Re: [PATCH, rs6000] Fold vector absolutes in GIMPLE

[Will Schmidt <will_schmidt@vnet.ibm.com>]

On Tue, 2017-05-30 at 09:00 +0200, Richard Biener wrote:
> On Mon, May 29, 2017 at 2:21 PM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > On Mon, May 29, 2017 at 01:35:22PM +0200, Richard Biener wrote:
> >> >> What's the documented behavior for vec_abs with respect to an
> >> >argument
> >> >> of value INT_MIN?
> >> >
> >> >The documentation says:
> >> >
> >> >     "For integer vectors, the arithmetic is modular."
> >>
> >> This means that folding as ABS_EXPR is not safe for !TYPE_OVERFLOW_WRAPS
> >> Integral vector types.
> >
> > Is it still fine if TYPE_OVERFLOW_UNDEFINED?  So essentially always
> > except with -ftrapv?
> 
> The docs say it needs to wrap so the correct check is TYPE_OVERFLOW_WRAPS.
> It's not fine with TYPE_OVERFLOW_UNDEFINED as we will conclude the result
> can never be INT_MIN while the spec says it can.

Ok, thanks for the review.

So it looks like I should bail with something like: 
    ...
    case VSX_BUILTIN_XVABSDP:
      {
	arg0 = gimple_call_arg (stmt, 0);
	lhs = gimple_call_lhs (stmt);
	if (TYPE_OVERFLOW_WRAPS(TREE_TYPE(arg1))
	   return false;
	...

How can I test this scenario?  At a glance, a testcase snippet doesn't
appear to error out.  Am I quietly losing an overflow indicator?

vector signed int
test1_min (vector signed int x)
{
  vector signed int y = {INT_MIN,INT_MIN,INT_MIN,INT_MIN};
  return vec_abs (y);
}

generates gimple code:
  y = { -2147483648, -2147483648, -2147483648, -2147483648 };
  D.2579 = __builtin_altivec_abs_v4si (y);
or after folding:
  y = { -2147483648, -2147483648, -2147483648, -2147483648 };
  D.2579 = ABS_EXPR <y>;




> 
> Richard.
> 
> >
> >
> > Segher
>