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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id t12-20020a05620a004c00b0078d43da0be3sm2733181qkt.5.2024.04.12.12.05.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Apr 2024 12:05:28 -0700 (PDT) Message-ID: Date: Fri, 12 Apr 2024 15:05:26 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] c++: Fix constexpr evaluation of parameters passed by invisible reference [PR111284] To: Jakub Jelinek Cc: gcc-patches@gcc.gnu.org References: 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=-6.5 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,RCVD_IN_SORBS_WEB,SPF_HELO_NONE,SPF_NONE,TXREP 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 3/8/24 03:56, Jakub Jelinek wrote: > Hi! > > My r9-6136 changes to make a copy of constexpr function bodies before > genericization modifies it broke the constant evaluation of non-POD > arguments passed by value. > In the callers such arguments are passed as reference to usually a > TARGET_EXPR, but on the callee side until genericization they are just > direct uses of a PARM_DECL with some class type. > In cxx_bind_parameters_in_call I've used convert_from_reference to > pretend it is passed by value and then cxx_eval_constant_expression > is called there and evaluates that as an rvalue, followed by > adjust_temp_type if the types don't match exactly (e.g. const Foo > argument and passing to it reference to Foo TARGET_EXPR). > > The reason this doesn't work is that when the TARGET_EXPR in the caller > is constant initialized, this for it is the address of the TARGET_EXPR_SLOT, > but if the code later on pretends the PARM_DECL is just initialized to the > rvalue of the constant evaluation of the TARGET_EXPR, it is as if there > is a bitwise copy of the TARGET_EXPR to the callee, so this in the callee > is then address of the PARM_DECL in the callee. > > The following patch attempts to fix that by constexpr evaluation of such > arguments in the caller as an lvalue instead of rvalue, and on the callee > side when seeing such a PARM_DECL, if we want an lvalue, lookup the value > (lvalue) saved in ctx->globals (if any), and if wanting an rvalue, > recursing with vc_prvalue on the looked up value (because it is there > as an lvalue, nor rvalue). > > adjust_temp_type doesn't work for lvalues of non-scalarish types, for > such types it relies on changing the type of a CONSTRUCTOR, but on the > other side we know what we pass to the argument is addressable, so > the patch on type mismatch takes address of the argument value, casts > to reference to the desired type and dereferences it. > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? > > 2024-03-08 Jakub Jelinek > > PR c++/111284 > * constexpr.cc (cxx_bind_parameters_in_call): For PARM_DECLs with > TREE_ADDRESSABLE types use vc_glvalue rather than vc_prvalue for > cxx_eval_constant_expression and if it doesn't have the same > type as it should, cast the reference type to reference to type > before convert_from_reference and instead of adjust_temp_type > take address of the arg, cast to reference to type and then > convert_from_reference. > (cxx_eval_constant_expression) : For lval case > on parameters with TREE_ADDRESSABLE types lookup result in > ctx->globals if possible. Otherwise if lookup in ctx->globals > was successful for parameter with TREE_ADDRESSABLE type, > recurse with vc_prvalue on the returned value. > > * g++.dg/cpp1z/constexpr-111284.C: New test. > * g++.dg/cpp1y/constexpr-lifetime7.C: Expect one error on a different > line. > > --- gcc/cp/constexpr.cc.jj 2024-02-13 10:29:57.979155641 +0100 > +++ gcc/cp/constexpr.cc 2024-03-07 19:35:01.032412221 +0100 > @@ -1877,13 +1877,21 @@ cxx_bind_parameters_in_call (const const > x = build_address (x); > } > if (TREE_ADDRESSABLE (type)) > - /* Undo convert_for_arg_passing work here. */ > - x = convert_from_reference (x); > - /* Normally we would strip a TARGET_EXPR in an initialization context > - such as this, but here we do the elision differently: we keep the > - TARGET_EXPR, and use its CONSTRUCTOR as the value of the parm. */ > - arg = cxx_eval_constant_expression (ctx, x, vc_prvalue, > - non_constant_p, overflow_p); > + { > + /* Undo convert_for_arg_passing work here. */ > + if (TYPE_REF_P (TREE_TYPE (x)) > + && !same_type_p (type, TREE_TYPE (TREE_TYPE (x)))) > + x = cp_fold_convert (build_reference_type (type), x); > + x = convert_from_reference (x); > + arg = cxx_eval_constant_expression (ctx, x, vc_glvalue, > + non_constant_p, overflow_p); > + } > + else > + /* Normally we would strip a TARGET_EXPR in an initialization context > + such as this, but here we do the elision differently: we keep the > + TARGET_EXPR, and use its CONSTRUCTOR as the value of the parm. */ > + arg = cxx_eval_constant_expression (ctx, x, vc_prvalue, > + non_constant_p, overflow_p); It seems simpler to move the convert_for_reference after the cxx_eval_constant_expression rather than duplicate the call to cxx_eval_constant_expression. > /* Check we aren't dereferencing a null pointer when calling a non-static > member function, which is undefined behaviour. */ > if (i == 0 && DECL_OBJECT_MEMBER_FUNCTION_P (fun) > @@ -1909,7 +1917,16 @@ cxx_bind_parameters_in_call (const const > { > /* Make sure the binding has the same type as the parm. But > only for constant args. */ > - if (!TYPE_REF_P (type)) > + if (TREE_ADDRESSABLE (type)) > + { > + if (!same_type_p (type, TREE_TYPE (arg))) > + { > + arg = build_fold_addr_expr (arg); > + arg = cp_fold_convert (build_reference_type (type), arg); > + arg = convert_from_reference (arg); > + } > + } It shouldn't be necessary to convert both here and above? In fact, the testcase still passes with neither conversion, just moving the convert_for_reference and the change below. > + else if (!TYPE_REF_P (type)) > arg = adjust_temp_type (type, arg); > if (!TREE_CONSTANT (arg)) > *non_constant_args = true; > @@ -7499,9 +7516,19 @@ cxx_eval_constant_expression (const cons > > case PARM_DECL: > if (lval && !TYPE_REF_P (TREE_TYPE (t))) > - /* glvalue use. */; > + { > + /* glvalue use. */ > + if (TREE_ADDRESSABLE (TREE_TYPE (t))) > + if (tree v = ctx->global->get_value (t)) > + r = v; > + } > else if (tree v = ctx->global->get_value (t)) > - r = v; > + { > + r = v; > + if (TREE_ADDRESSABLE (TREE_TYPE (t))) > + r = cxx_eval_constant_expression (ctx, r, vc_prvalue, > + non_constant_p, overflow_p); > + } > else if (lval) > /* Defer in case this is only used for its type. */; > else if (ctx->global->is_outside_lifetime (t)) > --- gcc/testsuite/g++.dg/cpp1z/constexpr-111284.C.jj 2024-03-07 16:27:48.113651999 +0100 > +++ gcc/testsuite/g++.dg/cpp1z/constexpr-111284.C 2024-03-07 16:26:49.565466606 +0100 > @@ -0,0 +1,19 @@ > +// PR c++/111284 > +// { dg-do compile { target c++17 } } > + > +struct S { > + S () = default; > + constexpr S (const S &) noexcept : s{this} {} > + constexpr S & operator= (const S &) noexcept { return *this; } > + constexpr bool foo () const noexcept { return s == this; } > + S *s = this; > +}; > + > +constexpr bool > +bar (S x) noexcept > +{ > + return x.foo (); > +} > + > +static_assert (bar (S {}), ""); > +static_assert ([] (S x) { return x.foo (); } (S {}), ""); > --- gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime7.C.jj 2023-12-13 19:09:33.252657826 +0100 > +++ gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime7.C 2024-03-07 19:49:49.342334090 +0100 > @@ -87,7 +87,7 @@ constexpr bool n1 = test_access constexpr bool n2 = test_modification(); // { dg-message "in .constexpr." "" { target c++20 } } > constexpr bool n3 = test_scope(); // { dg-message "in .constexpr." "" { target c++20 } } > constexpr bool n4 = test_destroy_temp(); // { dg-message "in .constexpr." "" { target c++20 } } > -constexpr bool n5 = test_parameter(NonTrivial{}); // { dg-error "destroying" "" { target c++20 } } > +constexpr bool n5 = test_parameter(NonTrivial{}); // { dg-message "in .constexpr." "" { target c++20 } } > constexpr bool n6 = test_bindings(); > #endif > > > Jakub >