[PATCH] D92500: ADT: Replace guts of AlignedCharArrayUnion with std::aligned_union_t, NFC

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith created this revision.
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Instead of computing the alignment and size of the `char` buffer in
`AlignedCharArrayUnion`, rely on the math in `std::aligned_union_t`.
Because some users of this rely on the `buffer` field existing with a
type convertible to `char *`, we can't change the field type, but we can
still avoid duplicating the logic.

A potential follow up would be to delete `AlignedCharArrayUnion` after
updating its users to use `std::aligned_union_t` directly; or if we like
our template parameters better, could update users to stop peeking
inside and then replace the definition with:

  template <class T, class... T>
  using AlignedCharArrayUnion = std::aligned_union_t<1, T, Ts...>;


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92500

Files:
  llvm/include/llvm/Support/AlignOf.h


Index: llvm/include/llvm/Support/AlignOf.h
===================================================================
--- llvm/include/llvm/Support/AlignOf.h
+++ llvm/include/llvm/Support/AlignOf.h
@@ -13,32 +13,10 @@
 #ifndef LLVM_SUPPORT_ALIGNOF_H
 #define LLVM_SUPPORT_ALIGNOF_H
 
-#include "llvm/Support/Compiler.h"
-#include <cstddef>
+#include <type_traits>
 
 namespace llvm {
 
-namespace detail {
-
-template <typename T, typename... Ts> class AlignerImpl {
-  T t;
-  AlignerImpl<Ts...> rest;
-  AlignerImpl() = delete;
-};
-
-template <typename T> class AlignerImpl<T> {
-  T t;
-  AlignerImpl() = delete;
-};
-
-template <typename T, typename... Ts> union SizerImpl {
-  char arr[sizeof(T)];
-  SizerImpl<Ts...> rest;
-};
-
-template <typename T> union SizerImpl<T> { char arr[sizeof(T)]; };
-} // end namespace detail
-
 /// A suitably aligned and sized character array member which can hold elements
 /// of any type.
 ///
@@ -46,8 +24,8 @@
 /// `buffer` member which can be used as suitable storage for a placement new of
 /// any of these types.
 template <typename T, typename... Ts> struct AlignedCharArrayUnion {
-  alignas(::llvm::detail::AlignerImpl<T, Ts...>) char buffer[sizeof(
-      llvm::detail::SizerImpl<T, Ts...>)];
+  using AlignedUnion = std::aligned_union_t<1, T, Ts...>;
+  alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];
 };
 
 } // end namespace llvm


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