[Bug tree-optimization/92233] New: missed optimisation for multiplication when it's known that at least one of the arguments is 0

[SztfG at yandex dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92233

            Bug ID: 92233
           Summary: missed optimisation for multiplication when it's known
                    that at least one of the arguments is 0
           Product: gcc
           Version: 9.2.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: SztfG at yandex dot ru
  Target Milestone: ---

testcase:

unsigned test_mult(unsigned a, unsigned b)
{
  if ((a == 0) || (b == 0))
  {
    return a*b; // here a*0 or 0*b or 0*0 - always 0
  }
  return 0;
}

So this function should always return 0 no matter what, but GCC generate
comparisons and imul instruction, even with -O3