From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.kundenserver.de (mout.kundenserver.de [212.227.126.135]) by sourceware.org (Postfix) with ESMTPS id F335A382D83E for ; Fri, 5 Mar 2021 15:25:05 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org F335A382D83E Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=towo.net Authentication-Results: sourceware.org; spf=none smtp.mailfrom=towo@towo.net Received: from [192.168.178.74] ([91.65.218.78]) by mrelayeu.kundenserver.de (mreue010 [212.227.15.167]) with ESMTPSA (Nemesis) id 1MIxFi-1l2AoM2NGU-00KU4U for ; Fri, 05 Mar 2021 16:25:04 +0100 Subject: Re: stack grow direction wrongly detected To: cygwin@cygwin.com References: <6eded5d3-93f3-7c98-5055-ee5ac2566bc8@gmail.com> <20210305233104.782838da83161a90f56a5369@nifty.ne.jp> From: Thomas Wolff Message-ID: <7f5794ef-8e98-0caf-b2f2-fb9dd670bfb8@towo.net> Date: Fri, 5 Mar 2021 16:25:05 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210305233104.782838da83161a90f56a5369@nifty.ne.jp> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Provags-ID: V03:K1:U2lYI2GItALcFDCbM7BTRj/vlwkqgOMMQMCtYyWbMUoakYTWzP9 nJ5AMsVqnw52eSWn+5nZvKjZ15eobagxiaRKWjSXrzTlc3ZE6HzPQnViVoTKcnqB6Dgw3j1 m7EHXpQ7TuMLXA8uuM7tb7HF6hHBMBrch5elJxs0USlrBJX2iZldj+fAl3k6eaEy3hIvxze UXVhwLFCLfnSpvoFxssqw== X-UI-Out-Filterresults: notjunk:1;V03:K0:YUyzNiaM5Ww=:+qYk0wYOWD0D/AUbW4FO4m YgkAcpckXn/LvW/fMdu454mQvAPDH9XCmlpWPaisAAypjasPtenVecw+L+LedN2ZgAPJ/sDAo UBmujZEL5oDX6zA+Ie5AmtU822G6Qb4u3LzCHrYCtuIJjWI4ipWFpNhrjAJTcQ7/7fFioG//T /brh5jTvR8DGS18/1ZZ3IUikypzIAmbipAVpHNz0pbC+oIjosGP2Q9SKsP9BVIPjE8TWh5PX8 ZOx/Tc1SaNcBynwR4/NpDuQq4g1DliBzFzpOgFR//gWeaqz23Y4nl4cu64lE72c6T5xFKaf/P pCBxq0OMYshgIWEXCqyUqVXJIbR3acp0WX8fI3BAjztbM1XzZs2x/7am7/JEx6P7k41mW9+Nt uZk7x5hmp726TU4ynJh+O9fC0Wo209FfzrLf0B/lfgtdD25WIg4fIhcshudvc X-Spam-Status: No, score=-1.0 required=5.0 tests=BAYES_00, KAM_ASCII_DIVIDERS, KAM_DMARC_STATUS, KAM_LAZY_DOMAIN_SECURITY, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: cygwin@cygwin.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: General Cygwin discussions and problem reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 05 Mar 2021 15:25:07 -0000 Am 05.03.2021 um 15:31 schrieb Takashi Yano via Cygwin: > On Fri, 5 Mar 2021 13:18:38 +0100 > Marco Atzeri wrote: >> Hi Guys, >> noted trying to rebuild guile 1.8.8. >> >> The following piece of code in the past >> was setting SCM_I_GSC_STACK_GROWS_UP=0 >> and now produces SCM_I_GSC_STACK_GROWS_UP=1 >> >> I assume some change in the gcc compiler is causing the issue. >> I presume most of the programs and libraries do not care, >> but some special one like guile crashes during build for this issue, >> so be aware. >> >> Regards >> Marco >> >> >> #-------------------------------------------------------------------- >> # >> # Which way does the stack grow? >> # >> # Following code comes from Autoconf 2.61's internal _AC_LIBOBJ_ALLOCA >> # macro (/usr/share/autoconf/autoconf/functions.m4). Gnulib has >> # very similar code, so in future we could look at using that. >> # >> # An important detail is that the code involves find_stack_direction >> # calling _itself_ - which means that find_stack_direction (or at >> # least the second find_stack_direction() call) cannot be inlined. >> # If the code could be inlined, that might cause the test to give >> # an incorrect answer. >> #-------------------------------------------------------------------- >> >> SCM_I_GSC_STACK_GROWS_UP=0 >> AC_RUN_IFELSE([AC_LANG_SOURCE( >> [AC_INCLUDES_DEFAULT >> int >> find_stack_direction () >> { >> static char *addr = 0; >> auto char dummy; >> if (addr == 0) >> { >> addr = &dummy; >> return find_stack_direction (); >> } >> else >> return (&dummy > addr) ? 1 : -1; >> } >> >> int >> main () >> { >> return find_stack_direction () < 0; >> }])], >> [SCM_I_GSC_STACK_GROWS_UP=1], >> [], >> [AC_MSG_WARN(Guessing that stack grows down -- see >> scmconfig.h)]) > This seems to be a result of optimization. With gcc v10.2.0, > the return value of the code is: > -O0: 1 > -O1: 1 > -O2: 0 > -O3: 1 > -O4: 1 > > If find_stack_direction() is implemented as recursive call, > and auto variable is allocated in the stack every time, > in the first call, addr is initialized to the first stack > position, and in the second call, second address of dummy > is reduced because stack of x86 is reverse direction. > Therefore (&dummy > addr) ? 1 : -1; returns -1. > As a result, the return value find_stack_direction() < 0 > is 1. With -O0 or -O1 this implemented as recursive call, > so the return value is 1. > > So, IIUC, the setting SCM_I_GSC_STACK_GROUS_UP is completly > oposite. > > With the following modified code, > > #include > int > find_stack_direction (int n) > { > static char *addr = 0; > char dummy; > printf("%p\n", &dummy); > if (addr == 0) > addr = &dummy; > if (n) > return find_stack_direction (n - 1); > else > return (&dummy > addr) ? 1 : -1; > } > > int > main () > { > int ret = find_stack_direction (10) < 0; > printf("%d\n", ret); > return ret; > } > > the result with -O0 is > 0x62cc2f > 0x62cbff > 0x62cbcf > 0x62cb9f > 0x62cb6f > 0x62cb3f > 0x62cb0f > 0x62cadf > 0x62caaf > 0x62ca7f > 0x62ca4f > 1 > > This looks very reasonable. However, with -O2 > 0x62cc3d > 0x62cc3e > 0x62cc3f > 0x62cc0d > 0x62cc0e > 0x62cc0f > 0x62cbdd > 0x62cbde > 0x62cbdf > 0x62cbad > 0x62cbae > 1 > > This is very strange. The address is not decreased uniformly. > > Therefore, using -O0 and setting SCM_I_GSC_STACK_GROUS_UP > reversely is the right thing, I think. > The function calls for tail recursion optimization, so it's not really suitable to make observations on recursion. However, with real tail recursion easily performed, the address of the local variable should actually not change at all, unlike here. The reason for that is beyond me.