC diagnostic push
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#pragma GCC diagnostic ignored "-Wstrict-aliasing"
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#endif
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return reinterpret_cast<To &>(storage);
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#if defined(__GNUC__)
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#pragma GCC diagnostic pop
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#endif
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}
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}
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} // namespace llvm
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} // namespace llvm
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