[PATCH] arm: libbacktrace: Check if the compiler supports __sync atomics

[Richard Earnshaw]

Older versions of the Arm architecture lack support for __sync
operations directly in hardware and require calls into appropriate
operating-system hooks.  But such hooks obviously don't exist in a
freestanding environment.

Consquently, it is incorrect to assume during configure that such
functions will exist and we need a configure-time check to determine
whether or not these routines will work.

libbacktrace:

        * configure.ac: Always check if the compiler supports __sync
        operations.
        * configure: Regenerated.
---

A consequence of this assumption that these functions 'just work' (TM)
is that we build libbacktrace with threading support, but that then
fails at link time when we find out that they do not.  I'm not sure I
really understand why we can assume that having with_target_subdir set
is enough to assume that the operations exist: it isn't for Arm and it
looks like it wasn't for hpux either.  Perhaps the whole logic here
needs reconsideration...

 libbacktrace/configure    | 23 +++++++++++++++++++++++
 libbacktrace/configure.ac | 10 ++++++++++
 2 files changed, 33 insertions(+)

diff --git a/libbacktrace/configure b/libbacktrace/configure
index db491a78234..0ecdd3ec0a3 100755
--- a/libbacktrace/configure
+++ b/libbacktrace/configure
@@ -12760,6 +12760,29 @@ else
   if test -n "${with_target_subdir}"; then
    case "${host}" in
    hppa*-*-hpux*) libbacktrace_cv_sys_sync=no ;;
+   arm*-*-eabi*)
+     # Older versions of the Arm architecture lack the necessary instructions
+     # for these constructs, so check whether we can use them.
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+int i;
+int
+main ()
+{
+__sync_bool_compare_and_swap (&i, i, i);
+                         __sync_lock_test_and_set (&i, 1);
+                         __sync_lock_release (&i);
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  libbacktrace_cv_sys_sync=yes
+else
+  libbacktrace_cv_sys_sync=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext;;
    *) libbacktrace_cv_sys_sync=yes ;;
    esac
  else
diff --git a/libbacktrace/configure.ac b/libbacktrace/configure.ac
index b700bf9d4f9..75b3a7536f1 100644
--- a/libbacktrace/configure.ac
+++ b/libbacktrace/configure.ac
@@ -199,6 +199,16 @@ AC_CACHE_CHECK([__sync extensions],
 [if test -n "${with_target_subdir}"; then
    case "${host}" in
    hppa*-*-hpux*) libbacktrace_cv_sys_sync=no ;;
+   arm*-*-eabi*)
+     # Older versions of the Arm architecture lack the necessary instructions
+     # for these constructs, so check whether we can use them.
+     AC_LINK_IFELSE(
+       [AC_LANG_PROGRAM([int i;],
+                        [__sync_bool_compare_and_swap (&i, i, i);
+                         __sync_lock_test_and_set (&i, 1);
+                         __sync_lock_release (&i);])],
+       [libbacktrace_cv_sys_sync=yes],
+       [libbacktrace_cv_sys_sync=no]);;
    *) libbacktrace_cv_sys_sync=yes ;;
    esac
  else
-- 
2.34.1