[Bug tree-optimization/46957] New: http://blog.regehr.org/archives/320 Example 1

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46957

           Summary: http://blog.regehr.org/archives/320 Example 1
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: ja...@gcc.gnu.org
                CC: s...@gcc.gnu.org


void foo (void);
long a, b;
void
foo (void)
{
  int i;

  b = -1L;
  i = 0;
  while (i < 63)
    {
      b *= 2L;
      i++;
    }
  a = -(b + 1L);
}

doesn't have the loop optimized out, as we don't have {-1L, *, 2}_1 style
chrecs, only , +, style.  I wonder if we couldn't for << or multiplication by
power of two express {-1L, *, 2}_1 instead as -1L << ({0, +, 1}_1), or
for other multiplication, say {a, *, b}_1, as a * pow (b, {0, +, 1}_1) and use
that in compute_overall_effect_of_inner_loop etc. to sccp it.

Of course only if the resulting expression is cheap enough.