[libcxx-commits] [libcxx] [libc++] Remove alignment attributes from _LIBCPP_COMPRESSED_PAIR (PR #158595)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/158595

>From b2c7468339a3755bb866fb0ad20e2e1a3274d91c Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Mon, 15 Sep 2025 12:27:28 +0200
Subject: [PATCH] [libc++] Remove alignment attributes from
 _LIBCPP_COMPRESSED_PAIR

---
 libcxx/include/__memory/compressed_pair.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/libcxx/include/__memory/compressed_pair.h b/libcxx/include/__memory/compressed_pair.h
index d9d2720f9c9e4..0388d752ccc8b 100644
--- a/libcxx/include/__memory/compressed_pair.h
+++ b/libcxx/include/__memory/compressed_pair.h
@@ -28,8 +28,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // understand how it works).                                                                                          //
 // ================================================================================================================== //
 
-// The first member is aligned to the alignment of the second member to force padding in front of the compressed pair
-// in case there are members before it.
+// On GCC, the first member is aligned to the alignment of the second member to force padding in front of the compressed
+// pair in case there are members before it.
 //
 // For example:
 // (assuming x86-64 linux)
@@ -53,6 +53,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // Furthermore, that alignment must be the same as what was used in the old __compressed_pair layout, so we must
 // handle reference types specially since alignof(T&) == alignof(T).
 // See https://llvm.org/PR118559.
+//
+// On Clang, this is unnecessary, since we use anonymous structs instead, which automatically handle the alignment
+// correctly.
 
 #ifndef _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
 
@@ -104,8 +107,7 @@ class __compressed_pair_padding<_ToPad, true> {};
 #  else
 #    define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2)                                                \
       struct {                                                                                                         \
-        _LIBCPP_NO_UNIQUE_ADDRESS                                                                                      \
-        __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1;                          \
+        _LIBCPP_NO_UNIQUE_ADDRESS T1 Initializer1;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);      \
         _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _);      \
@@ -113,9 +115,7 @@ class __compressed_pair_padding<_ToPad, true> {};
 
 #    define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3)                            \
       struct {                                                                                                         \
-        _LIBCPP_NO_UNIQUE_ADDRESS                                                                                      \
-        __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>),                                            \
-                       __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1;                          \
+        _LIBCPP_NO_UNIQUE_ADDRESS T1 Initializer1;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);      \
         _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _);      \