On Sat, Feb 20, 2010 at 1:38 PM, Christoph Rupp <ch...@crupp.de> wrote: > 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.
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. 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 <jwakely....@gmail.com>: >> 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.) >> >
