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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id v1-20020ac87481000000b003bffe7fdc38sm165688qtq.9.2023.03.30.14.39.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Mar 2023 14:39:34 -0700 (PDT) Message-ID: Date: Thu, 30 Mar 2023 17:39:34 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [PATCH 1/2] c++: improve "NTTP argument considered unused" fix [PR53164, PR105848] To: Patrick Palka Cc: gcc-patches@gcc.gnu.org References: <20230323211803.396326-1-ppalka@redhat.com> <4a04306f-faa9-5dd8-c850-05aa4a21d36e@idea> From: Jason Merrill In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.2 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 3/30/23 14:53, Patrick Palka wrote: > On Wed, 29 Mar 2023, Jason Merrill wrote: > >> On 3/27/23 09:30, Patrick Palka wrote: >>> 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. >> >> Agreed. But why didn't we already wrap it in an ADDR_EXPR? Even for >> reference tparms convert_nontype_argument should do that. > > Indeed we do, the commit message was just rather sloppy/inaccurate... > I'll try to correct it. > >> >>>> 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) : 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) : 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); >> >> Do we really need to walk into the args? > > Hmm, I think we might need to some sort for walking for class NTTP > arguments since they could be nested CONSTRUCTORs. But for non-class > arguments we know that an entity will only appear as a pointer- or > reference-to-function/variable, so we could pattern match for such a > structure directly. And doing so should be preferable because > cp_walk_tree is relatively expensive and this is a relatively hot code > path. So maybe the following would be preferable? > > -- >8 -- > > Subject: [PATCH] c++: improve "NTTP argument considered unused" fix [PR53164, > PR105848] > > r13-995-g733a792a2b2e16 worked around the problem of (pointer to) > function NTTP arguments not always getting marked as odr-used by > redundantly calling mark_used on the substituted ADDR_EXPR callee of a > CALL_EXPR. That is just a narrow workaround however, since it assumes > the function is later called. But the use as a template argument alone > should constitute an odr-use of the function (since template arguments > are an evaluated context, and we need its address); we shouldn't need to > subsequently call or otherwise use the NTTP argument. > > 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 in 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. And as the second testcase > below illustrates, we also need to walk C++20 class NTTP arguments which > can in theory be large and thus expensive to walk repeatedly. The > change to invalid_tparm_referent_p is needed to avoid bogusly rejecting > the testcase's class NTTP arguments containing function pointers. > > (The third testcase is unrelated to this fix, but it helped rule out an > earlier approach I was considering and it seems we don't have existing > test coverage for this situation.) > > PR c++/53164 > PR c++/105848 > > gcc/cp/ChangeLog: > > * pt.cc (invalid_tparm_referent_p): Accept ADDR_EXPR of > FUNCTION_DECL. > (instantiate_class_template): Call mark_template_arguments_used. > (tsubst_copy_and_build) : 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-ptr3b.C: New test. > * g++.dg/template/fn-ptr4.C: New test. > --- > gcc/cp/pt.cc | 78 ++++++++++++++++++------ > gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 ++++++++ > gcc/testsuite/g++.dg/template/fn-ptr3b.C | 28 +++++++++ > gcc/testsuite/g++.dg/template/fn-ptr4.C | 14 +++++ > 4 files changed, 127 insertions(+), 18 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3a.C > create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3b.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 e514a277872..01ab220320c 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 > @@ -7142,12 +7143,13 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain) > decl = TREE_OPERAND (decl, 0); > } > > - if (!VAR_P (decl)) > + if (!VAR_OR_FUNCTION_DECL_P (decl)) > { > if (complain & tf_error) > error_at (cp_expr_loc_or_input_loc (expr), > "%qE is not a valid template argument of type %qT " > - "because %qE is not a variable", expr, type, decl); > + "because %qE is not a variable or function", > + expr, type, decl); > return true; > } > else if (cxx_dialect < cxx11 && !DECL_EXTERNAL_LINKAGE_P (decl)) > @@ -12152,6 +12154,9 @@ 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 +21197,10 @@ 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 +21895,53 @@ 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)) > + { > + /* A (pointer/reference to) function or variable NTTP argument. */ > + if (TREE_CODE (arg) == ADDR_EXPR > + || TREE_CODE (arg) == INDIRECT_REF) > + { > + while (TREE_CODE (arg) == ADDR_EXPR > + || REFERENCE_REF_P (arg) > + || CONVERT_EXPR_P (arg)) > + arg = TREE_OPERAND (arg, 0); > + if (DECL_P (arg)) > + mark_used (arg, tf_none); Passing tf_none and also ignoring the return value needs a comment justifying why you assume it can't fail. > + } > + /* A class NTTP argument. */ > + else if (VAR_P (arg) > + && DECL_NTTP_OBJECT_P (arg)) Maybe avoid doing this multiple times for the same NTTP object by gating it on DECL_ODR_USED (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 (&DECL_INITIAL (arg), > + 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 +22071,8 @@ 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 > +void f(T) { T::fail; } // { dg-error "fail" } > + > +template > +struct A { > + static void wrap(); > +}; > + > +template > +void wrap(); > + > +template > +void g() { > + A::wrap(); // { dg-message "required from here" } > + wrap(); // { dg-message "required from here" } > +} > + > +int main() { > + g<0>(); > +} > diff --git a/gcc/testsuite/g++.dg/template/fn-ptr3b.C b/gcc/testsuite/g++.dg/template/fn-ptr3b.C > new file mode 100644 > index 00000000000..90d988ce896 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/template/fn-ptr3b.C > @@ -0,0 +1,28 @@ > +// PR c++/53164 > +// PR c++/105848 > +// A C++20 version of fn-ptr3a.C using class NTTPs. > +// { dg-do compile { target c++20 } } > + > +template > +void f(T) { T::fail; } // { dg-error "fail" } > + > +struct B_int { void (*P)(int); }; > +struct B_char { void (*P)(char); }; > + > +template > +struct A { > + static void wrap(); > +}; > + > +template > +void wrap(); > + > +template > +void g() { > + A::wrap(); // { dg-message "required from here" } > + wrap(); // { 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 wrap() { > + P(); // OK, despite A::g not being accessible from here. > +} > + > +struct A { > + static void f() { > + wrap(); > + } > +private: > + static void g(); > +};