[llvm-branch-commits] [llvm] b0b5d38 - Document that AlignedCharArrayUnion exists to work around an MSVC bug

[Reid Kleckner via llvm-branch-commits]

Author: Reid Kleckner
Date: 2020-12-15T16:01:55-08:00
New Revision: b0b5d389635a54c5aeb490b1b067f3a38e038235

URL: https://github.com/llvm/llvm-project/commit/b0b5d389635a54c5aeb490b1b067f3a38e038235
DIFF: https://github.com/llvm/llvm-project/commit/b0b5d389635a54c5aeb490b1b067f3a38e038235.diff

LOG: Document that AlignedCharArrayUnion exists to work around an MSVC bug

Differential Revision: https://reviews.llvm.org/D93355

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/include/llvm/Support/AlignOf.h b/llvm/include/llvm/Support/AlignOf.h
index f9dcde4d4ff1..f586d7f182aa 100644
--- a/llvm/include/llvm/Support/AlignOf.h
+++ b/llvm/include/llvm/Support/AlignOf.h
@@ -20,9 +20,10 @@ namespace llvm {
 /// A suitably aligned and sized character array member which can hold elements
 /// of any type.
 ///
-/// These types may be arrays, structs, or any other types. This exposes a
-/// `buffer` member which can be used as suitable storage for a placement new of
-/// any of these types.
+/// This template is equivalent to std::aligned_union_t<1, ...>, but we cannot
+/// use it due to a bug in the MSVC x86 compiler:
+/// https://github.com/microsoft/STL/issues/1533
+/// Using `alignas` here works around the bug.
 template <typename T, typename... Ts> struct AlignedCharArrayUnion {
   using AlignedUnion = std::aligned_union_t<1, T, Ts...>;
   alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];