On Fri, Mar 08, 2019 at 11:36:36AM +0100, Richard Biener wrote:
> Shouldn't we somehow limit the number of stmt evaluations instead?
> I can imagine using constexpr templates you can do "loops" easily
> without actually writing loops ...
Maybe. The question is what exactly should we count. We could count only
in the cxx_eval_statement_list loop individual non-debug statements,
but that would miss statements that aren't in STATEMENT_LISTs.
Or we could count number of cxx_eval_constant_expression calls
(perhaps after the initial if (jump_target && *jump_target) skipping)
within a single cxx_eval_outermost_constant_expr, but we couldn't call that
number of statements even when it would be much more accurate on how long
actually something will take to evaluate, because one can have a single
statement with hundreds of thousands of operations in it.
So -fconstexpr-ops-limit= ? Probably should use Host_Wide_Int in that case
rather than UInteger though, so users can allow more than 4G ops.
Looking at what clang++ does, they have -fconstexpr-steps= limit which is
declared to count statements with a quite low default 1048576 and it rejects
already
constexpr unsigned
foo ()
{
unsigned int r = 0;
for (int i = 0; i < 65536; i++)
for (int j = 0; j < 14; j++)
r += (i + j);
return r;
}
constexpr auto x = foo ();
(but not 13 instead of 14). 13 * 65536 is 851968, so that would be count of
r += (i + j); statements, plus the inner for as whole probably 65536 times
and with 14 * 65536 + 65536 we are still below the limit, so it must count
something else, but e.g. can't count j++ already.
> > 2019-03-07 Jakub Jelinek <ja...@redhat.com>
> >
> > PR c++/87481
> > * doc/invoke.texi (-fconstexpr-loop-nest-limit=): Document.
> > (-fconstexpr-loop-limit=): Slightly adjust wording.
> >
> > * c.opt (-fconstexpr-loop-nest-limit=): New option.
> > (-fconstexpr-loop-limit=): Slightly adjust description.
> >
> > * constexpr.c (cxx_eval_loop_expr): Count also number of iterations
> > of all nested loops and punt if it is above
> > constexpr_loop_nest_limit.
> >
> > * g++.dg/cpp1y/constexpr-87481.C: New test.
Jakub
