Re: [RFA] [tree-optimization/80576] Handle non-constant sizes in DSE

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Sep 9, 2019 at 10:10 PM Jeff Law <law@redhat.com> wrote:
>
> On 8/26/19 3:00 AM, Richard Biener wrote:
> > On Fri, Aug 23, 2019 at 9:19 PM Jeff Law <law@redhat.com> wrote:
> >>
> >> On 8/22/19 4:46 AM, Richard Biener wrote:
> >>>>> Also you seem to use this info to constrain optimization when you
> >>>>> might remember that types of addresses do not carry such information...
> >>>>> Thus it should be "trivially" possible to write a testcase that is miscompiled
> >>>>> after your patch.  I also don't see this really exercised in the
> >>>>> testcases you add?
> >>>> Arggh.  You're absolutely correct.  I must be blocking out that entire
> >>>> discussion from last summer due to the trama :-)
> >>>>
> >>>> If the destination is the address of a _DECL node, can we use the size
> >>>> of the _DECL?
> >>>
> >>> Yes, but this should already happen for both invariant ones like &a.b.c
> >>> and variant ones like &a.b[i].c in ao_ref_init_from_ptr_and_size.
> >> I don't see that in ao_ref_init_from_ptr_and_size.  AFAICT if you don't
> >> know the size when you call that routine (size == NULL), then you end up
> >> with the ref->size and ref->max_size set to -1.
> >>
> >> Am I missing something here?
> >
> > Ah, of course.  ao_ref_from_ptr_and_size would need to be extended
> > to constrain max_size.  So what I was
> > saying is that ao_ref_init_from_ptr_and_size should get you
> > a DECL ao_ref_base () from which you could constrain max_size with.
> > Or rather ao_ref_from_ptr_and_size should be extended do that,
> > mimicing what get_ref_base_and_extent does at the end in the
> > if (DECL_P (exp)) case (mind flag_unconstrained_commons!).
> So I was going to use get_ref_base_and_extent from within
> ao_ref_init_from_ptr_and_size to capture these cases, but
> get_ref_base_and_extent internally uses TYPE_SIZE to get the maximum
> size of the referenced object.
>
> That likely represents a codegen bug waiting to happen.

Yeah, you can't use get_ref_base_and_extent literally here.

> I'll see if I can refactor just the bits we want so that we're not
> duplicating anything.

Not sure if that's too important.  But yes, splitting out

  if (DECL_P (exp))
    {
      if (VAR_P (exp)
          && ((flag_unconstrained_commons && DECL_COMMON (exp))
              || (DECL_EXTERNAL (exp) && seen_variable_array_ref)))
        {
          tree sz_tree = TYPE_SIZE (TREE_TYPE (exp));
          /* If size is unknown, or we have read to the end, assume there
             may be more to the structure than we are told.  */
          if (TREE_CODE (TREE_TYPE (exp)) == ARRAY_TYPE
              || (seen_variable_array_ref
                  && (sz_tree == NULL_TREE
                      || !poly_int_tree_p (sz_tree)
                      || maybe_eq (bit_offset + maxsize,
                                   wi::to_poly_offset (sz_tree)))))
            maxsize = -1;
        }
      /* If maxsize is unknown adjust it according to the size of the
         base decl.  */
      else if (!known_size_p (maxsize)
               && DECL_SIZE (exp)
               && poly_int_tree_p (DECL_SIZE (exp)))
        maxsize = wi::to_poly_offset (DECL_SIZE (exp)) - bit_offset;
    }
  else if (CONSTANT_CLASS_P (exp))
    {
      /* If maxsize is unknown adjust it according to the size of the
         base type constant.  */
      if (!known_size_p (maxsize)
          && TYPE_SIZE (TREE_TYPE (exp))
          && poly_int_tree_p (TYPE_SIZE (TREE_TYPE (exp))))
        maxsize = (wi::to_poly_offset (TYPE_SIZE (TREE_TYPE (exp)))
                   - bit_offset);
    }

into a helper with just the computed offset as argument
(plus maybe that seen_variable_array_ref which is meaningless
for the address case or rather has to be assumed true(?)) should be possible.

Richard.

>
> Jeff