ubsan: load/store of misaligned address & strange behavior with different -O levels (g++)

[Nekrolm]

I have faced to strange behavior (suppose UB) while trying to use compound template-generated type as unordered_map KeyType: https://gist.github.com/Nekrolm/47be0ea8644247ee7390f59a4ac3a778

In Debug build with ubsan instrumentation I got warning about misaligned access (robin_hood.h line 1991).
With -O2 it works fine. But with -O3 it crushes with segfault.

g++ -v

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.4.0-1ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1) 

[phprus]

Wrong optimization:
https://github.com/martinus/robin-hood-hashing/blob/master/src/include/robin_hood.h#L1983

[martinus]

Hi, thanks for reporting this! I see what I can do

[martinus]

I believe I have fixed this bug in the version 3.4.4. Can you give it a try? https://github.com/martinus/robin-hood-hashing/blob/master/src/include/robin_hood.h

[Nekrolm]

Now it works as expected!

[martinus]

Fixed