[Bug c/104983] New: gcc ignore side effect inside CommaExpr and failed to update value variable's value

[141242068 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104983

            Bug ID: 104983
           Summary: gcc ignore side effect inside CommaExpr and failed to
                    update value variable's value
           Product: gcc
           Version: 8.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 141242068 at smail dot nju.edu.cn
  Target Milestone: ---

The code:
```
int main() {
  int a = 0;
  unsigned long long one = 1;
  unsigned long long crf1 = 0;
  unsigned long long crf2 = 0;
  unsigned long long crf3 = 0;
  unsigned long long *cty1[] = {&crf1};
  unsigned long long *cty2[] = {&crf2};
  unsigned long long *cty3[] = {&crf3};
  ((-1ULL / (0xffffffffffffffffull + (*cty1[0]))) < a++,
      ((unsigned long long)((*cty2[0]) * (*cty3[0]))));
  if (a != 1) __builtin_abort();
  return 0;
}
```

Note that, a is updated only once in the above expression, and -1ull is
unsigned type, it should not trigger integer overflow.

But when compile the above code with -fopen-simd, the generated executable will
abort.

```
$ gcc -fopenmp-simd a.c -o a.out && ./a.out
[1]    2409988 abort (core dumped)  ./a.out
```

My gcc version is:
```
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc-8
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 8.4.0-3ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--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 --enable-bootstrap --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=auto --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 8.4.0 (Ubuntu 8.4.0-3ubuntu2)
```