gcc 4.4.1/linux 64bit: code crashes with -O3, works with -O2
Hi, i'd like to submit a bug report. I read the instructions on http://gcc.gnu.org/bugs, but i do not really know where the crash happens and what causes the crash to happen. I therefore just wrote the instructions below - they're simple. If you still need more specific stuff, please send me a private message and help me a little bit... I have a library which uses automake/autoconf; it does not have dependencies. If i build the non-debug version, files are compiled with -O3. Then the library causes a segfault. If i compile with -O2, everything works. Actually it's enough if i compile 2 files with -O2, all others with -O3, and everything's fine. If i run the program under valgrind, it runs without failure. gdb shows that a pointer is bogus. My system is a 64bit Ubuntu 9.04 linux, but i also saw it on other 64bit distributions (but it works fine on 32bit). uname -a says: Linux wintermute 2.6.31-17-generic #54-Ubuntu SMP Thu Dec 10 17:01:44 UTC 2009 x86_64 GNU/Linux My gcc is Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.1-4ubuntu8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu8) I did some tests with the following versions (again, only x86_64. x86 works fine): 4.1.3 20080704 (prerelease) (Debian 4.1.2-27) - works 4.3.4 (Debian 4.3.4-6) - works 4.4.1 on Gentoo - fails 4.4.3 20100108 (prerelease) (Ubuntu 4.4.2-9ubuntu3) - fails To reproduce, these steps are necessary: wget http://crupp.de/dl/hamsterdb-1.1.1.tar.gz tar -zxvf hamsterdb-1.1.1.tar.gz cd hamsterdb-1.1.1 ./configure --enable-internal make make test # this will segfault after a few tests Now change the CFLAGS to -O2 in src/Makefile, and rebuild the two files which depend on freelist.c cd src vi Makefile # -O3 -> -O2 touch freelist.c make # will build two files - freelist.c and freelist_v2.c cd ../unittests make ./test # this will run Best regards Christoph
Re: gcc 4.4.1/linux 64bit: code crashes with -O3, works with -O2
Hi Jonathan & list, this is a follow-up on an email from mid-january - i had crashes when compiling my library with -O3. I fixed all warnings "dereferencing type-punned pointer will break > strict-aliasing rules" and got it running. (at least i thought so - now i experience crashes again so maybe i just messed up my makefile and falsely thought that everything would work)... I fixed all warnings regarding dereferencing type-punned pointers and I compile with -O3 AND -fno-strict-aliasing. and i still get the same crash as earlier. it does not crash with -O2. >From what i understand the -fno-strict-aliasing should solve the problem, but it doesn't. The function which crashes basically is searching a bitmap for a pattern. When the search starts, it uses two pointers to the same memory location (u64 ptr for searching in 8byte-steps, char* ptr for searching byte-wise). If i understand the aliasing correctly then this might cause the problems. I just don't know how to rewrite the function - I want to use those two pointers for performance reasons. I don't even know if the problem is in this function or if it's somewhere else and the crash is just a side-effect of a completely different problem. To reproduce: wget http://crupp.de/hamsterdb-1.1.3.tar.gz tar -zxvf hamsterdb-1.1.3.tar.gz cd hamsterdb-1.1.3 ./configure make cd unittests ./test # <-- will segfault with a bad pointer Here's my gcc version: Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.1-4ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu9) Thanks for any help, Christoph 2010/1/14 Jonathan Wakely : > 2010/1/14 Christoph Rupp: >> >> To reproduce, these steps are necessary: >> >> wget http://crupp.de/dl/hamsterdb-1.1.1.tar.gz >> tar -zxvf hamsterdb-1.1.1.tar.gz >> cd hamsterdb-1.1.1 >> ./configure --enable-internal >> make > > There are lots of these warnings, which you ignore at your peril: > > freelist.c:3326: warning: dereferencing type-punned pointer will break > strict-aliasing rules > > ham_info.c:80: warning: dereferencing type-punned pointer will break > strict-aliasing rules > > env.cpp:1804: warning: dereferencing type-punned pointer will break > strict-aliasing rules > > You should probably either fix those warnings, avoid compiling at high > optimisation levels, or use -fno-strict-aliasing (which allows the > tests to run successfully.) >
Re: gcc 4.4.1/linux 64bit: code crashes with -O3, works with -O2
2010/2/20 Richard Guenther : > On Sat, Feb 20, 2010 at 1:38 PM, Christoph Rupp wrote: [...] >> I fixed all warnings regarding dereferencing type-punned pointers and >> I compile with -O3 AND -fno-strict-aliasing. >> >> and i still get the same crash as earlier. it does not crash with -O2. >> >> From what i understand the -fno-strict-aliasing should solve the >> problem, but it doesn't. >> >> The function which crashes basically is searching a bitmap for a >> pattern. When the search starts, it uses two pointers to the same >> memory location (u64 ptr for searching in 8byte-steps, char* ptr for >> searching byte-wise). If i understand the aliasing correctly then this >> might cause the problems. I just don't know how to rewrite the >> function - I want to use those two pointers for performance reasons. I >> don't even know if the problem is in this function or if it's >> somewhere else and the crash is just a side-effect of a completely >> different problem. > > With -fno-strict-aliasing this is perfectly valid so the problem must be > elsewhere. It might be alignment related if you do not make sure > that the u64 accesses are properly aligned. Try -O3 -fno-tree-vectorize > or analyze the crash. If i enable -fno-tree-vectorize for one file (that's the file which produces the crash) everything works. For me this workaround is fine, although this file is performance relevant and i'd like to have every optimization that's available. I don't really understand how alignment issues could cause this crash. i'm only building for x86_64 and i was not aware of any alignment requirements (i'd love to test on a SPARC, but sadly i don't have access to one...) BTW - if i execute this in valgrind then it doesn't crash or give warnings. For me this looks cheesy - there are no gcc warnings, -O2 works, -O3 crashes. MSVC works and older gcc versions also work fine. In this case it's hard for me not to blame gcc for the crash. Anyway - thanks for your help! Christoph > > Richard. > >> To reproduce: >> wget http://crupp.de/hamsterdb-1.1.3.tar.gz >> tar -zxvf hamsterdb-1.1.3.tar.gz >> cd hamsterdb-1.1.3 >> ./configure >> make >> cd unittests >> ./test # <-- will segfault with a bad pointer >> >> Here's my gcc version: >> Using built-in specs. >> Target: x86_64-linux-gnu >> Configured with: ../src/configure -v --with-pkgversion='Ubuntu >> 4.4.1-4ubuntu9' >> --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs >> --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr >> --enable-shared --enable-multiarch --enable-linker-build-id >> --with-system-zlib --libexecdir=/usr/lib --without-included-gettext >> --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 >> --program-suffix=-4.4 --enable-nls --enable-clocale=gnu >> --enable-libstdcxx-debug --enable-objc-gc --disable-werror >> --with-arch-32=i486 --with-tune=generic --enable-checking=release >> --build=x86_64-linux-gnu --host=x86_64-linux-gnu >> --target=x86_64-linux-gnu >> Thread model: posix >> gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu9) >> >> Thanks for any help, >> Christoph >> >> >> 2010/1/14 Jonathan Wakely : >>> 2010/1/14 Christoph Rupp: >>>> >>>> To reproduce, these steps are necessary: >>>> >>>> wget http://crupp.de/dl/hamsterdb-1.1.1.tar.gz >>>> tar -zxvf hamsterdb-1.1.1.tar.gz >>>> cd hamsterdb-1.1.1 >>>> ./configure --enable-internal >>>> make >>> >>> There are lots of these warnings, which you ignore at your peril: >>> >>> freelist.c:3326: warning: dereferencing type-punned pointer will break >>> strict-aliasing rules >>> >>> ham_info.c:80: warning: dereferencing type-punned pointer will break >>> strict-aliasing rules >>> >>> env.cpp:1804: warning: dereferencing type-punned pointer will break >>> strict-aliasing rules >>> >>> You should probably either fix those warnings, avoid compiling at high >>> optimisation levels, or use -fno-strict-aliasing (which allows the >>> tests to run successfully.) >>> >> >
