Require and limit features to C11

Error out if the compiler does not support C11 and limit
the standard to C11 if the compiler accepts a standard flag.
This limit prevents us from using features of newer standard versions.

Also removes support for `__thread` since we now rely on C11 `_Thread_local`.

Signed-off-by: Joseph Schuchart <[email protected]>

Author: devreal

config/opal_setup_cc.m4

@@ -170,7 +170,7 @@ AC_DEFUN([OPAL_SETUP_CC],[
     AC_REQUIRE([_OPAL_PROG_CC])
     AC_REQUIRE([AM_PROG_CC_C_O])
 
-    OPAL_VAR_SCOPE_PUSH([opal_prog_cc_c11_helper__Thread_local_available opal_prog_cc_c11_helper_atomic_var_available opal_prog_cc_c11_helper__Atomic_available opal_prog_cc_c11_helper__static_assert_available opal_prog_cc_c11_helper__Generic_available opal_prog_cc__thread_available opal_prog_cc_c11_helper_atomic_fetch_xor_explicit_available opal_prog_cc_c11_helper_proper__Atomic_support_in_atomics])
+    OPAL_VAR_SCOPE_PUSH([opal_prog_cc_c11_helper__Thread_local_available opal_prog_cc_c11_helper_atomic_var_available opal_prog_cc_c11_helper__Atomic_available opal_prog_cc_c11_helper__static_assert_available opal_prog_cc_c11_helper__Generic_available opal_prog_cc_c11_helper_atomic_fetch_xor_explicit_available opal_prog_cc_c11_helper_proper__Atomic_support_in_atomics])
 
     OPAL_PROG_CC_C11
 
@@ -179,33 +179,24 @@ AC_DEFUN([OPAL_SETUP_CC],[
     OPAL_C_COMPILER_VENDOR([opal_c_vendor])
 
     if test $opal_cv_c11_supported = no ; then
-        # It is not currently an error if C11 support is not available. Uncomment the
-        # following lines and update the warning when we require a C11 compiler.
-        # AC_MSG_WARNING([Open MPI requires a C11 (or newer) compiler])
-        # AC_MSG_ERROR([Aborting.])
-        # From Open MPI 1.7 on we require a C99 compliant compiler
-        dnl with autoconf 2.70 AC_PROG_CC makes AC_PROG_CC_C99 obsolete
-        m4_version_prereq([2.70],
-            [],
-            [AC_PROG_CC_C99])
-        # The result of AC_PROG_CC_C99 is stored in ac_cv_prog_cc_c99
-        if test "x$ac_cv_prog_cc_c99" = xno ; then
-            AC_MSG_WARN([Open MPI requires a C99 (or newer) compiler. C11 is recommended.])
-            AC_MSG_ERROR([Aborting.])
-        fi
-
-        # Get the correct result for C11 support flags now that the compiler flags have
-        # changed
-        OPAL_PROG_CC_C11_HELPER([], [], [])
+        # C11 is required
+        AC_MSG_WARN([Open MPI requires a C11 (or newer) compiler. C11 is required.])
+        AC_MSG_ERROR([Aborting.])
+    elif test $opal_cv_c11_flag_required = no ; then
+        # Check if the compiler accepts -std=c11
+        # this helps avoid accidentally using newer features
+        opal_prog_cc_c11_flags="-std=c11 -c11"
+        AC_MSG_NOTICE([checking if $CC supports C11 standard flag])
+        for flag in $(echo $opal_prog_cc_c11_flags | tr ' ' '\n') ; do
+            _OPAL_CHECK_SPECIFIC_CFLAGS($flag, std_c11)
+            if test "x$opal_cv_cc_std_c11" == "x1" ; then
+                OPAL_FLAGS_APPEND_UNIQ([CFLAGS], ["$flag"])
+                AC_MSG_NOTICE([using $flag to ensure C11 support])
+                break
+            fi
+        done
     fi
 
-    # Check if compiler support __thread
-    OPAL_CC_HELPER([if $CC $1 supports __thread], [opal_prog_cc__thread_available],
-               [],[[static __thread int  foo = 1;++foo;]])
-
-    OPAL_CC_HELPER([if $CC $1 supports C11 _Thread_local], [opal_prog_cc_c11_helper__Thread_local_available],
-                   [],[[static _Thread_local int  foo = 1;++foo;]])
-
     dnl At this time Open MPI only needs thread local and the atomic convenience types for C11 support. These
     dnl will likely be required in the future.
     AC_DEFINE_UNQUOTED([OPAL_C_HAVE__THREAD_LOCAL], [$opal_prog_cc_c11_helper__Thread_local_available],
@@ -223,9 +214,6 @@ AC_DEFUN([OPAL_SETUP_CC],[
     AC_DEFINE_UNQUOTED([OPAL_C_HAVE__STATIC_ASSERT], [$opal_prog_cc_c11_helper__static_assert_available],
                        [Whether C compiler support _Static_assert keyword])
 
-    AC_DEFINE_UNQUOTED([OPAL_C_HAVE___THREAD], [$opal_prog_cc__thread_available],
-                       [Whether C compiler supports __thread])
-
     # Check for standard headers, needed here because needed before
     # the types checks.  This is only necessary for Autoconf < v2.70.
     m4_version_prereq([2.70],

opal/mca/threads/thread_usage.h

@@ -250,17 +250,7 @@ OPAL_THREAD_DEFINE_ATOMIC_SWAP(int64_t, int64_t, 64)
 #    define OPAL_ATOMIC_SWAP_64 opal_thread_swap_64
 
 /* thread local storage */
-#if OPAL_C_HAVE__THREAD_LOCAL
 #    define opal_thread_local      _Thread_local
 #    define OPAL_HAVE_THREAD_LOCAL 1
 
-#elif OPAL_C_HAVE___THREAD /* OPAL_C_HAVE__THREAD_LOCAL */
-#    define opal_thread_local      __thread
-#    define OPAL_HAVE_THREAD_LOCAL 1
-#endif /* OPAL_C_HAVE___THREAD */
-
-#if !defined(OPAL_HAVE_THREAD_LOCAL)
-#    define OPAL_HAVE_THREAD_LOCAL 0
-#endif /* !defined(OPAL_HAVE_THREAD_LOCAL) */
-
 #endif /* OPAL_MCA_THREADS_THREAD_USAGE_H */