[libunwind] Properly align _Unwind_Exception.

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

Reviewers: mclow.lists, compnerd, kledzik, emaste

Subscribers: emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D22543

llvm-svn: 276215

diff --git a/libunwind/include/unwind.h b/libunwind/include/unwind.h
index 8f0ef92..0e01a2c 100644 (file)
--- a/libunwind/include/unwind.h
+++ b/libunwind/include/unwind.h
@@ -122,13 +122,16 @@ struct _Unwind_Exception {
   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,

diff --git a/libunwind/test/alignment.pass.cpp b/libunwind/test/alignment.pass.cpp
new file mode 100644 (file)
index 0000000..5ab5584
--- /dev/null
+++ b/libunwind/test/alignment.pass.cpp
@@ -0,0 +1,21 @@
+// -*- 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()
+{
+}