On Thu, 23 Mar 2023, Patrick Palka 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.
>
> 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);
Here's v2 which optimizes this function by not walking if
!PRIMARY_TEMPLATE_P (since in that case the innermost arguments have
already been walked when instantiating the context), and walking only
non-type arguments:
-- >8
Subject: [PATCH] c++: improve "NTTP argument considered unused" fix [PR53164,
PR105848]
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 | 57 +++++++++++++++++-------
gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 +++++++++++
gcc/testsuite/g++.dg/template/fn-ptr4.C | 14 ++++++
3 files changed, 80 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 3bb98ebeac1..a87366bb616 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, tree);
/* Make the current scope suitable for access checking when we are
processing T. T can be FUNCTION_DECL for instantiated function
@@ -12152,6 +12153,9 @@ instantiate_class_template (tree type)
cp_unevaluated_operand = 0;
c_inhibit_evaluation_warnings = 0;
}
+
+ mark_template_arguments_used (templ, CLASSTYPE_TI_ARGS (type));
+
/* Use #pragma pack from the template context. */
saved_maximum_field_alignment = maximum_field_alignment;
maximum_field_alignment = TYPE_PRECISION (pattern);
@@ -21192,22 +21196,10 @@ 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;
@@ -21902,6 +21894,37 @@ check_instantiated_args (tree tmpl, tree args,
tsubst_flags_t complain)
return result;
}
+/* Call mark_used on each entity within the non-type template arguments in
+ ARGS for a specialization of TMPL, 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 is set iff the template arguments are
+ dependent. */
+
+static void
+mark_template_arguments_used (tree tmpl, tree args)
+{
+ /* It suffices to do this only when fully specializing a primary template.
*/
+ if (processing_template_decl || !PRIMARY_TEMPLATE_P (tmpl))
+ return;
+
+ /* We already marked outer arguments when specializing the context. */
+ args = INNERMOST_TEMPLATE_ARGS (args);
+
+ for (tree arg : tree_vec_range (args))
+ if (!TYPE_P (arg))
+ {
+ 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. */
@@ -22031,6 +22054,8 @@ instantiate_template (tree tmpl, tree orig_args,
tsubst_flags_t complain)
push_nested_class (ctx);
}
+ mark_template_arguments_used (gen_tmpl, 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..36551ec5d7f
--- /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 here.
+}
+
+struct A {
+ static void f() {
+ wrap<A::g>();
+ }
+private:
+ static void g();
+};
--
2.40.0.130.g27d43aaaf5
