Automated rollback of commit ec126a5069.

PiperOrigin-RevId: 634496540

src/google/protobuf/arena.h

@@ -142,7 +142,7 @@ struct ArenaOptions {
 // This is a thread-safe implementation: multiple threads may allocate from the
 // arena concurrently. Destruction is not thread-safe and the destructing
 // thread must synchronize with users of the arena first.
-class PROTOBUF_EXPORT alignas(8) Arena final {
+class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final {
  public:
   // Default constructor with sensible default options, tuned for average
   // use-cases.

src/google/protobuf/port_def.inc

@@ -403,6 +403,39 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 #error PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION was previously defined
 #endif
 
+// Specify memory alignment for structs, classes, etc.
+// Use like:
+//   class PROTOBUF_ALIGNAS(16) MyClass { ... }
+//   PROTOBUF_ALIGNAS(16) int array[4];
+//
+// In most places you can use the C++11 keyword "alignas", which is preferred.
+//
+// But compilers have trouble mixing __attribute__((...)) syntax with
+// alignas(...) syntax.
+//
+// Doesn't work in clang or gcc:
+//   struct alignas(16) __attribute__((packed)) S { char c; };
+// Works in clang but not gcc:
+//   struct __attribute__((packed)) alignas(16) S2 { char c; };
+// Works in clang and gcc:
+//   struct alignas(16) S3 { char c; } __attribute__((packed));
+//
+// There are also some attributes that must be specified *before* a class
+// definition: visibility (used for exporting functions/classes) is one of
+// these attributes. This means that it is not possible to use alignas() with a
+// class that is marked as exported.
+#ifdef PROTOBUF_ALIGNAS
+#error PROTOBUF_ALIGNAS was previously defined
+#endif
+#if defined(_MSC_VER)
+#define PROTOBUF_ALIGNAS(byte_alignment) __declspec(align(byte_alignment))
+#elif defined(__GNUC__)
+#define PROTOBUF_ALIGNAS(byte_alignment) \
+  __attribute__((aligned(byte_alignment)))
+#else
+#define PROTOBUF_ALIGNAS(byte_alignment) alignas(byte_alignment)
+#endif
+
 #ifdef PROTOBUF_THREAD_LOCAL
 #error PROTOBUF_THREAD_LOCAL was previously defined
 #endif

src/google/protobuf/port_undef.inc

@@ -50,6 +50,7 @@
 #undef PROTOBUF_ASSUME
 #undef PROTOBUF_EXPORT_TEMPLATE_DECLARE
 #undef PROTOBUF_EXPORT_TEMPLATE_DEFINE
+#undef PROTOBUF_ALIGNAS
 #undef PROTOBUF_THREAD_LOCAL
 #undef PROTOBUF_CONSTINIT
 #undef PROTOBUF_CONSTEXPR