Re: [PATCH 2/2] Enable elimination of zext/sext

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jun 25, 2014 at 06:14:57PM +1000, Kugan wrote:
> For these flags, value ranges generated are not usable for extension
> eliminations. Therefore, without this some of the test cases in
> regression fails. For example:
> 
> short a;
> void
> foo (void)
> {
>   for (a = 0; a >= 0; a++)
>     ;
> }
> -Os  -fno-strict-overflow produces the following range for the index
> increment and hence goes into infinite loop.
> _10: [1, 32768]
> _10 = _4 + 1;

For -fwrapv I don't see why you'd get into trouble ever, the VRP computation
should be well aware of the -fwrapv semantics and the value ranges should
reflect that.

For -fno-strict-overflow, I have no idea since it is very weirdly defined.

In any case, for your example above, the loop is always well defined,
because for char/short a++ is performed as:
a = (short) ((int) a + 1)
So, if the patch turns it into infinite loop, with -Os -fno-strict-overflow
or -Os, it is simply a problem with the patch.  VR [1, 32768] looks correct,
a++ is performed only if a is >= 0, therefore before addition [0, 32767].
But from VR [1, 32768] you can't optimize away the sign extension, make sure
you don't have there off-by-one?

It would be nice if the patch contained some testcases, it is easy
to construct testcases where you have arbitrary VRs on some SSA_NAMEs,
you just need something to stick the VR on, so you can do something like:
type foo (type a)
{
  if (a < VR_min + 1 || a > VR_max + 1) return; // If VR_min is type minimum or VR_max type maximum this needs to be adjusted of course.
  a = a + 1;
  // now you can try some cast that your optimization would try to optimize
  return a;
}
Or void bar (type a) { a = (a & mask) + bias; (or similarly) }
Make sure to cover the boundary cases, where VR minimum or maximum still
allow optimizing away zero and/or sign extensions, and another case where
they are +- 1 and already don't allow it.

	Jakub