[Mesa-dev] [PATCH 1/2] configure: check if -latomic is needed for __atomic_*

[Matt Turner]

On Mon, Sep 18, 2017 at 12:28 PM, Grazvydas Ignotas <notasas at gmail.com> wrote:
> On some platforms, gcc generates library calls when __atomic_* functions
> are used, but does not link the required library automatically. Detect
> this and add the library when needed.
>
> This change was tested on armel (library was added) and on x86_64 (was
> not, as expected).
>
> Fixes: 8915f0c0 "util: use GCC atomic intrinsics with explicit memory model"
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102573
> Signed-off-by: Grazvydas Ignotas <notasas at gmail.com>
> ---
>  configure.ac         | 13 +++++++++++++
>  src/util/Makefile.am |  3 ++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/configure.ac b/configure.ac
> index 605c9b4..b46f29d 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -377,12 +377,25 @@ int main() {
>      int n;
>      return __atomic_load_n(&n, __ATOMIC_ACQUIRE);
>  }]])], GCC_ATOMIC_BUILTINS_SUPPORTED=1)
>  if test "x$GCC_ATOMIC_BUILTINS_SUPPORTED" = x1; then
>      DEFINES="$DEFINES -DUSE_GCC_ATOMIC_BUILTINS"
> +    dnl On some platforms, new-style atomics need a helper library
> +    AC_MSG_CHECKING(whether -latomic is needed)
> +    AC_LINK_IFELSE([AC_LANG_SOURCE([[
> +    #include <stdint.h>
> +    uint64_t v;
> +    int main() {
> +        return (int)__atomic_load_n(&v, __ATOMIC_ACQUIRE);
> +    }]])], GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=no, GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=yes)
> +    AC_MSG_RESULT($GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC)
> +    if test "x$GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC" = xyes; then
> +        LIBATOMIC_LIBS="-latomic"
> +    fi
>  fi
>  AM_CONDITIONAL([GCC_ATOMIC_BUILTINS_SUPPORTED], [test x$GCC_ATOMIC_BUILTINS_SUPPORTED = x1])
> +AC_SUBST([LIBATOMIC_LIBS])
>
>  dnl Check if host supports 64-bit atomics
>  dnl note that lack of support usually results in link (not compile) error
>  AC_MSG_CHECKING(whether __sync_add_and_fetch_8 is supported)
>  AC_LINK_IFELSE([AC_LANG_SOURCE([[
> diff --git a/src/util/Makefile.am b/src/util/Makefile.am
> index 9885bbe..c37afff 100644
> --- a/src/util/Makefile.am
> +++ b/src/util/Makefile.am
> @@ -46,11 +46,12 @@ libmesautil_la_SOURCES = \
>         $(MESA_UTIL_FILES) \
>         $(MESA_UTIL_GENERATED_FILES)
>
>  libmesautil_la_LIBADD = \
>         $(CLOCK_LIB) \
> -       $(ZLIB_LIBS)
> +       $(ZLIB_LIBS) \
> +       $(LIBATOMIC_LIBS)

I can't remember how this works -- will $(LIBATOMIC_LIBS) be added
transitively to anything that LDADDs libmesautil? I hope so...

Both are

Reviewed-by: Matt Turner <mattst88 at gmail.com>

FWIW, libatomic only exists since gcc-4.8, so we'll still have some gaps.