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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id u17-20020a05620a431100b006b9c355ed75sm17277232qko.70.2022.09.08.05.46.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Sep 2022 05:46:28 -0700 (PDT) Message-ID: <6f16b678-bee0-f2c4-0943-cc5103838237@redhat.com> Date: Thu, 8 Sep 2022 08:46:27 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0 Subject: Re: [PATCH] c++: unnecessary instantiation of constexpr var [PR99130] To: Patrick Palka Cc: gcc-patches@gcc.gnu.org References: <20220907194100.879066-1-ppalka@redhat.com> <38d432fb-f9db-6f0f-1587-1b8c0f5c75e7@redhat.com> 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=-14.5 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_LOW,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE 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 9/7/22 16:40, Patrick Palka wrote: > On Wed, 7 Sep 2022, Jason Merrill wrote: > >> On 9/7/22 15:41, Patrick Palka wrote: >>> Here the use of the constexpr member/variable specialization 'value' >>> from within an unevaluated context causes us to overeagerly instantiate >>> it, via maybe_instantiate_decl called from mark_used, despite only its >>> declaration not its definition being needed. >> >> If the issue is with unevaluated context, maybe maybe_instantiate_decl should >> guard the call to decl_maybe_constant_var_p with !cp_unevaluated_operand? > > Hmm, that seems to work too. But IIUC this would mean in an evaluated > (but non-constexpr) context we'd continue to instantiate constexpr > variables _immediately_ rather than ideally allowing mark_used to > postpone their instantiation until the end of TU processing (which is > what happens with the below approach). > > Another benefit of the below approach is that from within a template > definition we we now avoid instantiation altogether e.g. for > > template constexpr int value = /* blah */; > > template > int f() { return value; } > > we no longer instantiate value which IIUC is consistent with how we > handle other kinds of specializations used within a template definition. > So making mark_used no longer instantiate constexpr variables immediately > (in both evaluated and unevaluated contexts) seems to yield the most > benefits. Makes sense. The patch is OK. >>> We used to have the same issue for constexpr function specializations >>> until r6-1309-g81371eff9bc7ef made us delay their instantiation until >>> necessary during constexpr evaluation. >>> >>> So this patch makes us avoid unnecessarily instantiating constexpr >>> variable template specializations from mark_used as well. To that end >>> this patch pulls out the test in maybe_instantiate_decl >>> >>> (decl_maybe_constant_var_p (decl) >>> || (TREE_CODE (decl) == FUNCTION_DECL >>> && DECL_OMP_DECLARE_REDUCTION_P (decl)) >>> || undeduced_auto_decl (decl)) >>> >>> into each of its three callers (including mark_used) and refines the >>> test appropriately. The net result is that only mark_used is changed, >>> because the other two callers, resolve_address_of_overloaded_function >>> and decl_constant_var_p, already guard the call appropriately. And >>> presumably decl_constant_var_p will take care of instantiation when >>> needed for e.g. constexpr evaluation. >>> >>> Bootstrapped and regteste on x86_64-pc-linux-gnu, does this look OK for >>> trunk? >>> >>> PR c++/99130 >>> >>> gcc/cp/ChangeLog: >>> >>> * decl2.cc (maybe_instantiate_decl): Adjust function comment. >>> Check VAR_OR_FUNCTION_DECL_P. Pull out the disjunction into ... >>> (mark_used): ... here, removing the decl_maybe_constant_var_p >>> part of it. >>> gcc/testsuite/ChangeLog: >>> >>> * g++.dg/cpp1y/var-templ70.C: New test. >>> --- >>> gcc/cp/decl2.cc | 33 ++++++++---------------- >>> gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 19 ++++++++++++++ >>> 2 files changed, 30 insertions(+), 22 deletions(-) >>> create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C >>> >>> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc >>> index 89ab2545d64..cd188813bee 100644 >>> --- a/gcc/cp/decl2.cc >>> +++ b/gcc/cp/decl2.cc >>> @@ -5381,24 +5381,15 @@ possibly_inlined_p (tree decl) >>> return true; >>> } >>> -/* Normally, we can wait until instantiation-time to synthesize DECL. >>> - However, if DECL is a static data member initialized with a constant >>> - or a constexpr function, we need it right now because a reference to >>> - such a data member or a call to such function is not value-dependent. >>> - For a function that uses auto in the return type, we need to instantiate >>> - it to find out its type. For OpenMP user defined reductions, we need >>> - them instantiated for reduction clauses which inline them by hand >>> - directly. */ >>> +/* If DECL is a function or variable template specialization, instantiate >>> + its definition now. */ >>> void >>> maybe_instantiate_decl (tree decl) >>> { >>> - if (DECL_LANG_SPECIFIC (decl) >>> + if (VAR_OR_FUNCTION_DECL_P (decl) >>> + && DECL_LANG_SPECIFIC (decl) >>> && DECL_TEMPLATE_INFO (decl) >>> - && (decl_maybe_constant_var_p (decl) >>> - || (TREE_CODE (decl) == FUNCTION_DECL >>> - && DECL_OMP_DECLARE_REDUCTION_P (decl)) >>> - || undeduced_auto_decl (decl)) >>> && !DECL_DECLARED_CONCEPT_P (decl) >>> && !uses_template_parms (DECL_TI_ARGS (decl))) >>> { >>> @@ -5700,15 +5691,13 @@ mark_used (tree decl, tsubst_flags_t complain) >>> return false; >>> } >>> - /* Normally, we can wait until instantiation-time to synthesize DECL. >>> - However, if DECL is a static data member initialized with a constant >>> - or a constexpr function, we need it right now because a reference to >>> - such a data member or a call to such function is not value-dependent. >>> - For a function that uses auto in the return type, we need to >>> instantiate >>> - it to find out its type. For OpenMP user defined reductions, we need >>> - them instantiated for reduction clauses which inline them by hand >>> - directly. */ >>> - maybe_instantiate_decl (decl); >>> + /* If DECL has a deduced return type, we need to instantiate it now to >>> + find out its type. For OpenMP user defined reductions, we need them >>> + instantiated for reduction clauses which inline them by hand directly. >>> */ >>> + if (undeduced_auto_decl (decl) >>> + || (TREE_CODE (decl) == FUNCTION_DECL >>> + && DECL_OMP_DECLARE_REDUCTION_P (decl))) >>> + maybe_instantiate_decl (decl); >>> if (processing_template_decl || in_template_function ()) >>> return true; >>> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C >>> b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C >>> new file mode 100644 >>> index 00000000000..80965657c32 >>> --- /dev/null >>> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C >>> @@ -0,0 +1,19 @@ >>> +// PR c++/99130 >>> +// { dg-do compile { target c++14 } } >>> + >>> +template >>> +struct A { >>> + static constexpr int value = T::value; >>> +}; >>> + >>> +struct B { >>> + template >>> + static constexpr int value = T::value; >>> +}; >>> + >>> +template >>> +constexpr int value = T::value; >>> + >>> +using ty1 = decltype(A::value); >>> +using ty2 = decltype(B::value); >>> +using ty3 = decltype(value); >> >> >