Re: Add a constant_range_value_p function (PR 92033)

[Aldy Hernandez <aldyh@redhat.com>]

On 10/14/19 8:31 AM, Richard Sandiford wrote:
> Aldy Hernandez <aldyh@redhat.com> writes:
>> On 10/14/19 4:30 AM, Richard Sandiford wrote:
>>> Aldy Hernandez <aldyh@redhat.com> writes:
>>>> On 10/11/19 10:42 AM, Richard Sandiford wrote:
>>>>> The range-tracking code has a pretty hard-coded assumption that
>>>>> is_gimple_min_invariant is equivalent to "INTEGER_CST or invariant
>>>>> ADDR_EXPR".  It seems better to add a predicate specifically for
>>>>> that rather than contiually fight cases in which it can't handle
>>>>> other invariants.
>>>>
>>>> I was going to suggest we normalize ranges to numerics completely before
>>>> folding.  That is, replacing normalize_addresses() here:
>>>>
>>>>      *vr = op->fold_range (expr_type,
>>>> 			vr0.normalize_addresses (),
>>>> 			vr1.normalize_addresses ());
>>>>
>>>> ...into normalize_symbolics().  But I suppose getting the gate correct
>>>> is even better.  Thanks for taking the care of this extensive and manual
>>>> change.
>>>>
>>>> The patch looks good to me.  However, I do wonder if VRP and
>>>> subsidiaries can't handle non-integer invariants, if we shouldn't
>>>> disallow them from the setters as well:
>>>>
>>>> void
>>>> value_range_base::set (tree val)
>>>> {
>>>>      gcc_assert (TREE_CODE (val) == SSA_NAME || is_gimple_min_invariant
>>>> (val));
>>>>      if (TREE_OVERFLOW_P (val))
>>>>        val = drop_tree_overflow (val);
>>>>      set (VR_RANGE, val, val);
>>>> }
>>>>
>>>> void
>>>> value_range::set (tree val)
>>>> {
>>>>      gcc_assert (TREE_CODE (val) == SSA_NAME || is_gimple_min_invariant
>>>> (val));
>>>>      if (TREE_OVERFLOW_P (val))
>>>>        val = drop_tree_overflow (val);
>>>>      set (VR_RANGE, val, val, NULL);
>>>> }
>>>>
>>>> This would still allow setting of VARYING and UNDEFINED, but disallow
>>>> poly-ints, etc from a range.
>>>>
>>>> Was this a small oversight, or was there a reason you left those in?
>>>
>>> Yeah, this was intentional.  The patch is effectively treating
>>> POLY_INY_CST as symbolic rather than constant.  (It's really somewhere
>>> in the middle: it's at least a function invariant, but the invariant
>>> depends on a runtime target property.)  So places like here that
>>> can handle both symbolics and constants should be able to handle
>>> POLY_INT_CST wihout problems.  We just need to make sure that
>>> POLY_INT_CSTs aren't treated as constants for range tracking,
>>> because they're not "constant enough" there.
>>
>> Hmmm... We don't handle POLY_INT_CST value_range's anywhere, so perhaps
>> it's better to stop their creation at the source and fix the caller, to
>> inhibit their proliferation.  AFAICT, the only POLY_INT_CST ranges that
>> would be used are VARYING and UNDEFINED (for the lattice), and that
>> already works.
>>
>> If you don't agree, then at least a comment in ::set() would be nice, to
>> document that we're allowing their creation and why.
> 
> I don't think it makes sense to allow SSA_NAME (which is completely
> unconstrained) and not allow POLY_INT_CST.  Like I say, POLY_INT_CST
> can be conservatively treated as symbolic.

Oh, I see your change to ::symbolic_p() would effectively treat them as 
symbolics.

> 
> POLY_INT_CST can produce useful singleton ranges or be a minimum or
> a maximum, even if we don't use them to reduce ranges arithmetically.

In which case, singleton_p() should be adapted to return the singleton. 
I don't expect you to do this, unless you want to :).  Could you at 
least put a comment there, so we don't forget?

I'm feel a bit queasy allowing POLY_INT ranges throughout, when we have 
no code that handle them anywhere, but I won't object to your patch. 
Thanks for fixing this.

The patch with the comment on singleton is fine with me.

Aldy