dnl FP_CC_SUPPORTS__ATOMICS
dnl ------------------------
dnl Does C compiler support the __atomic_* family of builtins?
AC_DEFUN([FP_CC_SUPPORTS__ATOMICS],
[
    AC_REQUIRE([AC_PROG_CC])
    AC_MSG_CHECKING([whether C compiler supports __atomic_ builtins])
    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[int x, y;]], [[__atomic_load(&x, &y, __ATOMIC_SEQ_CST); return y]])],
    [
        AC_MSG_RESULT(yes)

        need_latomic=0
        AC_MSG_CHECKING(whether -latomic is needed for sub-word-sized atomic operations)
        AC_LINK_IFELSE([AC_LANG_PROGRAM([[unsigned char a;]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
        [
            AC_MSG_RESULT(no)
        ],
        [
            _save_LIBS="$LIBS"
            LIBS="-latomic"
            AC_LINK_IFELSE([AC_LANG_PROGRAM([[unsigned char a;]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
            [
                AC_MSG_RESULT(yes)
                need_latomic=1
            ],
            [
                AC_MSG_RESULT(failed)
                AC_MSG_ERROR([sub-word-sized atomic operations are not available.])
            ])
            LIBS="$_save_LIBS"
        ])
        AC_MSG_CHECKING(whether -latomic is needed for 64-bit atomic operations)
        AC_LINK_IFELSE([AC_LANG_PROGRAM(
            [[
            #include <inttypes.h>
            uint64_t a;
            ]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
        [
            AC_MSG_RESULT(no)
        ],
        [
            _save_LIBS="$LIBS"
            LIBS="-latomic"
            AC_LINK_IFELSE([AC_LANG_PROGRAM(
                [[
                #include <inttypes.h>
                uint64_t a;
                ]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
            [
                AC_MSG_RESULT(yes)
                need_latomic=1
            ],
            [
                AC_MSG_RESULT(failed)
                AC_MSG_ERROR([64-bit atomic operations are not available.])
            ])
            LIBS="$_save_LIBS"
        ])
    ],
    [
        AC_MSG_RESULT(no)
        AC_MSG_ERROR([C compiler needs to support __atomic primitives.])
    ])
    AC_DEFINE([HAVE_C11_ATOMICS], [1], [Does C compiler support __atomic primitives?])
    if test "$need_latomic" = 1; then
        AC_SUBST([NeedLibatomic],[YES])
    else
        AC_SUBST([NeedLibatomic],[NO])
    fi
    AC_DEFINE_UNQUOTED([NEED_ATOMIC_LIB], [$need_latomic],
        [Define to 1 if we need -latomic.])
])