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Mon, 23 Aug 2021 10:38:55 +0000 From: Darren Kenny To: Paul Eggert , bug-gnulib@gnu.org Subject: Re: [PATCH 3/3] lib/regexec: Resolve unused variable In-Reply-To: <4cec2520-759b-cb6d-172f-0120f42a4cfe@cs.ucla.edu> References: <3932753cd7929507688d341754fae944dac9a983.1624030621.git.darren.kenny@oracle.com> <4cec2520-759b-cb6d-172f-0120f42a4cfe@cs.ucla.edu> Date: Mon, 23 Aug 2021 11:38:50 +0100 Message-ID: Content-Type: text/plain X-ClientProxiedBy: DB3PR08CA0031.eurprd08.prod.outlook.com (2603:10a6:8::44) To BLAPR10MB5138.namprd10.prod.outlook.com (2603:10b6:208:322::8) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from oracle.com (46.7.162.180) by DB3PR08CA0031.eurprd08.prod.outlook.com (2603:10a6:8::44) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4436.19 via Frontend Transport; Mon, 23 Aug 2021 10:38:54 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 15348d2a-55e4-4d62-31f7-08d966223473 X-MS-TrafficTypeDiagnostic: MN2PR10MB4286: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:2512; 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envelope-from=darren.kenny@oracle.com; helo=mx0a-00069f02.pphosted.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_BL=0.01, RCVD_IN_MSPIKE_L3=0.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: bug-gnulib@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Gnulib discussion list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: bug-gnulib-bounces+normalperson=yhbt.net@gnu.org Sender: "bug-gnulib" Hi Paul, On Wednesday, 2021-08-11 at 00:24:25 -07, Paul Eggert wrote: > On 6/18/21 8:44 AM, Darren Kenny wrote: >> The reason for this issue is that we are not building with DEBUG set and >> this in turn means that the assert() that reads the value of the >> variable match_last is being processed out. > > Unfortunately I don't understand the scenario here. If not building with > DEBUG, 'DEBUG_ASSERT (match_last != 1)' should expand to 'assume > (match_last != 1)', which in turn should expand to something that > evaluates the expression 'match_last != 1'. Please see this commit, > which removed the "#ifdef" that you're proposing to re-add: > > https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=79f8ee4e389f8cb1339f8abed9a7d29816e2a2d4 The description of the problem by Coverity is: - An assigned value that is never used may represent unnecessary computation, an incorrect algorithm, or possibly the need for cleanup or refactoring. In re_search_internal: A value assigned to a variable is never used. (CWE-563) That it is referring to in this case is that this assignment is being made to set match_last = -1, but before ever testing it with-in the for-loop, it is overwiting that value. So, the only testing was occurring outside of the for-loop, and in what appeared to be an assertion only fired during DEBUG. What I did here was to not overwrite the value if DEBUG is set, since it appeared to never be checked anyway within the for-loop. I don't know about DEBUG_ASSERT() becoming an assume() - I can't even find a reference to assume() in the GCC manuals, only assert(), similarly for the gnulib manual's index at: - https://www.gnu.org/software/gnulib/manual/html_node/Index.html#Index_cp_letter-A I can see that LLVM has a __builtin_assume() though, but that's about the closest I could find. Searching through the gnulib sources did find verify.h and the assume macro defined in that - I can only assume (pun not intended) that this is ending up as a no-op for our build, which means that Coverity sees it as never being read after the for-loop. Thanks, Darren