On Thu, Mar 23, 2023 at 5:18 PM Patrick Palka <ppa...@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 >