[llvm-branch-commits] [llvm] 65c5c9f - ADT: Rely on std::aligned_union_t for math in AlignedCharArrayUnion, NFC

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

Author: Duncan P. N. Exon Smith
Date: 2020-12-02T15:56:12-08:00
New Revision: 65c5c9f92ec514ae41c8ea407d1c885737d699ec

URL: https://github.com/llvm/llvm-project/commit/65c5c9f92ec514ae41c8ea407d1c885737d699ec
DIFF: https://github.com/llvm/llvm-project/commit/65c5c9f92ec514ae41c8ea407d1c885737d699ec.diff

LOG: ADT: Rely on std::aligned_union_t for math in AlignedCharArrayUnion, NFC

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

Differential Revision: https://reviews.llvm.org/D92500

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Support/AlignOf.h b/llvm/include/llvm/Support/AlignOf.h
index eb42542b777f..f9dcde4d4ff1 100644
--- a/llvm/include/llvm/Support/AlignOf.h
+++ b/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 @@ template <typename T> union SizerImpl<T> { char arr[sizeof(T)]; };
 /// `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