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Mon, 08 Aug 2022 12:51:51 -0700 (PDT) X-Received: by 2002:a05:620a:1a9d:b0:6b5:bc5e:cc48 with SMTP id bl29-20020a05620a1a9d00b006b5bc5ecc48mr14865122qkb.400.1659988311430; Mon, 08 Aug 2022 12:51:51 -0700 (PDT) Received: from redhat.com ([2601:184:4780:4310::e531]) by smtp.gmail.com with ESMTPSA id v2-20020a05620a440200b006b978b521c8sm63245qkp.69.2022.08.08.12.51.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Aug 2022 12:51:50 -0700 (PDT) Date: Mon, 8 Aug 2022 15:51:48 -0400 To: Jason Merrill Subject: [PATCH v2] c++: Tweak for -Wpessimizing-move in templates [PR89780] Message-ID: References: <20220804184642.106756-1-polacek@redhat.com> <56c01b19-087c-b955-6528-61b4c1fd0ab2@redhat.com> MIME-Version: 1.0 In-Reply-To: <56c01b19-087c-b955-6528-61b4c1fd0ab2@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=-13.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, 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+ouuuleilei=gmail.com@gcc.gnu.org Sender: "Gcc-patches" X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1740257479534091202?= X-GMAIL-MSGID: =?utf-8?q?1740623954567758876?= On Sat, Aug 06, 2022 at 04:02:13PM -0700, Jason Merrill wrote: > On 8/4/22 11:46, Marek Polacek wrote: > > In my previous patches I've been extending our std::move warnings, > > but this tweak actually dials it down a little bit. As reported in > > bug 89780, it's questionable to warn about expressions in templates > > that were type-dependent, but aren't anymore because we're instantiating > > the template. As in, > > > > template > > Dest withMove() { > > T x; > > return std::move(x); > > } > > > > template Dest withMove(); // #1 > > template Dest withMove(); // #2 > > > > Saying that the std::move is pessimizing for #1 is not incorrect, but > > it's not useful, because removing the std::move would then pessimize #2. > > So the user can't really win. At the same time, disabling the warning > > just because we're in a template would be going too far, I still want to > > warn for > > > > template > > Dest withMove() { > > Dest x; > > return std::move(x); > > } > > > > because the std::move therein will be pessimizing for any instantiation. > > > > So I'm using the suppress_warning machinery to that effect. > > Problem: I had to add a new group to nowarn_spec_t, otherwise > > suppressing the -Wpessimizing-move warning would disable a whole bunch > > of other warnings, which we really don't want. > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > > > PR c++/89780 > > > > gcc/cp/ChangeLog: > > > > * pt.cc (tsubst_copy_and_build) : Maybe suppress > > -Wpessimizing-move. > > * typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings > > if they are suppressed. > > (check_return_expr): Disable -Wpessimizing-move when returning > > a dependent expression. > > > > gcc/ChangeLog: > > > > * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle > > OPT_Wpessimizing_move and OPT_Wredundant_move. > > * diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator. > > > > gcc/testsuite/ChangeLog: > > > > * g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning. > > * g++.dg/cpp0x/Wpessimizing-move7.C: Likewise. > > * g++.dg/cpp0x/Wredundant-move2.C: Likewise. > > * g++.dg/cpp0x/Wpessimizing-move9.C: New test. > > --- > > gcc/cp/pt.cc | 3 + > > gcc/cp/typeck.cc | 20 +++-- > > gcc/diagnostic-spec.cc | 7 +- > > gcc/diagnostic-spec.h | 4 +- > > .../g++.dg/cpp0x/Wpessimizing-move3.C | 2 +- > > .../g++.dg/cpp0x/Wpessimizing-move7.C | 2 +- > > .../g++.dg/cpp0x/Wpessimizing-move9.C | 89 +++++++++++++++++++ > > gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C | 4 +- > > 8 files changed, 119 insertions(+), 12 deletions(-) > > create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C > > > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc > > index 6c581fe0fb7..fe7e809fc2d 100644 > > --- a/gcc/cp/pt.cc > > +++ b/gcc/cp/pt.cc > > @@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t, > > CALL_EXPR_ORDERED_ARGS (call) = ord; > > CALL_EXPR_REVERSE_ARGS (call) = rev; > > } > > + if (warning_suppressed_p (t, OPT_Wpessimizing_move)) > > + /* This also suppresses -Wredundant-move. */ > > + suppress_warning (ret, OPT_Wpessimizing_move); > > } > > RETURN (ret); > > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc > > index 2650beb780e..70a5efc45de 100644 > > --- a/gcc/cp/typeck.cc > > +++ b/gcc/cp/typeck.cc > > @@ -10430,9 +10430,10 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) > > if (can_do_rvo_p (arg, type)) > > { > > auto_diagnostic_group d; > > - if (warning_at (loc, OPT_Wpessimizing_move, > > - "moving a temporary object prevents copy " > > - "elision")) > > + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) > > I don't think we ever want to suppress this warning; moving it to a > different warning flag (as I suggested on the other patch) would accomplish > that. Agreed. I just removed the warning_suppressed_p check though, a new flag would need another NW_ group etc. > > + && 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 > > @@ -10443,14 +10444,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) > > else if (can_do_nrvo_p (arg, type)) > > { > > auto_diagnostic_group d; > > - if (warning_at (loc, OPT_Wpessimizing_move, > > - "moving a local object in a return statement " > > - "prevents copy elision")) > > + 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 > > @@ -10695,6 +10698,11 @@ check_return_expr (tree retval, bool *no_warning) > > /* We don't know if this is an lvalue or rvalue use, but > > either way we can mark it as read. */ > > mark_exp_read (retval); > > + /* Disable our std::move warnings when we're returning > > + a dependent expression (c++/89780). */ > > + if (retval && TREE_CODE (retval) == CALL_EXPR) > > Should this check type_dependent_expression_p? We already check that, this block is guarded by: if (dependent_type_p (functype) || type_dependent_expression_p (retval)) { dependent: > > --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C > > +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C > > @@ -46,7 +46,7 @@ template > > T1 > > fn3 () > > { > > - return std::move (T2{}); // { dg-warning "moving a temporary object prevents copy elision" } > > + return std::move (T2{}); > > As mentioned above, we want to keep this warning; the dangling reference > problem is not type-dependent. Fixed. Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- In my previous patches I've been extending our std::move warnings, but this tweak actually dials it down a little bit. As reported in bug 89780, it's questionable to warn about expressions in templates that were type-dependent, but aren't anymore because we're instantiating the template. As in, template Dest withMove() { T x; return std::move(x); } template Dest withMove(); // #1 template Dest withMove(); // #2 Saying that the std::move is pessimizing for #1 is not incorrect, but it's not useful, because removing the std::move would then pessimize #2. So the user can't really win. At the same time, disabling the warning just because we're in a template would be going too far, I still want to warn for template Dest withMove() { Dest x; return std::move(x); } because the std::move therein will be pessimizing for any instantiation. So I'm using the suppress_warning machinery to that effect. Problem: I had to add a new group to nowarn_spec_t, otherwise suppressing the -Wpessimizing-move warning would disable a whole bunch of other warnings, which we really don't want. PR c++/89780 gcc/cp/ChangeLog: * pt.cc (tsubst_copy_and_build) : Maybe suppress -Wpessimizing-move. * typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings if they are suppressed. (check_return_expr): Disable -Wpessimizing-move when returning a dependent expression. gcc/ChangeLog: * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle OPT_Wpessimizing_move and OPT_Wredundant_move. * diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning. * g++.dg/cpp0x/Wredundant-move2.C: Likewise. --- gcc/cp/pt.cc | 3 + gcc/cp/typeck.cc | 13 ++- gcc/diagnostic-spec.cc | 7 +- gcc/diagnostic-spec.h | 4 +- .../g++.dg/cpp0x/Wpessimizing-move3.C | 2 +- .../g++.dg/cpp0x/Wpessimizing-move9.C | 89 +++++++++++++++++++ gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C | 4 +- 7 files changed, 114 insertions(+), 8 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C base-commit: 4b0253b019943abf2cc5f4db0b7ed67caedffe4a prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5 prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4 diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index 6c581fe0fb7..fe7e809fc2d 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t, CALL_EXPR_ORDERED_ARGS (call) = ord; CALL_EXPR_REVERSE_ARGS (call) = rev; } + if (warning_suppressed_p (t, OPT_Wpessimizing_move)) + /* This also suppresses -Wredundant-move. */ + suppress_warning (ret, OPT_Wpessimizing_move); } RETURN (ret); diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index 2650beb780e..f4104c75fbe 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -10443,14 +10443,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) else if (can_do_nrvo_p (arg, type)) { auto_diagnostic_group d; - if (warning_at (loc, OPT_Wpessimizing_move, - "moving a local object in a return statement " - "prevents copy elision")) + 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 @@ -10695,6 +10697,11 @@ check_return_expr (tree retval, bool *no_warning) /* We don't know if this is an lvalue or rvalue use, but either way we can mark it as read. */ mark_exp_read (retval); + /* Disable our std::move warnings when we're returning + a dependent expression (c++/89780). */ + if (retval && TREE_CODE (retval) == CALL_EXPR) + /* This also suppresses -Wredundant-move. */ + suppress_warning (retval, OPT_Wpessimizing_move); return retval; } diff --git a/gcc/diagnostic-spec.cc b/gcc/diagnostic-spec.cc index 4341ccfaae9..aece89619e7 100644 --- a/gcc/diagnostic-spec.cc +++ b/gcc/diagnostic-spec.cc @@ -96,7 +96,7 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt) case OPT_Winit_self: case OPT_Wuninitialized: case OPT_Wmaybe_uninitialized: - m_bits = NW_UNINIT; + m_bits = NW_UNINIT; break; case OPT_Wdangling_pointer_: @@ -105,6 +105,11 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt) m_bits = NW_DANGLING; break; + case OPT_Wpessimizing_move: + case OPT_Wredundant_move: + m_bits = NW_REDUNDANT; + break; + default: /* A catchall group for everything else. */ m_bits = NW_OTHER; diff --git a/gcc/diagnostic-spec.h b/gcc/diagnostic-spec.h index 28e5e5ccc75..e5f1c127d4f 100644 --- a/gcc/diagnostic-spec.h +++ b/gcc/diagnostic-spec.h @@ -45,9 +45,11 @@ public: NW_DANGLING = 1 << 5, /* All other unclassified warnings. */ NW_OTHER = 1 << 6, + /* Warnings about redundant calls. */ + NW_REDUNDANT = 1 << 7, /* All groups of warnings. */ NW_ALL = (NW_ACCESS | NW_LEXICAL | NW_NONNULL - | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_OTHER) + | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_REDUNDANT | NW_OTHER) }; nowarn_spec_t (): m_bits () { } diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C index a1af1230b68..c81f29a4ba6 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C @@ -39,7 +39,7 @@ Tp1 fn2 () { Tp2 t; - return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" } + return std::move (t); } template diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C new file mode 100644 index 00000000000..898040e6bfc --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C @@ -0,0 +1,89 @@ +// PR c++/89780 +// { dg-do compile { target c++11 } } +// { dg-options "-Wpessimizing-move -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 Dest { + Dest() = default; + Dest(Dest&&); + Dest(const Dest&); +}; +struct Source : Dest {}; + +template +Dest withMove() { + T x; + return std::move(x); +} + +template Dest withMove(); +template Dest withMove(); + +template +Dest bar () { + return std::move(T()); // { dg-warning "moving a temporary object prevents copy elision" } +} + +template Dest bar(); +template Dest bar(); + +template +Dest baz (T x) { + return std::move(x); +} + +void +call_baz () +{ + Dest d; + Source s; + baz (d); + baz (s); +} + +template +Dest foo () +{ + Dest d; + return std::move(d); // { dg-warning "moving a local object in a return statement prevents copy elision" } +} + +template Dest foo(); + +template +Dest qux () { + return std::move(Dest()); // { dg-warning "moving a temporary object prevents copy elision" } +} + +template Dest qux(); + +template +Dest qui (Dest x) { + return std::move(x); // { dg-warning "redundant move in return statement" } +} + +void +call_qui () +{ + Dest d; + qui (d); +} diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C index f181afeeb84..6e0aa4b0277 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C @@ -37,14 +37,14 @@ template Tp1 fn2 (Tp2 t) { - return std::move (t); // { dg-warning "redundant move in return statement" } + return std::move (t); } template Tp1 fn3 (Tp2 t) { - return std::move (t); // { dg-warning "redundant move in return statement" } + return std::move (t); } int