[Bug target/84524] -O3 causes behavior change

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84524

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-02-23
                 CC|                            |jakub at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Adjusted testcase for the testsuite:

void
foo (unsigned short *x)
{
  unsigned short i, v;
  unsigned char j;
  for (i = 0; i < 256; i++)
    {
      v = i << 8;
      for (j = 0; j < 8; j++)
        if (v & 0x8000)
          v = (v << 1) ^ 0x1021;
        else
          v = v << 1;
      x[i] = v;
    }
}

int
main (void)
{
  unsigned short a[256];

  foo (a);
  for (int i = 0; i < 256; i++)
    {
      unsigned short v = i << 8;
      for (int j = 0; j < 8; j++)
        {
          asm volatile ("" : "+r" (v));
          if (v & 0x8000)
            v = (v << 1) ^ 0x1021;
          else
            v = v << 1;
        }
      if (a[i] != v)
        __builtin_abort ();
    }
  return 0;
}

I can confirm this is miscompiled even with current trunk at -O3 -mavx512bw.