Support building with older compilers.

Add a check for __builtin_unreachable.

configure.ac

@@ -285,6 +285,17 @@ if test x$have__builtin___clear_cache = xyes; then
 fi
 AC_MSG_RESULT([$have__builtin___clear_cache])
 
+AC_MSG_CHECKING([for __builtin_unreachable])
+AC_LINK_IFELSE(
+  [AC_LANG_PROGRAM([[]], [[__builtin_unreachable()]])],
+  [have__builtin_unreachable=yes],
+  [have__builtin_unreachable=no])
+if test x$have__builtin_unreachable = xyes; then
+  AC_DEFINE([HAVE__BUILTIN_UNREACHABLE], [1],
+            [Defined if __builtin_unreachable() is available])
+fi
+AC_MSG_RESULT([$have__builtin_unreachable])
+
 AC_MSG_CHECKING([for __sync atomics])
 AC_LINK_IFELSE(
   [AC_LANG_PROGRAM([[]], [[

include/libunwind_i.h

@@ -72,6 +72,12 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 # endif
 #endif
 
+#if defined(HAVE__BUILTIN_UNREACHABLE)
+# define unreachable() __builtin_unreachable()
+#else
+# define unreachable() do { } while (1)
+#endif
+
 #ifdef DEBUG
 # define UNW_DEBUG	1
 #else

src/arm/Gresume.c

@@ -96,7 +96,7 @@ arm_local_resume (unw_addr_space_t as, unw_cursor_t *cursor, void *arg)
 	: : "r" (c->sigcontext_sp), "r" (c->sigcontext_pc)
       );
    }
-  __builtin_unreachable();
+  unreachable();
 #else
   printf ("%s: implement me\n", __FUNCTION__);
 #endif

src/sh/Gresume.c

@@ -109,7 +109,7 @@ sh_local_resume (unw_addr_space_t as, unw_cursor_t *cursor, void *arg)
 	  "r" (c->sigcontext_pc)
       );
    }
-  __builtin_unreachable();
+  unreachable();
 #endif
   return -UNW_EINVAL;
 }