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Document that AlignedCharArrayUnion exists to work around an MSVC bug 

Differential Revision: https://reviews.llvm.org/D93355
This commit is contained in:
Reid Kleckner 2020-12-15 15:59:21 -08:00
parent f43e67cc6c
commit b0b5d38963
1 changed files with 4 additions and 3 deletions
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7
llvm/include/llvm/Support/AlignOf.h
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@ -20,9 +20,10 @@ namespace llvm {
/// A suitably aligned and sized character array member which can hold elements /// A suitably aligned and sized character array member which can hold elements
/// of any type. /// of any type.
/// ///
/// These types may be arrays, structs, or any other types. This exposes a /// This template is equivalent to std::aligned_union_t<1, ...>, but we cannot
/// `buffer` member which can be used as suitable storage for a placement new of /// use it due to a bug in the MSVC x86 compiler:
/// any of these types. /// https://github.com/microsoft/STL/issues/1533
/// Using `alignas` here works around the bug.
template <typename T, typename... Ts> struct AlignedCharArrayUnion { template <typename T, typename... Ts> struct AlignedCharArrayUnion {
using AlignedUnion = std::aligned_union_t<1, T, Ts...>; using AlignedUnion = std::aligned_union_t<1, T, Ts...>;
alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)]; alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];