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gatomic: introduce G_ATOMIC_LOCK_FREE

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We clean up the detection of if we should do 'real' atomic operations or
mutex-emulated ones with the introduction of a new (public) macro:
G_ATOMIC_LOCK_FREE.  If defined, our atomic operations are guaranteed to
be done in hardware.

We need to use __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 to determine if our
compiler supports GCC-style atomic operations from the gatomic.h header
because we might be building a program against GLib using a different
set of compiler options (or a different compiler) than was used to build
GLib itself.

Unfortunately, this macro is not available on clang, so it has currently
regressed to using the mutex emulation.  A bug about that has been
opened here:

  http://llvm.org/bugs/show_bug.cgi?id=11174
This commit is contained in:
Ryan Lortie
2011-10-18 16:21:50 -04:00
parent c9b6c3c85a
commit aba0f0c38b
6 changed files with 89 additions and 50 deletions
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77
configure.ac
View File

@@ -2407,35 +2407,58 @@ dnl ************************
dnl *** g_atomic_* tests ***
dnl ************************
case $host_cpu in
i?86|x86_64|s390|s390x|arm*|crisv32*|etrax*)
glib_memory_barrier_needed=no
;;
sparc*|alpha*|powerpc*|ia64)
glib_memory_barrier_needed=yes
;;
*)
glib_memory_barrier_needed=yes
;;
esac
dnl We need to decide at configure time if GLib will use real atomic
dnl operations ("lock free") or emulated ones with a mutex. This is
dnl because we must put this information in glibconfig.h so we know if
dnl it is safe or not to inline using compiler intrinsics directly from
dnl the header.
dnl
dnl We also publish the information via G_ATOMIC_LOCK_FREE in case the
dnl user is interested in knowing if they can use the atomic ops across
dnl processes.
dnl
dnl We can currently support the atomic ops natively when building GLib
dnl with recent versions of GCC or MSVC. MSVC doesn't run ./configure,
dnl so we skip that case here and define G_ATOMIC_LOCK_FREE exactly when
dnl we are using GCC.
dnl
dnl Note that the atomic ops are only available with GCC on x86 when
dnl using -march=i486 or higher. If we detect that the atomic ops are
dnl not available but would be available given the right flags, we want
dnl to abort and advise the user to fix their CFLAGS. It's better to do
dnl that then to silently fall back on emulated atomic ops just because
dnl the user had the wrong build environment.
glib_cv_gcc_has_builtin_atomic_operations=no
if test x"$GCC" = xyes; then
AC_MSG_CHECKING([whether GCC supports built-in atomic intrinsics])
AC_TRY_LINK([],
[int i;
__sync_synchronize ();
__sync_bool_compare_and_swap (&i, 0, 1);
__sync_fetch_and_add (&i, 1);
],
[glib_cv_gcc_has_builtin_atomic_operations=yes],
[glib_cv_gcc_has_builtin_atomic_operations=no])
dnl We may add other compilers here in the future...
AC_MSG_CHECKING([for lock-free atomic intrinsics])
AC_TRY_COMPILE([],
[__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4;],
[g_atomic_lock_free=yes],
[g_atomic_lock_free=no])
AC_MSG_RESULT($g_atomic_lock_free)
AC_MSG_RESULT($glib_cv_gcc_has_builtin_atomic_operations)
if test "$g_atomic_lock_free" = "no"; then
SAVE_CFLAGS="${CFLAGS}"
CFLAGS="-march=i486"
AC_TRY_COMPILE([],
[__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4;],
[AC_MSG_ERROR([GLib must be build with -march=i486 or later.])],
[])
CFLAGS="${SAVE_CFLAGS}"
fi
AM_CONDITIONAL(HAVE_GCC_BUILTINS_FOR_ATOMIC_OPERATIONS,
[test $glib_cv_gcc_has_builtin_atomic_operations = yes])
dnl We need a more robust approach here...
case $host_cpu in
i?86|x86_64|s390|s390x|arm*|crisv32*|etrax*)
glib_memory_barrier_needed=no
;;
sparc*|alpha*|powerpc*|ia64)
glib_memory_barrier_needed=yes
;;
*)
glib_memory_barrier_needed=yes
;;
esac
dnl ************************
dnl ** Check for futex(2) **
@@ -3085,9 +3108,9 @@ _______EOF
echo >>$outfile
echo "#define G_ATOMIC_OP_MEMORY_BARRIER_NEEDED 1" >>$outfile
fi
if test x"$g_gcc_atomic_ops" != xno; then
if test x"$g_atomic_lock_free" = xyes; then
echo >>$outfile
echo "#define G_ATOMIC_OP_USE_GCC_BUILTINS 1" >>$outfile
echo "#define G_ATOMIC_LOCK_FREE" >>$outfile
fi
echo >>$outfile
g_bit_sizes="16 32 64"