Re: [PATCH 1/2] c++: improve "NTTP argument considered unused" fix [PR53164, PR105848]

[Patrick Palka <ppalka@redhat.com>]

On Thu, Mar 23, 2023 at 5:18 PM Patrick Palka <ppalka@redhat.com> wrote:
>
> r13-995-g733a792a2b2e16 worked around the problem of FUNCTION_DECL
> template arguments not always getting marked as odr-used by redundantly
> calling mark_used on the substituted ADDR_EXPR callee of a CALL_EXPR.
> This is just a narrow workaround however, since using a FUNCTION_DECL as
> a template argument alone should constitutes an odr-use; we shouldn't
> need to subsequently e.g. call the function or take its address.
>
> This patch fixes this in a more general way at template specialization
> time by walking the template arguments of the specialization and calling
> mark_used on all entities used within.  As before, the call to mark_used
> as it worst a no-op, but it compensates for the situation where we end up
> forming a specialization from a template context in which mark_used is
> inhibited.  Another approach would be to call mark_used whenever we
> substitute a TEMPLATE_PARM_INDEX, but that would result in many more
> redundant calls to mark_used compared to this approach.

Note we previously discussed this TEMPLATE_PARM_INDEX approach
here https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596257.html
though I never pushed it since it felt somewhat overkill to me every single
substituted use of an NTTP would be considered for marking;
 perhaps this approach might be preferable? Yet another approach would
be to do it from tsubst_template_args..

>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
>
>         PR c++/53164
>         PR c++/105848
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (instantiate_class_template): Call
>         mark_template_arguments_used.
>         (tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995 change.
>         (mark_template_arguments_used): Define.
>         (instantiate_template): Call mark_template_arguments_used.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/template/fn-ptr3a.C: New test.
>         * g++.dg/template/fn-ptr4.C: New test.
> ---
>  gcc/cp/pt.cc                             | 51 ++++++++++++++++--------
>  gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 ++++++++++++
>  gcc/testsuite/g++.dg/template/fn-ptr4.C  | 14 +++++++
>  3 files changed, 74 insertions(+), 16 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3a.C
>  create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr4.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 7e4a8de0c8b..9b3cc33331c 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -220,6 +220,7 @@ static tree make_argument_pack (tree);
>  static tree enclosing_instantiation_of (tree tctx);
>  static void instantiate_body (tree pattern, tree args, tree d, bool nested);
>  static tree maybe_dependent_member_ref (tree, tree, tsubst_flags_t, tree);
> +static void mark_template_arguments_used (tree);
>
>  /* Make the current scope suitable for access checking when we are
>     processing T.  T can be FUNCTION_DECL for instantiated function
> @@ -12142,6 +12143,9 @@ instantiate_class_template (tree type)
>        cp_unevaluated_operand = 0;
>        c_inhibit_evaluation_warnings = 0;
>      }
> +
> +  mark_template_arguments_used (INNERMOST_TEMPLATE_ARGS (args));
> +
>    /* Use #pragma pack from the template context.  */
>    saved_maximum_field_alignment = maximum_field_alignment;
>    maximum_field_alignment = TYPE_PRECISION (pattern);
> @@ -21173,22 +21177,10 @@ tsubst_copy_and_build (tree t,
>           }
>
>         /* Remember that there was a reference to this entity.  */
> -       if (function != NULL_TREE)
> -         {
> -           tree inner = function;
> -           if (TREE_CODE (inner) == ADDR_EXPR
> -               && TREE_CODE (TREE_OPERAND (inner, 0)) == FUNCTION_DECL)
> -             /* We should already have called mark_used when taking the
> -                address of this function, but do so again anyway to make
> -                sure it's odr-used: at worst this is a no-op, but if we
> -                obtained this FUNCTION_DECL as part of ahead-of-time overload
> -                resolution then that call to mark_used wouldn't have marked it
> -                odr-used yet (53164).  */
> -             inner = TREE_OPERAND (inner, 0);
> -           if (DECL_P (inner)
> -               && !mark_used (inner, complain) && !(complain & tf_error))
> -             RETURN (error_mark_node);
> -         }
> +       if (function != NULL_TREE
> +           && DECL_P (function)
> +           && !mark_used (function, complain) && !(complain & tf_error))
> +         RETURN (error_mark_node);
>
>         if (!maybe_fold_fn_template_args (function, complain))
>           return error_mark_node;
> @@ -21883,6 +21875,31 @@ check_instantiated_args (tree tmpl, tree args, tsubst_flags_t complain)
>    return result;
>  }
>
> +/* Call mark_used on each entity within the template arguments ARGS of some
> +   template specialization, to ensure that each such entity is considered
> +   odr-used regardless of whether the specialization was first formed in a
> +   template context.
> +
> +   This function assumes push_to_top_level has been called beforehand, and
> +   that processing_template_decl has been set iff the template arguments
> +   are dependent.  */
> +
> +static void
> +mark_template_arguments_used (tree args)
> +{
> +  gcc_checking_assert (TMPL_ARGS_DEPTH (args) == 1);
> +
> +  if (processing_template_decl)
> +    return;
> +
> +  auto mark_used_r = [](tree *tp, int *, void *) {
> +    if (DECL_P (*tp))
> +      mark_used (*tp, tf_none);
> +    return NULL_TREE;
> +  };
> +  cp_walk_tree_without_duplicates (&args, mark_used_r, nullptr);
> +}
> +
>  /* We're out of SFINAE context now, so generate diagnostics for the access
>     errors we saw earlier when instantiating D from TMPL and ARGS.  */
>
> @@ -22012,6 +22029,8 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
>        push_nested_class (ctx);
>      }
>
> +  mark_template_arguments_used (INNERMOST_TEMPLATE_ARGS (targ_ptr));
> +
>    tree pattern = DECL_TEMPLATE_RESULT (gen_tmpl);
>
>    fndecl = NULL_TREE;
> diff --git a/gcc/testsuite/g++.dg/template/fn-ptr3a.C b/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> new file mode 100644
> index 00000000000..7456be5d51f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> @@ -0,0 +1,25 @@
> +// PR c++/53164
> +// PR c++/105848
> +// A stricter version of fn-ptr3.C that verifies using f as a template
> +// argument alone constitutes an odr-use.
> +
> +template<class T>
> +void f(T) { T::fail; } // { dg-error "fail" }
> +
> +template<void (*P)(int)>
> +struct A {
> +  static void wrap();
> +};
> +
> +template<void (&P)(char)>
> +void wrap();
> +
> +template<int>
> +void g() {
> +  A<f>::wrap(); // { dg-message "required from here" }
> +  wrap<f>(); // { dg-message "required from here" }
> +}
> +
> +int main() {
> +  g<0>();
> +}
> diff --git a/gcc/testsuite/g++.dg/template/fn-ptr4.C b/gcc/testsuite/g++.dg/template/fn-ptr4.C
> new file mode 100644
> index 00000000000..e7425ba96cc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/fn-ptr4.C
> @@ -0,0 +1,14 @@
> +// { dg-do compile }
> +
> +template<void (*P)()>
> +void wrap() {
> +  P(); // OK, despite A::g not being accessible from foo.
> +}
> +
> +struct A {
> +  static void f() {
> +    wrap<A::g>();
> +  }
> +private:
> +  static void g();
> +};
> --
> 2.40.0.130.g27d43aaaf5
>