On Thu, Jan 09, 2014 at 02:13:39PM +0100, Richard Biener wrote:
> On Thu, 9 Jan 2014, Jakub Jelinek wrote:
>
> > On Thu, Jan 09, 2014 at 01:30:53PM +0100, Richard Biener wrote:
> > > > gimplify_modify_expr has:
> > > >
> > > > if (!gimple_call_noreturn_p (assign))
> > > > gimple_call_set_lhs (assign, *to_p);
> > >
> > > Ok, it seems to be too early then - move it after the folding.
> >
> > That wouldn't help all the other early calls of fold_stmt though.
> > E.g. lower_omp. Plus, even in gimplify_modify_expr, doing it
> > after fold_stmt would mean having to walk all stmts created by the folding?,
> > check if they are calls (because a call can fold into nothing or something
> > completely different). Isn't it better then fold_stmt does that instead?
>
> Hmm, maybe. Not sure why we are this anal about requiring noreturn
> calls not to have a LHS. But if we require callers in SSA form
> to update the stmt and properly cleanup the cfg if fold_stmt returns
> true then it's reasonable to require at least "something" for callers
> from non-SSA/CFG code.
>
> That is, I don't like this special-casing. If so, then rather
> don't fold at this point - thus if (... !inplace && in_ssa_form (cfun)
> ...) (or rather if we have a CFG - cfun && cfun->curr_properties &
> PROP_cfg).
But, isn't right now gimplification the only guaranteed folding of all
stmts? I mean, other passes fold_stmt only if they propagate something into
them, don't they? Also, in most cases the call actually isn't noreturn,
so stopping all the devirtualization just for the unlikely case doesn't look
like a good idea to me.
Perhaps, if you don't like the !gimple_in_ssa_p (cfun) in the condition
we can just drop the lhs always in that case, just doing what we do for
__builtin_unreachable if lhs is SSA_NAME:
tree var = create_tmp_var (TREE_TYPE (lhs), NULL);
tree def = get_or_create_ssa_default_def (cfun, var);
gsi_insert_after (gsi, gimple_build_assign (lhs, def), GSI_NEW_STMT);
Jakub
