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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id y12-20020a05620a25cc00b006bb78d095c5sm2441524qko.79.2022.08.18.17.33.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 18 Aug 2022 17:33:48 -0700 (PDT) Message-ID: Date: Thu, 18 Aug 2022 20:33:47 -0400 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 v2] c++: Implement -Wself-move warning [PR81159] To: Marek Polacek Cc: GCC Patches References: <20220809163719.381319-1-polacek@redhat.com> <1ca5b35f-0fa6-be28-290f-701c5ac43807@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=-26.0 required=5.0 tests=BAYES_00, DKIM_INVALID, DKIM_SIGNED, GIT_PATCH_0, KAM_DMARC_NONE, KAM_DMARC_STATUS, 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 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: Fri, 19 Aug 2022 00:33:56 -0000 On 8/18/22 13:19, Marek Polacek wrote: > On Mon, Aug 15, 2022 at 03:54:05PM -0400, Jason Merrill wrote: >> On 8/9/22 09:37, Marek Polacek wrote: >>> + /* We're looking for *std::move ((T &) &arg), or >>> + *std::move ((T &) (T *) r) if the argument it a reference. */ >>> + if (!REFERENCE_REF_P (rhs) >>> + || TREE_CODE (TREE_OPERAND (rhs, 0)) != CALL_EXPR) >>> + return; >>> + tree fn = TREE_OPERAND (rhs, 0); >>> + if (!is_std_move_p (fn)) >>> + return; >>> + tree arg = CALL_EXPR_ARG (fn, 0); >>> + if (TREE_CODE (arg) != NOP_EXPR) >>> + return; >>> + /* Strip the (T &). */ >>> + arg = TREE_OPERAND (arg, 0); >>> + /* Strip the (T *) or &. */ >>> + arg = TREE_OPERAND (arg, 0); >> >> Are you sure these are the only two expressions that can make it here? What >> if the argument to move is *Tptr? > > Not 100% sure but I couldn't find any other form. For *Tptr we get > *std::move ((int * &) &Tptr) That likes like what you'd get when the argument is Tptr, not when it's *Tptr. And indeed that's what I see in the testcase: > + Tptr = std::move (Tptr); // { dg-warning "moving a variable to itself" } is missing the * >>> @@ -6826,6 +6827,26 @@ of a declaration: >>> This warning is enabled by @option{-Wall}. >>> +@item -Wno-self-move @r{(C++ and Objective-C++ only)} >>> +@opindex Wself-move >>> +@opindex Wno-self-move >>> +This warning warns when a value is moved to itself with @code{std::move}. >>> +Such a @code{std::move} has no effect. >> >> ...unless it naively breaks the object, like >> >> T(T&& ot): data(ot.data) { ot.data = nullptr; } // oops > > "If you try to move me I'll disappear!" > > I've added the weasel word: "typically has no effect." Or do we want to say > more? > > -- >8 -- > About 5 years ago we got a request to implement -Wself-move, which > warns about useless moves like this: > > int x; > x = std::move (x); > > This patch implements that warning. > > PR c++/81159 > > gcc/c-family/ChangeLog: > > * c.opt (Wself-move): New option. > > gcc/cp/ChangeLog: > > * typeck.cc (maybe_warn_self_move): New. > (cp_build_modify_expr): Call maybe_warn_self_move. > > gcc/ChangeLog: > > * doc/invoke.texi: Document -Wself-move. > > gcc/testsuite/ChangeLog: > > * g++.dg/warn/Wself-move1.C: New test. > --- > gcc/c-family/c.opt | 4 + > gcc/cp/typeck.cc | 48 ++++++++++- > gcc/doc/invoke.texi | 23 +++++- > gcc/testsuite/g++.dg/warn/Wself-move1.C | 105 ++++++++++++++++++++++++ > 4 files changed, 178 insertions(+), 2 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/warn/Wself-move1.C > > diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt > index dfdebd596ef..f776efd39d8 100644 > --- a/gcc/c-family/c.opt > +++ b/gcc/c-family/c.opt > @@ -1229,6 +1229,10 @@ Wselector > ObjC ObjC++ Var(warn_selector) Warning > Warn if a selector has multiple methods. > > +Wself-move > +C++ ObjC++ Var(warn_self_move) Warning LangEnabledBy(C++ ObjC++, Wall) > +Warn when a value is moved to itself with std::move. > + > Wsequence-point > C ObjC C++ ObjC++ Var(warn_sequence_point) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall) > Warn about possible violations of sequence point rules. > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc > index 992ebfd99fb..cbc32a7c8ca 100644 > --- a/gcc/cp/typeck.cc > +++ b/gcc/cp/typeck.cc > @@ -8897,7 +8897,51 @@ cp_build_c_cast (location_t loc, tree type, tree expr, > > return error_mark_node; > } > - > + > +/* Warn when a value is moved to itself with std::move. LHS is the target, > + RHS may be the std::move call, and LOC is the location of the whole > + assignment. */ > + > +static void > +maybe_warn_self_move (location_t loc, tree lhs, tree rhs) > +{ > + if (!warn_self_move) > + return; > + > + /* C++98 doesn't know move. */ > + if (cxx_dialect < cxx11) > + return; > + > + if (processing_template_decl) > + return; > + > + /* We're looking for *std::move ((T &) &arg), or > + *std::move ((T &) (T *) r) if the argument it a reference. */ > + if (!REFERENCE_REF_P (rhs) > + || TREE_CODE (TREE_OPERAND (rhs, 0)) != CALL_EXPR) > + return; > + tree fn = TREE_OPERAND (rhs, 0); > + if (!is_std_move_p (fn)) > + return; > + tree arg = CALL_EXPR_ARG (fn, 0); > + if (TREE_CODE (arg) != NOP_EXPR) > + return; > + /* Strip the (T &). */ > + arg = TREE_OPERAND (arg, 0); > + /* Strip the (T *) or &. */ > + arg = TREE_OPERAND (arg, 0); > + arg = convert_from_reference (arg); > + /* So that we catch (i) = std::move (i);. */ > + lhs = maybe_undo_parenthesized_ref (lhs); > + STRIP_ANY_LOCATION_WRAPPER (lhs); > + if (cp_tree_equal (lhs, arg)) > + { > + auto_diagnostic_group d; > + if (warning_at (loc, OPT_Wself_move, "moving a variable to itself")) > + inform (loc, "remove % call"); > + } > +} > + > /* For use from the C common bits. */ > tree > build_modify_expr (location_t location, > @@ -9101,6 +9145,8 @@ cp_build_modify_expr (location_t loc, tree lhs, enum tree_code modifycode, > > if (modifycode == NOP_EXPR) > { > + maybe_warn_self_move (loc, lhs, rhs); > + > if (c_dialect_objc ()) > { > result = objc_maybe_build_modify_expr (lhs, rhs); > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi > index f65d351a5fc..5dea3fee124 100644 > --- a/gcc/doc/invoke.texi > +++ b/gcc/doc/invoke.texi > @@ -264,7 +264,7 @@ in the following sections. > -Wreorder -Wregister @gol > -Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates @gol > -Wno-non-template-friend -Wold-style-cast @gol > --Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo @gol > +-Woverloaded-virtual -Wno-pmf-conversions -Wself-move -Wsign-promo @gol > -Wsized-deallocation -Wsuggest-final-methods @gol > -Wsuggest-final-types -Wsuggest-override @gol > -Wno-terminate -Wuseless-cast -Wno-vexing-parse @gol > @@ -5843,6 +5843,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}. > -Wreorder @gol > -Wrestrict @gol > -Wreturn-type @gol > +-Wself-move @r{(only for C++)} @gol > -Wsequence-point @gol > -Wsign-compare @r{(only in C++)} @gol > -Wsizeof-array-div @gol > @@ -6828,6 +6829,26 @@ of a declaration: > > This warning is enabled by @option{-Wall}. > > +@item -Wno-self-move @r{(C++ and Objective-C++ only)} > +@opindex Wself-move > +@opindex Wno-self-move > +This warning warns when a value is moved to itself with @code{std::move}. > +Such a @code{std::move} typically has no effect. > + > +@smallexample > +struct T @{ > +@dots{} > +@}; > +void fn() > +@{ > + T t; > + @dots{} > + t = std::move (t); > +@} > +@end smallexample > + > +This warning is enabled by @option{-Wall}. > + > @item -Wsequence-point > @opindex Wsequence-point > @opindex Wno-sequence-point > diff --git a/gcc/testsuite/g++.dg/warn/Wself-move1.C b/gcc/testsuite/g++.dg/warn/Wself-move1.C > new file mode 100644 > index 00000000000..b12f0d43dad > --- /dev/null > +++ b/gcc/testsuite/g++.dg/warn/Wself-move1.C > @@ -0,0 +1,105 @@ > +// PR c++/81159 > +// { dg-do compile { target c++11 } } > +// { dg-options "-Wself-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); } > +} > + > +int g; > + > +struct S { > + int x; > + S(S&& o) { > + x = std::move (x); // { dg-warning "moving a variable to itself" } > + x = std::move (o.x); > + o.x = std::move (x); > + o.x = std::move (o.x); // { dg-warning "moving a variable to itself" } > + } > + void foo (int x) { > + x = std::move (x); // { dg-warning "moving a variable to itself" } > + } > +}; > + > +struct X { > + int x; > + X(int x) : x(std::move (x)) { } > +}; > + > +struct A {}; > +struct B { A a; }; > +struct C { C(); ~C(); }; > +struct D { D(); D(const D&); D(D&&); D& operator=(const D&); }; > + > +void > +test () > +{ > + int i = 42; > + i = std::move (i); // { dg-warning "moving a variable to itself" } > + (i) = std::move (i); // { dg-warning "moving a variable to itself" } > + > + g = std::move (g); // { dg-warning "moving a variable to itself" } > + (g) = std::move (g); // { dg-warning "moving a variable to itself" } > + > + A a; > + a = std::move (a); // { dg-warning "moving a variable to itself" } > + > + B b; > + b = std::move (b); // { dg-warning "moving a variable to itself" } > + b.a = std::move (b.a); // { dg-warning "moving a variable to itself" } > + > + C c; > + c = std::move (c); // { dg-warning "moving a variable to itself" } > + D d; > + d = std::move (d); // { dg-warning "moving a variable to itself" } > +} > + > +template > +void ttest () > +{ > + T t; > + t = std::move (t); // { dg-warning "moving a variable to itself" } > +} > + > +template void ttest(); > + > +void > +testref (int &r, int &&rr) > +{ > + r = std::move (r); // { dg-warning "moving a variable to itself" } > + rr = std::move (rr); // { dg-warning "moving a variable to itself" } > +} > + > +// Test various other arguments to std::move. > +template > +void > +testargs (T *Tptr, T& Tref, T&& Trref, const T *Tcptr) > +{ > + Tptr = std::move (Tptr); // { dg-warning "moving a variable to itself" } > + Tref = std::move (Tref); // { dg-warning "moving a variable to itself" } > + Trref = std::move (Trref); // { dg-warning "moving a variable to itself" } > + Tcptr = std::move (Tcptr); // { dg-warning "moving a variable to itself" } > +} > + > +void > +call_testargs () > +{ > + int i = 42; > + testargs(&i, i, 42, &i); > +} > > base-commit: ca170ed9f8a086ca7e1eec841882b6bed9ec1a3a