From 24484e80b3e329c9edee1995e102f8612eedb79c Mon Sep 17 00:00:00 2001
From: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Date: Tue, 13 May 2014 23:32:27 +0200
Subject: [PATCH] Add AO_REQUIRE_CAS to fix build on ARM < v6

ARM earlier than ARMv6, such as ARMv4 and ARMv5 do not provide
optimize atomic operations in libatomic_ops. Since libunwind is using
such operations, it should define AO_REQUIRE_CAS before including
<atomic_ops.h> so that libatomic_ops knows it should use emulated
atomic operations instead (even though they are obviously a lot more
expensive).

Also, while real atomic operations are all inline functions and
therefore linking against libatomic_ops was not required, the emulated
atomic operations actually require linking against libatomic_ops, so
the commented AC_CHECK_LIB test in acinclude.m4 is uncommented to make
sure we link against libatomic_ops.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---
 acinclude.m4          | 8 +-------
 include/libunwind_i.h | 1 +
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/acinclude.m4 b/acinclude.m4
index 497f7c2..9c15af1 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -22,11 +22,5 @@ fi])
 AC_DEFUN([CHECK_ATOMIC_OPS],
 [dnl Check whether the system has the atomic_ops package installed.
   AC_CHECK_HEADERS(atomic_ops.h)
-#
-# Don't link against libatomic_ops for now.  We don't want libunwind
-# to depend on libatomic_ops.so.  Fortunately, none of the platforms
-# we care about so far need libatomic_ops.a (everything is done via
-# inline macros).
-#
-#  AC_CHECK_LIB(atomic_ops, main)
+  AC_CHECK_LIB(atomic_ops, main)
 ])
diff --git a/include/libunwind_i.h b/include/libunwind_i.h
index 23f615e..deabdfd 100644
--- a/include/libunwind_i.h
+++ b/include/libunwind_i.h
@@ -95,6 +95,7 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 	(pthread_mutex_unlock != NULL ? pthread_mutex_unlock (l) : 0)
 
 #ifdef HAVE_ATOMIC_OPS_H
+# define AO_REQUIRE_CAS
 # include <atomic_ops.h>
 static inline int
 cmpxchg_ptr (void *addr, void *old, void *new)
-- 
1.9.2

