On Tue, Oct 26, 2021 at 03:29:02PM -0400, Jason Merrill wrote:
> On Tue, Oct 26, 2021 at 2:46 PM Jakub Jelinek via Gcc-patches <
> gcc-patches@gcc.gnu.org> wrote:
>
> > On Tue, Oct 26, 2021 at 01:44:18PM -0400, Patrick Palka via Gcc-patches
> > wrote:
> > > In the testcase below the two left fold expressions each expand into a
> > > logical expression with 1024 terms, for which potential_const_expr_1
> > > takes more than a minute to return true. This is because p_c_e_1
> > > performs trial evaluation of the first operand of a &&/|| in order to
> > > determine whether to consider its second operand. And because the
> > > expanded expression is left-associated, this trial evaluation causes
> > > p_c_e_1 to be quadratic in the number of terms of the expression.
> > >
> > > This patch fixes this quadratic behavior by making p_c_e_1 recurse only
> > > into the first operand of a &&/|| when checking for potentiality. This
> > > means we'll consider more non-constant expressions as potentially
> > > constant compared to the trial evaluation approach, but that should be
> > > harmless as later constexpr evaluation of the expression will deem it
> > > non-constant anyway.
> >
>
> Fair, especially now that it seems that C++23 is removing the diagnostic
> for a constexpr function that can't produce a constant expression. And as
> more functions become constexpr, the trial evaluation gets more expensive
> to get to the point of failure. I wonder if we want to stop calling
> cxx_eval_outermost_constant_expression from pot_c_e_1 entirely, only check
> whether the operand is already constant.
That would make p_c_e_1 surely much faster, but on the other side we would
stop diagnosing some functions that can't be valid constant expressions
no matter with what arguments they are called.
Another option would be to do these cxx_eval_outermost_constant_expression
calls from p_c_e_1 only in a cheap mode, i.e. with constexpr_ops_limit
temporarily lowered to something very small, 100-ish or 1000-ish or so, so it
would
handle small expressions like before, but would quietly punt on large
expressions.
Jakub
