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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1705005281; x=1705610081; h=cc:to:subject:message-id:date:from:mime-version:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=UlQTp+9Zvn02A6tNy8qitVFkDyJv1RKWm4ZeJcq3+7k=; b=UpSvjSqZ4ceeLiNpEjK4KPcOLieFC8y2Z4zsUoy8uURlOt3/21Yo3MyAt8GMImBVdU T1QcYjGIc481zk2rJHa3z6CFpTKJSqgb3GthX9lkXY4W2F9RCVI/HCUzBOQfQNxeRQkf 9wJUmJhbv8zImLNHFKP7WCX+DdoMURDQO7DRbR0i2xtc+d1kL0pzsc4gwHoZirhxslKh Rzw6GNtanJ5e7vbr5KSrZiZbCEW+GmUMSq5UN5hwUlRLjiO521e4Rj/jM7IGingj3lEb GDJilRZY4PZ4U/tZrjgDjMfGUspzMftVCCnieKvmhASv3Gsq6QTnPSBE4H86lK+HS8Jt YW5Q== X-Gm-Message-State: AOJu0YxNiIU/z9VHkNzrYESgF6BanSvjm3ja+0zaIEkHxp9J3DyeiELf mJYim4evvagCaNIPzfn0jOQAWWpCeT8Dk6m2bUmbsunlSrOoHA== X-Google-Smtp-Source: AGHT+IF8qV2sr8vwNdxXyrTz/TCTcP8dzuZnsr0zmfnG6q0t9lfvXc9Rl8uV4JhVLyWc7i4Poc6EP6hT4qJyj9OfaDs= X-Received: by 2002:a05:6512:38c7:b0:50d:1c5e:7982 with SMTP id p7-20020a05651238c700b0050d1c5e7982mr66825lft.182.1705005280911; Thu, 11 Jan 2024 12:34:40 -0800 (PST) MIME-Version: 1.0 From: Dmitry Drozodv Date: Thu, 11 Jan 2024 23:34:33 +0300 Message-ID: Subject: [PATCH v2] c++: side effect in nullptr_t conversion fix To: jakub@redhat.com Cc: gcc-patches@gcc.gnu.org X-Spam-Status: No, score=-0.4 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_ENVFROM_END_DIGIT, FREEMAIL_FROM, GIT_PATCH_0, HTML_MESSAGE, RCVD_IN_DNSWL_NONE, SPAM_FROM, SPF_HELO_NONE, SPF_PASS, 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: 2.1.30 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: gcc-patches-bounces+incoming=patchwork.ozlabs.org@gcc.gnu.org Hello, You are absolutely right, we can't throw all side-effects away. Example: ```c++ auto __attribute__ ((noinline)) bar() { volatile int* b = (int *)0xff; *b = 10; volatile auto n = nullptr; return n; // Problem (1) } int* foo_2() { volatile auto a = bar(); // Problem (2) return a; } ``` There are 2 additional places, where std::nullptr_t causes side effects. (1) nullptr_t identical conversion produces: mov QWORD PTR [rsp-8], 0 mov rax, QWORD PTR [rsp-8] // load n ret (2) Initialization by not constant expression: call bar() mov QWORD PTR [rsp+8], rax mov rax, QWORD PTR [rsp+8] xor eax, eax diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc index cbed847b343..24c1ceb4404 100644 --- a/gcc/cp/cvt.cc +++ b/gcc/cp/cvt.cc @@ -218,8 +218,11 @@ cp_convert_to_pointer (tree type, tree expr, bool dofold, ? build_int_cst_type (type, -1) : build_int_cst (type, 0)); - return (TREE_SIDE_EFFECTS (expr) - ? build2 (COMPOUND_EXPR, type, expr, val) : val); + /* C++ [conv.lval]p3: + If T is cv std::nullptr_t, the result is a null pointer constant. */ + return ((TREE_SIDE_EFFECTS (expr) && !CONVERT_EXPR_P (expr)) + ? build2 (COMPOUND_EXPR, type, expr, val) + : val); } else if (TYPE_PTRMEM_P (type) && INTEGRAL_CODE_P (form)) { @@ -743,8 +746,9 @@ ocp_convert (tree type, tree expr, int convtype, int flags, { if (complain & tf_warning) maybe_warn_zero_as_null_pointer_constant (e, loc); - - if (!TREE_SIDE_EFFECTS (e)) + /* C++ [conv.lval]p3: + Convert expr from nullptr to nullptr doesn't have side effect. */ + if (!TREE_SIDE_EFFECTS (e) || CONVERT_EXPR_P (expr)) return nullptr_node; } diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc index ac0fefa24f2..9c7866d319f 100644 --- a/gcc/cp/typeck2.cc +++ b/gcc/cp/typeck2.cc @@ -770,6 +770,15 @@ split_nonconstant_init (tree dest, tree init) && array_of_runtime_bound_p (TREE_TYPE (dest))) code = build_vec_init (dest, NULL_TREE, init, /*value-init*/false, /*from array*/1, tf_warning_or_error); + else if (TREE_CODE (TREE_TYPE(init)) == NULLPTR_TYPE) + { + /* C++ [conv.lval]p3: + If T is cv std::nullptr_t, the result is a null pointer constant. */ + tree val = build_int_cst(TREE_TYPE(dest), 0); + tree ie = cp_build_init_expr(dest, val); + // remain expr with side effect + code = add_stmt_to_compound(init, ie); + } else code = cp_build_init_expr (dest, init);