[libunwind] Properly align _Unwind_Exception.

[EricWF]

Summary

_Unwind_Exception is required to be double word aligned. Currently the struct is under aligned.

[compnerd]

LGTM; we should make sure Nick is okay with it as well (for Darwin). This should also make its way into 3.9 I think.

[kledzik]

LGTM

[emaste]

I've committed the alignment change to the copy of llvm libunwind in the FreeBSD base system now.

include/unwind.h

 
 struct _Unwind_Exception {
   uint64_t exception_class;
   void (*exception_cleanup)(_Unwind_Reason_Code reason,
                             _Unwind_Exception *exc);
   uintptr_t private_1; // non-zero means forced unwind
   uintptr_t private_2; // holds sp that phase1 found for phase2 to use
 #ifndef __LP64__
-  // The gcc implementation of _Unwind_Exception used attribute mode on the
-  // above fields which had the side effect of causing this whole struct to
+  // The implementation of _Unwind_Exception uses an attribute mode on the
+  // above fields which has the side effect of causing this whole struct to
   // round up to 32 bytes in size. To be more explicit, we add pad fields
   // added for binary compatibility.
   uint32_t reserved[3];
 #endif
-};
+  // The Itanium ABI requires that _Unwind_Exception objects are "double-word
+  // aligned".  GCC has interpreted this to mean "use the maximum useful
+  // alignment for the target"; so do we.
+} __attribute__((__aligned__));
 
 typedef _Unwind_Reason_Code (*_Unwind_Stop_Fn)
     (int version,
      _Unwind_Action actions,
      uint64_t exceptionClass,
      _Unwind_Exception* exceptionObject,
      struct _Unwind_Context* context,
      void* stop_parameter );

test/alignment.pass.cpp

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// The Itanium ABI requires that _Unwind_Exception objects are "double-word
+// aligned".
+
+#include <unwind.h>
+
+struct MaxAligned {} __attribute__((aligned));
+static_assert(alignof(_Unwind_Exception) == alignof(MaxAligned), "");
+
+int main()
+{
+}