[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

llvm-svn: 303175

libcxxabi/src/cxa_exception.hpp

@@ -61,7 +61,21 @@ struct _LIBCXXABI_HIDDEN __cxa_exception {
     size_t referenceCount;
 #endif
 
+    // 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 in unwind.h 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.
+    //
+    // For ARM EHABI, we do not align this field since _Unwind_Exception is an
+    // alias of _Unwind_Control_Block, which is not annotated with
+    // __attribute__((aligned).
+#if defined(_LIBCXXABI_ARM_EHABI)
     _Unwind_Exception unwindHeader;
+#else
+    _Unwind_Exception unwindHeader __attribute__((aligned));
+#endif
 };
 
 // http://sourcery.mentor.com/archives/cxx-abi-dev/msg01924.html
@@ -96,7 +110,13 @@ struct _LIBCXXABI_HIDDEN __cxa_dependent_exception {
     void* primaryException;
 #endif
 
+    // See the comment in __cxa_exception as to why this field has attribute
+    // aligned.
+#if defined(_LIBCXXABI_ARM_EHABI)
     _Unwind_Exception unwindHeader;
+#else
+    _Unwind_Exception unwindHeader __attribute__((aligned));
+#endif
 };
 
 struct _LIBCXXABI_HIDDEN __cxa_eh_globals {

libcxxabi/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;
+}