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[125.209.144.50]) by smtp.gmail.com with ESMTPSA id r1-20020a62e401000000b005a8ba70315bsm21811004pfh.6.2023.03.28.19.33.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Mar 2023 19:33:19 -0700 (PDT) From: Nathaniel Shead To: gcc-patches@gcc.gnu.org Cc: Nathaniel Shead Subject: [PATCH v2 2/3] c++: Improve constexpr error for dangling local variables Date: Wed, 29 Mar 2023 13:32:57 +1100 Message-Id: <20230329023258.13487-3-nathanieloshead@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230329023258.13487-1-nathanieloshead@gmail.com> References: <20230329023258.13487-1-nathanieloshead@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-12.3 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,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: Currently, when typeck discovers that a return statement will refer to a local variable it rewrites to return a null pointer. This causes the error messages for using the return value in a constant expression to be unhelpful, especially for reference return values. This patch removes this "optimisation". Relying on this raises a warning by default and causes UB anyway, so there should be no issue in doing so. We also suppress additional warnings from later passes that detect this as a dangling pointer, since we've already indicated this anyway. gcc/cp/ChangeLog: * semantics.cc (finish_return_stmt): Suppress dangling pointer reporting on return statement if already reported. * typeck.cc (check_return_expr): Don't set return expression to zero for dangling addresses. gcc/testsuite/ChangeLog: * g++.dg/cpp1y/constexpr-lifetime5.C: Test reported message is correct. * g++.dg/warn/Wreturn-local-addr-6.C: Remove check for return value optimisation. Signed-off-by: Nathaniel Shead --- gcc/cp/semantics.cc | 5 ++++- gcc/cp/typeck.cc | 5 +++-- gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C | 4 ++-- gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C | 3 --- 4 files changed, 9 insertions(+), 8 deletions(-) diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc index 87c2e8a7111..14b4b7f4ce1 100644 --- a/gcc/cp/semantics.cc +++ b/gcc/cp/semantics.cc @@ -1246,7 +1246,10 @@ finish_return_stmt (tree expr) r = build_stmt (input_location, RETURN_EXPR, expr); if (no_warning) - suppress_warning (r, OPT_Wreturn_type); + { + suppress_warning (r, OPT_Wreturn_type); + suppress_warning (r, OPT_Wdangling_pointer_); + } r = maybe_cleanup_point_expr_void (r); r = add_stmt (r); diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index afb956087ce..a7d642e2029 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -11235,8 +11235,9 @@ check_return_expr (tree retval, bool *no_warning) else if (!processing_template_decl && maybe_warn_about_returning_address_of_local (retval, loc) && INDIRECT_TYPE_P (valtype)) - retval = build2 (COMPOUND_EXPR, TREE_TYPE (retval), retval, - build_zero_cst (TREE_TYPE (retval))); + /* Suppress the Wdangling-pointer warning in the return statement + that would otherwise occur. */ + *no_warning = true; } if (processing_template_decl) diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C index a4bc71d890a..ad3ef579f63 100644 --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C @@ -1,11 +1,11 @@ // { dg-do compile { target c++14 } } // { dg-options "-Wno-return-local-addr" } -constexpr const int& id(int x) { return x; } +constexpr const int& id(int x) { return x; } // { dg-message "note: declared here" } constexpr bool test() { const int& y = id(3); return y == 3; } -constexpr bool x = test(); // { dg-error "" } +constexpr bool x = test(); // { dg-error "accessing object outside its lifetime" } diff --git a/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C b/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C index fae8b7e766f..ec8e241d83e 100644 --- a/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C +++ b/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C @@ -24,6 +24,3 @@ return_addr_local_as_intref (void) return (const intptr_t&)a; // { dg-warning "\\\[-Wreturn-local-addr]" } */ } - -/* Verify that the return value has been replaced with zero: - { dg-final { scan-tree-dump-times "return 0;" 2 "optimized" } } */ -- 2.34.1