[libcxxabi] Align unwindHeader on a double-word boundary.

r276215 made a change to annotate _Unwind_Exception with attribute
"aligned" so that an exception object following field __cxa_exception
is sufficiently aligned. This fix hasn't been incorporated to unwind.h
on Darwin since it is an ABI breaking change.

Instead of annotating struct _Unwind_Exception with the attribute, this
commit annotates field unwindHeader of __cxa_exception. This ensures the
exception object is sufficiently aligned without breaking the ABI.

rdar://problem/25364625

Differential Revision: https://reviews.llvm.org/D33030

llvm-svn: 302763

libcxxabi/src/cxa_exception.hpp

@@ -61,7 +61,13 @@ struct _LIBCXXABI_HIDDEN __cxa_exception {
     size_t referenceCount;
 #endif
 
-    _Unwind_Exception unwindHeader;
+    // This field is annotated with attribute aligned so that the exception
+    // object following the field is sufficiently aligned and there is no
+    // gap between the field and the exception object. r276215 made a change to
+    // annotate _Unwind_Exception with __attribute__((aligned)), but we cannot
+    // incorporate the fix on Darwin since it is an ABI-breaking change, which
+    // is why we need the attribute on this field.
+    _Unwind_Exception unwindHeader __attribute__((aligned));
 };
 
 // http://sourcery.mentor.com/archives/cxx-abi-dev/msg01924.html
@@ -96,7 +102,9 @@ struct _LIBCXXABI_HIDDEN __cxa_dependent_exception {
     void* primaryException;
 #endif
 
-    _Unwind_Exception unwindHeader;
+    // See the comment in __cxa_exception as to why this field has attribute
+    // aligned.
+    _Unwind_Exception unwindHeader __attribute__((aligned));
 };
 
 struct _LIBCXXABI_HIDDEN __cxa_eh_globals {

libcxxabi/test/exception_object_alignment.sh.cpp

@@ -0,0 +1,46 @@
+//===---------------- exception_object_alignment.sh.cpp -------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcxxabi-no-exceptions
+
+// RUN: %cxx %flags %compile_flags -O1 %s -o %t.exe
+// RUN: %exec %t.exe
+
+// This test used to segfault on Darwin because field unwindHeader of struct
+// __cxa_exception was not 16B aligned.
+
+namespace {
+
+struct S {
+  int a;
+  int __attribute__((aligned(16))) b;
+};
+
+class base1 {
+protected:
+  virtual ~base1() throw() {}
+};
+
+class derived: public base1 {
+public:
+  derived() : member() {}
+private:
+  S member;
+};
+
+}
+
+int main() {
+  try {
+    throw derived();
+  }
+  catch(...) {
+  }
+  return 0;
+}