On Mon, Feb 10, 2020 at 8:40 PM Andrew Pinski <pins...@gmail.com> wrote:
>
> On Mon, Feb 10, 2020 at 8:35 PM Fangrui Song <i...@maskray.me> wrote:
> >
> > GCC never evaluates __atomic_is_lock_free to 0.
> > (gcc/builtins.c:fold_builtin_atomic_always_lock_free)
>
> Huh?
Oh it is this, you quoted the wrong function which made things more complicated.
/* Return a one or zero if it can be determined that object ARG1 of size ARG
is lock free on this architecture. */
static tree
fold_builtin_atomic_is_lock_free (tree arg0, tree arg1)
{
if (!flag_inline_atomics)
return NULL_TREE;
/* If it isn't always lock free, don't generate a result. */
if (fold_builtin_atomic_always_lock_free (arg0, arg1) == boolean_true_node)
return boolean_true_node;
return NULL_TREE;
}
--- CUT ---
Thanks,
Andrew Pinski
> /* We need a corresponding integer mode for the access to be lock-free. */
> size = INTVAL (expand_normal (arg0)) * BITS_PER_UNIT;
> if (!int_mode_for_size (size, 0).exists (&mode))
> return boolean_false_node;
> ...
> /* If the object has smaller alignment, the lock free routines cannot
> be used. */
> if (type_align < mode_align)
> return boolean_false_node;
> /* Check if a compare_and_swap pattern exists for the mode which represents
> the required size. The pattern is not allowed to fail, so the existence
> of the pattern indicates support is present. Also require that an
> atomic load exists for the required size. */
> if (can_compare_and_swap_p (mode, true) && can_atomic_load_p (mode))
> return boolean_true_node;
> else
> return boolean_false_node;
>
>
> Thanks,
> Andrew Pinski
>
>
>
> > I'd like to change clang to eagerly evaluate __atomic_is_lock_free to 0 for
> > apparently oversized types.
> > This helps some platforms to avoid a dependency on libatomic.
> >
> > Either of the following choices can move my patch
> > https://reviews.llvm.org/D72579 forward:
> >
> > * GCC will like do the same
> > * GCC is committed to not extend __atomic_is_lock_free in a clang
> > incompatible way.
