[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.

This recommits r302978 and r302981, which were reverted in r303016
because a libcxx test was failing on an AArch64 bot. I also modified the
libcxxabi test case to check the alignment of the pointer returned by
__cxa_allocate_exception rather than compiling the test with -O1 and
checking whether it segfaults.

rdar://problem/25364625

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@303175 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 33 insertions, 0 deletions

diff --git a/test/exception_object_alignment.pass.cpp b/test/exception_object_alignment.pass.cpp
new file mode 100644
index 0000000..addef18
--- /dev/null
+++ b/test/exception_object_alignment.pass.cpp
@@ -0,0 +1,33 @@
+//===---------------- exception_object_alignment.pass.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
+
+// Check that the pointer __cxa_allocate_exception returns is aligned to the
+// default alignment for the target architecture.
+
+#include <cassert>
+#include <cstdint>
+#include <cxxabi.h>
+#include <type_traits>
+#include <__cxxabi_config.h>
+
+struct S {
+  int a[4];
+} __attribute__((aligned));
+
+int main() {
+#if !defined(_LIBCXXABI_ARM_EHABI)
+  void *p = __cxxabiv1::__cxa_allocate_exception(16);
+  auto i = reinterpret_cast<uintptr_t>(p);
+  auto a = std::alignment_of<S>::value;
+  assert(i % a == 0);
+#endif
+  return 0;
+}