[Bug fortran/64530] New: [4.9 Regression] Incorrect calculation when assigning to array with -O3

["jkaus at ucsd dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64530

            Bug ID: 64530
           Summary: [4.9 Regression] Incorrect calculation when assigning
                    to array with -O3
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jkaus at ucsd dot edu

Created attachment 34398
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34398&action=edit
Minimal example illustrating the issue.

With gfortran 4.9 using -O3, the assignment to an array
can occur before the rest of the calculation, giving
an incorrect result. I have attached a minimal example
which shows the bug along with modified code that 
produces the correct results.

With O3 we get the wrong result (0.0):
gfortran -O3 a.F90
./a.out
> buggy   0.0000000000000000        0.0000000000000000        0.0000000000000000     
> works   3.5000000000000000        3.5000000000000000        3.5000000000000000  

With O2 we get the correct result (3.5):
gfortran -O2 a.F90
./a.out 
> buggy   3.5000000000000000        3.5000000000000000        3.5000000000000000     
> works   3.5000000000000000        3.5000000000000000        3.500000000000000

gfortran -v
Using built-in specs.
COLLECT_GCC=gfortran
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
4.9.2-0ubuntu1~12.04' --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 --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 4.9.2 (Ubuntu 4.9.2-0ubuntu1~12.04)