On Thu, Mar 7, 2019 at 8:22 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> Hi!
>
> We have -fconstexpr-loop-limit= option to have an upper bound for constexpr
> evaluation of a single loop. Even with that limit in place, if we have
> several nested loops during constexpr evaluation, even when each could have
> a few hundred iterations, the whole loop nest could take years to evaluate.
> And the loops can be split across several constexpr function calls even.
>
> The following patch adds another, slightly larger, limit on total number of
> iterations in the whole loop nest.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
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 ...
Richard.
> 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.
>
> --- gcc/doc/invoke.texi.jj 2019-03-02 09:03:25.822755663 +0100
> +++ gcc/doc/invoke.texi 2019-03-07 14:25:34.881197149 +0100
> @@ -210,7 +210,7 @@ in the following sections.
> @gccoptlist{-fabi-version=@var{n} -fno-access-control @gol
> -faligned-new=@var{n} -fargs-in-order=@var{n} -fchar8_t -fcheck-new @gol
> -fconstexpr-depth=@var{n} -fconstexpr-loop-limit=@var{n} @gol
> --fno-elide-constructors @gol
> +-fconstexpr-loop-nest-limit=@var{n} -fno-elide-constructors @gol
> -fno-enforce-eh-specs @gol
> -fno-gnu-keywords @gol
> -fno-implicit-templates @gol
> @@ -2522,10 +2522,19 @@ is 512.
>
> @item -fconstexpr-loop-limit=@var{n}
> @opindex fconstexpr-loop-limit
> -Set the maximum number of iterations for a loop in C++14 constexpr functions
> -to @var{n}. A limit is needed to detect infinite loops during
> +Set the maximum number of iterations for a single loop in C++14 constexpr
> +functions to @var{n}. A limit is needed to detect infinite loops during
> constant expression evaluation. The default is 262144 (1<<18).
>
> +@item -fconstexpr-loop-nest-limit=@var{n}
> +@opindex fconstexpr-loop-nest-limit
> +Set the maximum number of iterations for all loops in a loop nest in C++14
> +constexpr functions to @var{n}. Even when number of iterations of a single
> +loop is limited with the above limit, if there are several nested loops and
> +each of them has many iterations but still smaller than the above limit,
> +the resulting evaluation of the loop nest might take too long.
> +The default is 1048576 (1<<20).
> +
> @item -fdeduce-init-list
> @opindex fdeduce-init-list
> Enable deduction of a template type parameter as
> --- gcc/c-family/c.opt.jj 2019-02-25 23:56:58.149055806 +0100
> +++ gcc/c-family/c.opt 2019-03-07 14:25:07.782639790 +0100
> @@ -1414,7 +1414,11 @@ C++ ObjC++ Joined RejectNegative UIntege
>
> fconstexpr-loop-limit=
> C++ ObjC++ Joined RejectNegative UInteger Var(constexpr_loop_limit)
> Init(262144)
> --fconstexpr-loop-limit=<number> Specify maximum constexpr loop
> iteration count.
> +-fconstexpr-loop-limit=<number> Specify maximum constexpr loop
> iteration count of a single loop.
> +
> +fconstexpr-loop-nest-limit=
> +C++ ObjC++ Joined RejectNegative UInteger Var(constexpr_loop_nest_limit)
> Init(1048576)
> +-fconstexpr-loop-nest-limit=<number> Specify maximum constexpr loop
> iteration count for all nested loops.
>
> fdebug-cpp
> C ObjC C++ ObjC++
> --- gcc/cp/constexpr.c.jj 2019-03-06 19:45:40.360751756 +0100
> +++ gcc/cp/constexpr.c 2019-03-07 14:03:00.040329107 +0100
> @@ -4190,8 +4190,20 @@ cxx_eval_loop_expr (const constexpr_ctx
> default:
> gcc_unreachable ();
> }
> +
> + /* True on entry to cxx_eval_loop_expr if called indirectly from another
> + cxx_eval_loop_expr. */
> + static bool constexpr_loop_nested;
> +
> + /* Number of evaluated loop iterations in the whole current loop nest. */
> + static int constexpr_loop_nest_count;
> +
> + bool save_constexpr_loop_nested = constexpr_loop_nested;
> + constexpr_loop_nested = true;
> +
> hash_set<tree> save_exprs;
> new_ctx.save_exprs = &save_exprs;
> +
> do
> {
> if (count != -1)
> @@ -4248,6 +4260,24 @@ cxx_eval_loop_expr (const constexpr_ctx
> *non_constant_p = true;
> break;
> }
> +
> + /* In nested loops, don't count the first iteration, as it has been
> + counted in the parent loop already. */
> + if (count > (save_constexpr_loop_nested ? 1 : 0))
> + {
> + if (++constexpr_loop_nest_count >= constexpr_loop_nest_limit)
> + {
> + if (!ctx->quiet)
> + error_at (cp_expr_loc_or_loc (t, input_location),
> + "%<constexpr%> loop iteration count in all nested "
> + "loops exceeds limit of %d (use "
> + "-fconstexpr-loop-nest-limit= to increase the
> limit)",
> + constexpr_loop_nest_limit);
> + constexpr_loop_nest_count = 0;
> + *non_constant_p = true;
> + break;
> + }
> + }
> }
> while (!returns (jump_target)
> && !breaks (jump_target)
> @@ -4260,6 +4290,12 @@ cxx_eval_loop_expr (const constexpr_ctx
> iter != save_exprs.end(); ++iter)
> new_ctx.values->remove (*iter);
>
> + if (!save_constexpr_loop_nested)
> + {
> + constexpr_loop_nested = false;
> + constexpr_loop_nest_count = 0;
> + }
> +
> return NULL_TREE;
> }
>
> --- gcc/testsuite/g++.dg/cpp1y/constexpr-87481.C.jj 2019-03-07
> 14:28:45.664080874 +0100
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-87481.C 2019-03-07
> 14:36:05.622894508 +0100
> @@ -0,0 +1,16 @@
> +// PR c++/87481
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-fconstexpr-loop-limit=98304
> -fconstexpr-loop-nest-limit=131072" } */
> +
> +constexpr unsigned
> +foo ()
> +{
> + unsigned int r = 0;
> + for (int i = 0; i < 65536; i++)
> + for (int j = 0; j < 65536; j++)
> + for (int k = 0; k < 65536; k++) // { dg-error "'constexpr' loop
> iteration count in all nested loops exceeds limit of 131072" }
> + r += (i + j + k);
> + return r;
> +}
> +
> +constexpr auto x = foo (); // { dg-message "in 'constexpr'
> expansion of" }
>
> Jakub
