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Re: gcc 4.4.1/linux 64bit: code crashes with -O3, works with -O2
On Mon, 2010-02-22 at 21:17 +0100, Christoph Rupp wrote:
> 2010/2/20 Richard Guenther <richard.guenther@gmail.com>:
> > On Sat, Feb 20, 2010 at 1:38 PM, Christoph Rupp <chris@crupp.de> 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
Vector instructions usually require aligned data. Perhaps the
vectorizer assumes that a pointer to 64-bit data will only be used
to access data that is 64-bit aligned.
If you can reproduce the problem with a small, self-contained test then
please file a bug report. It might be possible to issue a warning or
to detect that the loop should not be vectorized. If not, maybe the
compiler should disable vectorization for -fno-strict-aliasing.
Janis
> >
> > 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.gcc@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.)
> >>>
> >>
> >