Re: Target dependence of conditional expression gimplification

[Florian Weimer <fweimer at redhat dot com>]

On 03/03/2014 01:03 PM, Florian Weimer wrote:
> I have code like this:
>
>       if (auth_info->attrs.pin.flags & SC_PKCS15_PIN_FLAG_SO_PIN) {
>          preferred = 1;
>      } else {
>          preferred = current;
>          /* PINs are even numbered, PUKs are odd */
>          if (!(preferred & 1))
>              preferred++;
>          if (preferred >= 126)
>              return SC_ERROR_TOO_MANY_OBJECTS;
>      }
>
>      if (current > preferred || preferred > CARDOS_PIN_ID_MAX)
>          return SC_ERROR_TOO_MANY_OBJECTS;
>
> On x86_64, the condition in the final if statement is gimplified as:
>
> -  D.8003 = current > preferred;
> -  D.8004 = preferred > 15;
> -  D.8005 = D.8003 | D.8004;
> -  if (D.8005 != 0) goto <D.8006>; else goto <D.8007>;
>
> On rs6000 (-mtune=power7 -mcpu=power7), I get this instead:
>
> +  if (current > preferred) goto <D.8376>; else goto <D.8378>;
> +  <D.8378>:
> +  if (preferred > 15) goto <D.8376>; else goto <D.8377>;

Here's a self-contained reproducer:

int g(void);

int
f(int flags, int current)
{
  int preferred;
  if (flags & 0x80) {
    preferred = 1;
  } else {
    preferred = current;
    if (!(preferred & 1))
      preferred++;
    if (preferred >= 126)
      return 55;
  }

  if (current > preferred || preferred > 17)
    return 55;
  return g();
}

> Obviously, this affects optimization-dependent warnings later in the
> compilation.
>
> I wonder why this happens.  Shouldn't gimplification be roughly
> target-independent?

It turns out that it's not gimplification, but fold which is the 
culprit.  In fold-const.c:fold_truth_andor, we have this code:

  if (LOGICAL_OP_NON_SHORT_CIRCUIT
      && (code == TRUTH_AND_EXPR
          || code == TRUTH_ANDIF_EXPR
          || code == TRUTH_OR_EXPR
          || code == TRUTH_ORIF_EXPR))
    {
…
      /* Transform (A AND-IF B) into (A AND B), or (A OR-IF B)
	 into (A OR B).
	 For sequence point consistancy, we need to check for trapping,
	 and side-effects.  */
      else if (code == icode && simple_operand_p_2 (arg0)
               && simple_operand_p_2 (arg1))
	return fold_build2_loc (loc, ncode, type, arg0, arg1);

rs6000 defines LOGICAL_OP_NON_SHORT_CIRCUIT as 0, so this code never 
runs, and the gimplifier produces two separate if statements.

--
Florian Weimer / Red Hat Product Security Team