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Mon, 08 Aug 2022 12:28:01 -0700 (PDT) X-Google-Smtp-Source: AA6agR6chhLubvoxGBFXbEDoltbV5PZ3iKZwXCKazssjsnZVa8Ah5VeBVxMDIl8qpVwCDR45L/W0ZA== X-Received: by 2002:a05:6214:2b09:b0:477:519:696d with SMTP id jx9-20020a0562142b0900b004770519696dmr17054569qvb.127.1659986881657; Mon, 08 Aug 2022 12:28:01 -0700 (PDT) Received: from redhat.com ([2601:184:4780:4310::e531]) by smtp.gmail.com with ESMTPSA id y17-20020a05620a44d100b006b8c575ce27sm240943qkp.109.2022.08.08.12.28.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Aug 2022 12:28:01 -0700 (PDT) Date: Mon, 8 Aug 2022 15:27:59 -0400 To: Jason Merrill Subject: [PATCH v2] c++: Extend -Wpessimizing-move for class prvalues [PR106276] Message-ID: References: <20220728001424.396849-1-polacek@redhat.com> <50df8e96-3894-b9d2-dbce-a886c04e9815@redhat.com> <242f9ff9-18d8-1da0-f9c5-51615862d22a@redhat.com> MIME-Version: 1.0 In-Reply-To: <242f9ff9-18d8-1da0-f9c5-51615862d22a@redhat.com> User-Agent: Mutt/2.2.6 (2022-06-05) X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-Spam-Status: No, score=-11.9 required=5.0 tests=BAYES_00, BODY_8BITS, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, KAM_SHORT, 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-Patchwork-Original-From: Marek Polacek via Gcc-patches From: Marek Polacek Reply-To: Marek Polacek Cc: GCC Patches Errors-To: gcc-patches-bounces+patchwork=sourceware.org@gcc.gnu.org Sender: "Gcc-patches" On Sat, Aug 06, 2022 at 04:07:54PM -0700, Jason Merrill wrote: > On 8/6/22 15:49, Jason Merrill wrote: > > On 7/27/22 17:14, Marek Polacek wrote: > > > We already have a warning that warns about pessimizing std::move > > > in a return statement, when it prevents the NRVO: > > > > > >    T fn() > > >    { > > >      T t; > > >      return std::move (t); // warning \o/ > > >    } > > > > > > However, the warning doesn't warn when what we are returning is a class > > > prvalue, that is, when std::move prevents the RVO: > > > > > >    T fn() > > >    { > > >      T t; > > >      return std::move (T{}); // no warning :-( > > >    } > > > > > > This came up recently in GCC: > > > . > > > > > > This patch fixes that.  I would like to extend the warning further, so > > > that it warns in more contexts, e.g.: > > > > > >    T t = std::move(T()); > > > > > > or > > > > > >    void foo (T); > > >    foo (std::move(T())); > > > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > > > > >     PR c++/106276 > > > > > > gcc/cp/ChangeLog: > > > > > >     * typeck.cc (can_do_rvo_p): New. > > >     (maybe_warn_pessimizing_move): Warn when moving a temporary object > > >     in a return statement prevents copy elision. > > > > > > gcc/testsuite/ChangeLog: > > > > > >     * g++.dg/cpp0x/Wpessimizing-move7.C: New test. > > > --- > > >   gcc/cp/typeck.cc                              | 31 ++++++++- > > >   .../g++.dg/cpp0x/Wpessimizing-move7.C         | 63 +++++++++++++++++++ > > >   2 files changed, 91 insertions(+), 3 deletions(-) > > >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C > > > > > > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc > > > index 6e4f23af982..9500c4e2fe8 100644 > > > --- a/gcc/cp/typeck.cc > > > +++ b/gcc/cp/typeck.cc > > > @@ -10287,12 +10287,29 @@ can_do_nrvo_p (tree retval, tree functype) > > >         /* The cv-unqualified type of the returned value must be the > > >            same as the cv-unqualified return type of the > > >            function.  */ > > > -      && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))), > > > -              (TYPE_MAIN_VARIANT (functype))) > > > +      && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)), > > > +              TYPE_MAIN_VARIANT (functype)) > > >         /* And the returned value must be non-volatile.  */ > > >         && !TYPE_VOLATILE (TREE_TYPE (retval))); > > >   } > > > +/* Like can_do_nrvo_p, but we check if we're trying to move a class > > > +   prvalue.  */ > > > + > > > +static bool > > > +can_do_rvo_p (tree retval, tree functype) > > > +{ > > > +  if (functype == error_mark_node) > > > +    return false; > > > +  if (retval) > > > +    STRIP_ANY_LOCATION_WRAPPER (retval); > > > +  return (retval != NULL_TREE > > > +      && TREE_CODE (retval) == TARGET_EXPR > > > > Maybe use !glvalue_p instead of specifically checking for TARGET_EXPR? I > > don't feel strongly about this. OK, this version uses !glvalue_p. > > > +      && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)), > > > +              TYPE_MAIN_VARIANT (functype)) > > > +      && !TYPE_VOLATILE (TREE_TYPE (retval))); > > > +} > > > + > > >   /* If we should treat RETVAL, an expression being returned, as if > > > it were > > >      designated by an rvalue, returns it adjusted accordingly; > > > otherwise, returns > > >      NULL_TREE.  See [class.copy.elision].  RETURN_P is true if this > > > is a return > > > @@ -10401,12 +10418,20 @@ maybe_warn_pessimizing_move (tree retval, > > > tree functype) > > >                     "prevents copy elision")) > > >           inform (loc, "remove % call"); > > >           } > > > +      else if (can_do_rvo_p (arg, functype)) > > > +        { > > > +          auto_diagnostic_group d; > > > +          if (warning_at (loc, OPT_Wpessimizing_move, > > > +                  "moving a temporary object in a return statement " > > > +                  "prevents copy elision")) > > > > It doesn't just prevent copy elision, it produces a dangling reference. > >  This is a special case of the warning we talked about passing a > > temporary to a function that returns a reference argument unchanged, and > > should probably use a different warning flag. > > Wait, no, I'm confused, the temporary does live long enough to get copied. > > I still don't think we want to suppress this warning in the other patch. Yeah, that makes sense. I'll change it there. Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- We already have a warning that warns about pessimizing std::move in a return statement, when it prevents the NRVO: T fn() { T t; return std::move (t); // warning \o/ } However, the warning doesn't warn when what we are returning is a class prvalue, that is, when std::move prevents the RVO: T fn() { T t; return std::move (T{}); // no warning :-( } This came up recently in GCC: . This patch fixes that. I would like to extend the warning further, so that it warns in more contexts, e.g.: T t = std::move(T()); or void foo (T); foo (std::move(T())); PR c++/106276 gcc/cp/ChangeLog: * typeck.cc (can_do_rvo_p): New. (maybe_warn_pessimizing_move): Warn when moving a temporary object in a return statement prevents copy elision. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/Wpessimizing-move7.C: New test. --- gcc/cp/typeck.cc | 31 ++++++++- .../g++.dg/cpp0x/Wpessimizing-move7.C | 63 +++++++++++++++++++ 2 files changed, 91 insertions(+), 3 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C base-commit: 4b0253b019943abf2cc5f4db0b7ed67caedffe4a diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index 6e4f23af982..15548e5091f 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -10287,12 +10287,29 @@ can_do_nrvo_p (tree retval, tree functype) /* The cv-unqualified type of the returned value must be the same as the cv-unqualified return type of the function. */ - && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))), - (TYPE_MAIN_VARIANT (functype))) + && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)), + TYPE_MAIN_VARIANT (functype)) /* And the returned value must be non-volatile. */ && !TYPE_VOLATILE (TREE_TYPE (retval))); } +/* Like can_do_nrvo_p, but we check if we're trying to move a class + prvalue. */ + +static bool +can_do_rvo_p (tree retval, tree functype) +{ + if (functype == error_mark_node) + return false; + if (retval) + STRIP_ANY_LOCATION_WRAPPER (retval); + return (retval != NULL_TREE + && !glvalue_p (retval) + && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)), + TYPE_MAIN_VARIANT (functype)) + && !TYPE_VOLATILE (TREE_TYPE (retval))); +} + /* If we should treat RETVAL, an expression being returned, as if it were designated by an rvalue, returns it adjusted accordingly; otherwise, returns NULL_TREE. See [class.copy.elision]. RETURN_P is true if this is a return @@ -10401,12 +10418,20 @@ maybe_warn_pessimizing_move (tree retval, tree functype) "prevents copy elision")) inform (loc, "remove % call"); } + else if (can_do_rvo_p (arg, functype)) + { + auto_diagnostic_group d; + if (warning_at (loc, OPT_Wpessimizing_move, + "moving a temporary 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 && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true))) { - /* Make sure that the overload resolution would actually succeed + /* Make sure that overload resolution would actually succeed if we removed the std::move call. */ tree t = convert_for_initialization (NULL_TREE, functype, moved, diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C new file mode 100644 index 00000000000..cd4eaa09aae --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C @@ -0,0 +1,63 @@ +// PR c++/106276 +// { dg-do compile { target c++11 } } +// { dg-options "-Wpessimizing-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 A { A(); A(const A&) = delete; A(A&&); }; +struct B { B(A); }; + +static A foo (); + +A +fn1 () +{ + return std::move (A{}); // { dg-warning "moving a temporary object in a return statement prevents copy elision" } + return std::move (A()); // { dg-warning "moving a temporary object in a return statement prevents copy elision" } + return std::move (foo ()); // { dg-warning "moving a temporary object in a return statement prevents copy elision" } +} + +B fn2 () +{ + return std::move (A()); + return std::move (A{}); + return std::move (foo ()); +} + +template +T1 +fn3 () +{ + return std::move (T2{}); // { dg-warning "moving a temporary object in a return statement prevents copy elision" } +} + +void +do_fn3 () +{ + fn3(); + fn3(); +} + +char take_buffer; +struct label_text { + label_text take() { return std::move(label_text(&take_buffer)); } // { dg-warning "moving a temporary object in a return statement prevents copy elision" } + label_text(char *); +};