[Bug tree-optimization/82703] New: Wrong addition of std::array components with -O2 -ftree-loop-vectorize -ftree-slp-vectorize (works fine with -O2)

[frederic.bron at m4x dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82703

            Bug ID: 82703
           Summary: Wrong addition of std::array components with -O2
                    -ftree-loop-vectorize -ftree-slp-vectorize (works fine
                    with -O2)
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frederic.bron at m4x dot org
  Target Milestone: ---

Created attachment 42463
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42463&action=edit
Minimal example to reproduce the bug

The small program attached to this message should always pass, i.e. not print
any error. This is the case with -O2. This is also the case with clang++ 4.0.1
with -O3.

However with g++ and "-O2 -ftree-loop-vectorize -ftree-slp-vectorize", v3[1] is
2.0 instead of 4.0.

Note that if I change double to int, long, long long, float or long double, I
have no error.
Also if I do not construct Array from int[3] but from double[3], it works and
if I remove a useless line, it works...

Tested on linux Fedora 26 x86_64 with g++ 7.2.0 and 7.2.1 on westmere,
broadwell, skylake.

To reproduce failure:
g++ -O2 -g -std=c++14 bug.cpp -ftree-loop-vectorize -ftree-slp-vectorize &&
./a.out

g++ 7.2.0 that shows the bug:
Using built-in specs.
COLLECT_GCC=g++-7.2.0
COLLECT_LTO_WRAPPER=/softs/gcc-7.2.0/libexec/gcc/x86_64-pc-linux-gnu/7.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /softs/build/gcc-7.2.0/configure --prefix=/softs/gcc-7.2.0
--program-suffix=-7.2.0 --with-gmp-include=/softs/gmp-6.1.2/include
--with-gmp-lib=/softs/gmp-6.1.2/lib
--with-mpfr-include=/softs/mpfr-3.1.5/include
--with-mpfr-lib=/softs/mpfr-3.1.5/lib
--with-mpc-include=/softs/mpc-1.0.3/include --with-mpc-lib=/softs/mpc-1.0.3/lib
--with-isl-include=/softs/isl-0.16.1/include
--with-isl-lib=/softs/isl-0.16.1/lib --disable-isl-version-check --enable-lto
--enable-nls --disable-multilib
Thread model: posix
gcc version 7.2.0 (GCC) 


g++ 7.2.1 that shows the bug:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-libmpx
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 7.2.1 20170915 (Red Hat 7.2.1-2) (GCC)