diff --git a/contrib/libcxxrt/exception.cc b/contrib/libcxxrt/exception.cc
index 0fb26ddb4ed2..0de878e9e6db 100644
--- a/contrib/libcxxrt/exception.cc
+++ b/contrib/libcxxrt/exception.cc
@@ -572,6 +572,19 @@ static void free_exception(char *e)
 	}
 }
 
+#ifdef __LP64__
+/**
+ * There's an ABI bug in __cxa_exception: unwindHeader requires 16-byte
+ * alignment but it was broken by the addition of the referenceCount.
+ * The unwindHeader is at offset 0x58 in __cxa_exception.  In order to keep
+ * compatibility with consumers of the broken __cxa_exception, explicitly add
+ * padding on allocation (and account for it on free).
+ */
+static const int exception_alignment_padding = 8;
+#else
+static const int exception_alignment_padding = 0;
+#endif
+
 /**
  * Allocates an exception structure.  Returns a pointer to the space that can
  * be used to store an object of thrown_size bytes.  This function will use an
@@ -580,16 +593,19 @@ static void free_exception(char *e)
  */
 extern "C" void *__cxa_allocate_exception(size_t thrown_size)
 {
-	size_t size = thrown_size + sizeof(__cxa_exception);
+	size_t size = exception_alignment_padding + sizeof(__cxa_exception) +
+	    thrown_size;
 	char *buffer = alloc_or_die(size);
-	return buffer+sizeof(__cxa_exception);
+	return buffer + exception_alignment_padding + sizeof(__cxa_exception);
 }
 
 extern "C" void *__cxa_allocate_dependent_exception(void)
 {
-	size_t size = sizeof(__cxa_dependent_exception);
+	size_t size = exception_alignment_padding +
+	    sizeof(__cxa_dependent_exception);
 	char *buffer = alloc_or_die(size);
-	return buffer+sizeof(__cxa_dependent_exception);
+	return buffer + exception_alignment_padding +
+	    sizeof(__cxa_dependent_exception);
 }
 
 /**
@@ -617,7 +633,8 @@ extern "C" void __cxa_free_exception(void *thrown_exception)
 		}
 	}
 
-	free_exception(reinterpret_cast<char*>(ex));
+	free_exception(reinterpret_cast<char*>(ex) -
+	    exception_alignment_padding);
 }
 
 static void releaseException(__cxa_exception *exception)
@@ -644,7 +661,8 @@ void __cxa_free_dependent_exception(void *thrown_exception)
 	{
 		releaseException(realExceptionFromException(reinterpret_cast<__cxa_exception*>(ex)));
 	}
-	free_exception(reinterpret_cast<char*>(ex));
+	free_exception(reinterpret_cast<char*>(ex) -
+	    exception_alignment_padding);
 }
 
 /**
diff --git a/contrib/libcxxrt/unwind-arm.h b/contrib/libcxxrt/unwind-arm.h
index ec81237e573b..6eb9d9e45981 100644
--- a/contrib/libcxxrt/unwind-arm.h
+++ b/contrib/libcxxrt/unwind-arm.h
@@ -97,7 +97,7 @@ struct _Unwind_Exception
 	} pr_cache;
 	/** Force alignment of next item to 8-byte boundary */
 	long long int :0;
-} __attribute__((__aligned__(8)));
+};
 
 /* Unwinding functions */
 _Unwind_Reason_Code _Unwind_RaiseException(struct _Unwind_Exception *ucbp);
diff --git a/contrib/libcxxrt/unwind-itanium.h b/contrib/libcxxrt/unwind-itanium.h
index 199d91de283d..1ee0cf0e81c4 100644
--- a/contrib/libcxxrt/unwind-itanium.h
+++ b/contrib/libcxxrt/unwind-itanium.h
@@ -79,12 +79,9 @@ struct _Unwind_Exception
   {
     uint64_t exception_class;
     _Unwind_Exception_Cleanup_Fn exception_cleanup;
-    uintptr_t private_1;
-    uintptr_t private_2;
-#if __SIZEOF_POINTER__ == 4
-    uint32_t reserved[3];
-#endif
-  } __attribute__((__aligned__));
+    unsigned long private_1;
+    unsigned long private_2;
+  } ;
 
 extern _Unwind_Reason_Code _Unwind_RaiseException (struct _Unwind_Exception *);
 extern _Unwind_Reason_Code _Unwind_ForcedUnwind (struct _Unwind_Exception *,