From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id C00633858415 for ; Sat, 6 Aug 2022 23:58:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org C00633858415 Received: from mail-qt1-f199.google.com (mail-qt1-f199.google.com [209.85.160.199]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-460--CRAioq9NFOk2Ti7WPR3IA-1; Sat, 06 Aug 2022 19:58:24 -0400 X-MC-Unique: -CRAioq9NFOk2Ti7WPR3IA-1 Received: by mail-qt1-f199.google.com with SMTP id bc10-20020a05622a1cca00b00342e824c23fso3100373qtb.23 for ; Sat, 06 Aug 2022 16:58:24 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:references:from:in-reply-to :content-transfer-encoding; bh=En4KtRLVLLAVW61AqLNC2d97Gbtvov3Pd2rpzWsbybc=; b=1hEYLT5f6gUocsPnggYL4F98b/HhVb9xySmxAvvbo+m64QQTbW9rakYsL1/HtTMyab aYV1FNYwCVMwJ3VWqXnCM5ccY/zMHglG64p9mGTp1GuWZXR0z/ybGygobj0sicRa2Ivo hOD8oPexYKF5uN2LpugIHtD1oNI1kMfnNTZLuOBqynQPI/oChfliDJkV/PlRvFbyFHbm u2x17shAkBoQwZPESXlbSZchZorNvPV8Ev5nN4R0jxX0kcM2vB2Wdp8WnoH8JY09ervI WKVB7d3bvRzxxMESUrb1nIPvzdLjh/kurEA928lYypfXbOxNptXFSeL+QKNAA9DC+ZkU NvNg== X-Gm-Message-State: ACgBeo2BM+jNP61VxTVHzgXVuI3VEEPVwjDT6flUjO+sRXft5VNOzwwY n1SYsKBht89byrd5KTpS80vwV9z3OUViHhGwqeC8mQxbEy8+DKyj4grrOjSd24As5ux4i+BkLj+ InACV7jpJTp8HwLqsXg== X-Received: by 2002:a05:6214:2aac:b0:474:8b64:8f56 with SMTP id js12-20020a0562142aac00b004748b648f56mr10955162qvb.0.1659830304039; Sat, 06 Aug 2022 16:58:24 -0700 (PDT) X-Google-Smtp-Source: AA6agR7MJ1b/u4Lm5ltHMhqaizgBWrIG/1E36PAprorkxnUFB6l76gpLaKGSUoWn7v6OSj96k04y7w== X-Received: by 2002:a05:6214:2aac:b0:474:8b64:8f56 with SMTP id js12-20020a0562142aac00b004748b648f56mr10955154qvb.0.1659830303590; Sat, 06 Aug 2022 16:58:23 -0700 (PDT) Received: from [192.168.44.44] ([72.2.237.35]) by smtp.gmail.com with ESMTPSA id u12-20020a05620a454c00b006b91d06044asm4045745qkp.127.2022.08.06.16.58.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 06 Aug 2022 16:58:23 -0700 (PDT) Message-ID: <4c50b759-f207-7c80-b3f0-efa00a9b73a6@redhat.com> Date: Sat, 6 Aug 2022 15:58:13 -0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH] c++: Extend -Wredundant-move for const-qual objects [PR90428] To: Marek Polacek , GCC Patches References: <20220806181313.337246-1-polacek@redhat.com> From: Jason Merrill In-Reply-To: <20220806181313.337246-1-polacek@redhat.com> 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=-12.7 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, 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 X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 06 Aug 2022 23:58:28 -0000 On 8/6/22 11:13, Marek Polacek wrote: > In this PR, Jon suggested extending the -Wredundant-move warning > to warn when the user is moving a const object as in: > > struct T { }; > > T f(const T& t) > { > return std::move(t); > } > > where the std::move is redundant, because T does not have > a T(const T&&) constructor (which is very unlikely). Even with > the std::move, T(T&&) would not be used because it would mean > losing the const. Instead, T(const T&) will be called. > > I had to restructure the function a bit, but it's better now. This patch > depends on my other recent patches to maybe_warn_pessimizing_move. > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > PR c++/90428 > > gcc/cp/ChangeLog: > > * typeck.cc (maybe_warn_pessimizing_move): Extend the > -Wredundant-move warning to warn about std::move on a > const-qualified object. > > gcc/testsuite/ChangeLog: > > * g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning. > * g++.dg/cpp0x/Wredundant-move9.C: Likewise. > * g++.dg/cpp0x/Wredundant-move10.C: New test. > --- > gcc/cp/typeck.cc | 157 +++++++++++------- > gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C | 3 +- > .../g++.dg/cpp0x/Wredundant-move10.C | 61 +++++++ > gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C | 3 +- > 4 files changed, 162 insertions(+), 62 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C > > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc > index 70a5efc45de..802bc9c43fb 100644 > --- a/gcc/cp/typeck.cc > +++ b/gcc/cp/typeck.cc > @@ -10411,72 +10411,109 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) > return; > } > > - /* We're looking for *std::move ((T &) &arg). */ > - if (REFERENCE_REF_P (expr) > - && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR) > - { > - tree fn = TREE_OPERAND (expr, 0); > - if (is_std_move_p (fn)) > - { > - tree arg = CALL_EXPR_ARG (fn, 0); > - tree moved; > - if (TREE_CODE (arg) != NOP_EXPR) > - return; > - arg = TREE_OPERAND (arg, 0); > - if (TREE_CODE (arg) != ADDR_EXPR) > - return; > - arg = TREE_OPERAND (arg, 0); > - arg = convert_from_reference (arg); > - if (can_do_rvo_p (arg, type)) > - { > - auto_diagnostic_group d; > - if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) > - && warning_at (loc, OPT_Wpessimizing_move, > - "moving a temporary object prevents copy " > - "elision")) > - inform (loc, "remove % call"); > - } > - /* The rest of the warnings is only relevant for when we are > - returning from a function. */ > - else if (!return_p) > - return; > - /* Warn if we could do copy elision were it not for the move. */ > - else if (can_do_nrvo_p (arg, type)) > + /* First, check if this is a call to std::move. */ > + if (!REFERENCE_REF_P (expr) > + || TREE_CODE (TREE_OPERAND (expr, 0)) != CALL_EXPR) > + return; > + tree fn = TREE_OPERAND (expr, 0); > + if (!is_std_move_p (fn)) > + return; > + tree arg = CALL_EXPR_ARG (fn, 0); > + if (TREE_CODE (arg) != NOP_EXPR) > + return; > + /* If we're looking at *std::move ((T &) &arg), do the pessimizing N/RVO > + and implicitly-movable warnings. */ > + if (TREE_CODE (TREE_OPERAND (arg, 0)) == ADDR_EXPR) > + { > + arg = TREE_OPERAND (arg, 0); > + arg = TREE_OPERAND (arg, 0); > + arg = convert_from_reference (arg); > + if (can_do_rvo_p (arg, type)) Incidentally, this function should probably have a different name if we're checking it in non-return situations. > + { > + auto_diagnostic_group d; > + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) > + && warning_at (loc, OPT_Wpessimizing_move, > + "moving a temporary object prevents copy elision")) > + inform (loc, "remove % call"); > + } > + /* The rest of the warnings is only relevant for when we are returning > + from a function. */ > + if (!return_p) > + return; > + > + tree moved; > + /* Warn if we could do copy elision were it not for the move. */ > + if (can_do_nrvo_p (arg, type)) > + { > + auto_diagnostic_group d; > + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) > + && warning_at (loc, OPT_Wpessimizing_move, > + "moving a local object in a return statement " > + "prevents copy elision")) > + inform (loc, "remove % call"); > + } > + /* Warn if the move is redundant. It is redundant when we would > + do maybe-rvalue overload resolution even without std::move. */ > + else if (warn_redundant_move > + && !warning_suppressed_p (expr, OPT_Wredundant_move) > + && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true))) > + { > + /* Make sure that overload resolution would actually succeed > + if we removed the std::move call. */ > + tree t = convert_for_initialization (NULL_TREE, type, > + moved, > + (LOOKUP_NORMAL > + | LOOKUP_ONLYCONVERTING > + | LOOKUP_PREFER_RVALUE), > + ICR_RETURN, NULL_TREE, 0, > + tf_none); > + /* If this worked, implicit rvalue would work, so the call to > + std::move is redundant. */ > + if (t != error_mark_node > + /* Trying to move something const will never succeed unless > + there's T(const T&&), which it almost never is, and if > + so, T wouldn't be error_mark_node now. So do warn. */ > + || (CP_TYPE_CONST_P (TREE_TYPE (arg)) > + && same_type_ignoring_top_level_qualifiers_p > + (TREE_TYPE (arg), type))) I'm confused by the || case here. Overload resolution fails if we drop the move call, but we're still going to warn? > { > auto_diagnostic_group d; > - if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) > - && warning_at (loc, OPT_Wpessimizing_move, > - "moving a local object in a return statement " > - "prevents copy elision")) > + if (warning_at (loc, OPT_Wredundant_move, > + "redundant move in return statement")) > inform (loc, "remove % call"); > } > - /* Warn if the move is redundant. It is redundant when we would > - do maybe-rvalue overload resolution even without std::move. */ > - else if (warn_redundant_move > - && !warning_suppressed_p (expr, OPT_Wredundant_move) > - && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true))) > - { > - /* Make sure that overload resolution would actually succeed > - if we removed the std::move call. */ > - tree t = convert_for_initialization (NULL_TREE, type, > - moved, > - (LOOKUP_NORMAL > - | LOOKUP_ONLYCONVERTING > - | LOOKUP_PREFER_RVALUE), > - ICR_RETURN, NULL_TREE, 0, > - tf_none); > - /* If this worked, implicit rvalue would work, so the call to > - std::move is redundant. */ > - if (t != error_mark_node) > - { > - auto_diagnostic_group d; > - if (warning_at (loc, OPT_Wredundant_move, > - "redundant move in return statement")) > - inform (loc, "remove % call"); > - } > - } > } > } > + /* Also try to warn about redundant std::move in code such as > + T f (const T& t) > + { > + return std::move(t); > + } > + for which EXPR will be something like > + *std::move ((const struct T &) (const struct T *) t) > + and where the std::move does nothing if T does not have a T(const T&&) > + constructor, because the argument is const. It will not use T(T&&) > + because that would mean losing the const. */ > + else if (TREE_CODE (TREE_TYPE (arg)) == REFERENCE_TYPE > + && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg)))) > + { > + tree rtype = TREE_TYPE (TREE_TYPE (arg)); > + if (!same_type_ignoring_top_level_qualifiers_p (rtype, type)) > + return; > + /* Check for the unlikely case there's T(const T&&) (we don't care if > + it's deleted). */ > + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (rtype))) > + if (move_fn_p (fn)) > + { > + tree t = TREE_VALUE (FUNCTION_FIRST_USER_PARMTYPE (fn)); > + if (UNLIKELY (CP_TYPE_CONST_P (TREE_TYPE (t)))) > + return; > + } > + auto_diagnostic_group d; > + if (warning_at (loc, OPT_Wredundant_move, > + "redundant move in return statement")) > + inform (loc, "remove % call"); > + } > } > > /* Check that returning RETVAL from the current function is valid. > diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C > index ce4087b476f..c227019cce1 100644 > --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C > +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C > @@ -60,7 +60,8 @@ fn4 (const T t) > { > // t is const: will decay into copy despite std::move, so it's redundant. > // We used to warn about this, but no longer since c++/87378. > - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } } > + // Now we warn again since c++/90428. > + return std::move (t); // { dg-warning "redundant move" } > } > > int > diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C > new file mode 100644 > index 00000000000..a215a4774d6 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C > @@ -0,0 +1,61 @@ > +// PR c++/90428 > +// { dg-do compile { target c++11 } } > +// { dg-options "-Wredundant-move" } > + > +// Define std::move. > +namespace std { > + template > + struct remove_reference > + { typedef _Tp type; }; > + > + template > + struct remove_reference<_Tp&> > + { typedef _Tp type; }; > + > + template > + struct remove_reference<_Tp&&> > + { typedef _Tp type; }; > + > + template > + constexpr typename std::remove_reference<_Tp>::type&& > + move(_Tp&& __t) noexcept > + { return static_cast::type&&>(__t); } > +} > + > +struct T { T(); T(const T&); T(T&&) = delete; }; > +struct S : T { }; > +struct W { W(const W&); W(W&&) = delete; W(const W&&); }; > + > +T f1(T t) > +{ > + const T& rt = t; > + return std::move(rt); // { dg-warning "redundant move" } > +} > + > +T f2(const T& t) > +{ > + return std::move(t); // { dg-warning "redundant move" } > +} > + > +W f3(const W& w) > +{ > + return std::move(w); > +} > + > +T f4(const S& s) > +{ > + return std::move(s); > +} > + > +T f5(const T t) > +{ > + return std::move(t); // { dg-warning "redundant move" } > +} > + > +struct S1 { S1(S1 &&) = delete; S1(const S1&); }; > +struct S2: S1 {}; > + > +S1 f3(const S2 s) > +{ > + return std::move(s); // { dg-warning "redundant move" "" { target c++20 } } > +} > diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C > index ca1e23b7a4b..489ecd2b9c9 100644 > --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C > +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C > @@ -61,7 +61,8 @@ fn4 (const T t) > { > // t is const: will decay into copy despite std::move, so it's redundant. > // We used to warn about this, but no longer since c++/87378. > - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } } > + // Now we warn again since c++/90428. > + return std::move (t); // { dg-warning "redundant move" } > } > > int > > base-commit: 36e96748edef21eecfe8ffc51dba8ec06e8533ec > prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5 > prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4 > prerequisite-patch-id: bf551f0fe75fa8f6775fe53469b61ae94d8cab81