[Bug c++/65311] New: Segmentation fault when doing unaligned assignment in a loop

[emil.styrke at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65311

            Bug ID: 65311
           Summary: Segmentation fault when doing unaligned assignment in
                    a loop
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: emil.styrke at gmail dot com

Created attachment 34948
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34948&action=edit
Demonstration of bug (no includes)

The attached code segfaults when compiled with -O3.  I realize the code is
dodgy (and breaks strict aliasing rules according to my understanding), but
even with -fno-strict-aliasing it breaks.  It works fine on -O2.

$ g++-4.9 -o test test.cpp -Wall -Wextra -O3 -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations
$ ./test
Segmenteringsfel (minnesutskrift skapad)
$ g++-4.9 -v
Using built-in specs.
COLLECT_GCC=g++-4.9
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.9.1-16ubuntu6'
--with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.9 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--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 4.9.1 (Ubuntu 4.9.1-16ubuntu6)