[Bug target/70450] New: [6 Regression] Wrong code with -O0 and -O1.

[vsevolod.livinskij at frtk dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70450

            Bug ID: 70450
           Summary: [6 Regression] Wrong code with -O0 and -O1.
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vsevolod.livinskij at frtk dot ru
  Target Milestone: ---

Created attachment 38127
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38127&action=edit
Reproducer.

Test case produces incorrect result with -O0 and -O1 on all x86 arch.
Everything works fine with gcc version 4.9.4 20160325 (prerelease)
(Revision=234483) and gcc version 5.3.1 20160325 (Revision=234483).

Output:
> g++ repr.cpp -o out -O2 -mavx2; ./out
18446744073709551584
> g++ repr.cpp -o out -O0 -mavx2; ./out
68719476704

GCC version:
> g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/export/users/vlivinsk/gcc-trunk/bin/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /export/users/vlivinsk/gcc-trunk/gcc/configure
--with-arch=corei7 --with-cpu=corei7 --enable-clocale=gnu --with-system-zlib
--enable-shared --with-demangler-in-ld --enable-cloog-backend=isl
--with-fpmath=sse --enable-checking=release --enable-languages=c,c++,lto
--with-gmp=/export/users/vlivinsk/gcc-trunk/gmp-6.1.0/bin
--with-mpfr=/export/users/vlivinsk/gcc-trunk/mpfr-3.1.3/bin
--with-mpc=/export/users/vlivinsk/gcc-trunk/mpc-1.0.3/bin
--prefix=/export/users/vlivinsk/gcc-trunk/bin
Thread model: posix
gcc version 6.0.0 20160330 (experimental) (Revision=234545)

Testcase:
#include <cstdlib>

unsigned long int a = 2UL;
int b = 2;
unsigned long int c = 2UL;

void foo () {
    c = 2 * ((2 * a) * (2 * (-b)));
}

int main () {
    foo();
    if (c != 18446744073709551584UL)
        abort();
    return 0;
}