Re: types for VR_VARYING

[Jeff Law <law@redhat.com>]

On 8/13/19 6:39 PM, Aldy Hernandez wrote:
> 
> 
> On 8/12/19 7:46 PM, Jeff Law wrote:
>> On 8/12/19 12:43 PM, Aldy Hernandez wrote:
>>> This is a fresh re-post of:
>>>
>>> https://gcc.gnu.org/ml/gcc-patches/2019-07/msg00006.html
>>>
>>> Andrew gave me some feedback a week ago, and I obviously don't remember
>>> what it was because I was about to leave on PTO.  However, I do remember
>>> I addressed his concerns before getting drunk on rum in tropical islands.
>>>
>> FWIW found a great coffee infused rum while in Kauai last week.  I'm not
>> a coffee fan, but it was wonderful.  The one bottle we brought back
>> isn't going to last until Cauldron and I don't think I can get a special
>> order filled before I leave :(
> 
> You must bring some to Cauldron before we believe you. :)
That's the problem.  The nearest place I can get it is in Vegas and
there's no distributor in Montreal.   I can special order it in our
state run stores, but it won't be here in time.

Of course, I don't mind if you don't believe me.  More for me in that
case...


>> Is the supports_type_p stuff there to placate the calls from ipa-cp?  I
>> can live with it in the short term, but it really feels like there
>> should be something in the ipa-cp client that avoids this silliness.
> 
> I am not happy with this either, but there are various places where
> statements that are !stmt_interesting_for_vrp() are still setting a
> range of VARYING, which is then being ignored at a later time.
> 
> For example, vrp_initialize:
> 
>       if (!stmt_interesting_for_vrp (phi))
>         {
>           tree lhs = PHI_RESULT (phi);
>           set_def_to_varying (lhs);
>           prop_set_simulate_again (phi, false);
>         }
> 
> Also in evrp_range_analyzer::record_ranges_from_stmt(), where we if the
> statement is interesting for VRP but extract_range_from_stmt() does not
> produce a useful range, we also set a varying for a range we will never
> use.  Similarly for a statement that is not interesting in this hunk.
Ugh.  One could perhaps argue that setting any kind of range in these
circumstances is silly.   But I suspect it's necessary due to the
optimistic handling of VR_UNDEFINED in value_range_base::union_helper.
It's all coming back to me now...


> 
> Then there is vrp_prop::visit_stmt() where we also set VARYING for types
> that VRP will never handle:
> 
>       case IFN_ADD_OVERFLOW:
>       case IFN_SUB_OVERFLOW:
>       case IFN_MUL_OVERFLOW:
>       case IFN_ATOMIC_COMPARE_EXCHANGE:
>     /* These internal calls return _Complex integer type,
>        which VRP does not track, but the immediate uses
>        thereof might be interesting.  */
>     if (lhs && TREE_CODE (lhs) == SSA_NAME)
>       {
>         imm_use_iterator iter;
>         use_operand_p use_p;
>         enum ssa_prop_result res = SSA_PROP_VARYING;
> 
>         set_def_to_varying (lhs);
> 
> I've adjusted the patch so that set_def_to_varying will set the range to
> VR_UNDEFINED if !supports_type_p.  This is a fail safe, as we can't
> really do anything with a nonsensical range.  I just don't want to leave
> the range in an indeterminate state.
> 
I think VR_UNDEFINED is unsafe due to value_range_base::union_helper.
And that's a more general than this patch.  VR_UNDEFINED is _not_ a safe
range to set something to if we can't handle it.  We have to use VR_VARYING.

Why?  See the beginning of value_range_base::union_helper:

   /* VR0 has the resulting range if VR1 is undefined or VR0 is varying.  */
   if (vr1->undefined_p ()
       || vr0->varying_p ())
     return *vr0;

   /* VR1 has the resulting range if VR0 is undefined or VR1 is varying.  */
   if (vr0->undefined_p ()
       || vr1->varying_p ())
     return *vr1;
This can get called for something like

  a = <cond> ? name1 : name2;

If name1 was set to VR_UNDEFINED thinking that VR_UNDEFINED was a safe
value for something we can't handle, then we'll incorrectly return the
range for name2.

VR_UNDEFINED can only be used for the ranges of objects we haven't
processed.  If we can't produce a range for an object because the
statement is something we don't handle or just doesn't produce anythign
useful, then the right result is VR_VARYING.

This may be worth commenting at the definition site for VR_*.

> 
> I also noticed that Andrew's patch was setting num_vr_values to
> num_ssa_names + num_ssa_names / 10.  I think he meant num_vr_values +
> num_vr_values / 10.  Please verify the current incantation makes sense.
Going to assume this will be adjusted per the other messages in this thread.


> diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> index 39ea22f0554..663dd6e2398 100644
> --- a/gcc/tree-ssa-threadedge.c
> +++ b/gcc/tree-ssa-threadedge.c
> @@ -182,8 +182,10 @@ record_temporary_equivalences_from_phis (edge e,
>  	    new_vr->deep_copy (vr_values->get_value_range (src));
>  	  else if (TREE_CODE (src) == INTEGER_CST)
>  	    new_vr->set (src);
> +	  else if (value_range_base::supports_type_p (TREE_TYPE (src)))
> +	    new_vr->set_varying (TREE_TYPE (src));
>  	  else
> -	    new_vr->set_varying ();
> +	    new_vr->set_undefined ();
So I think this can cause problems.  VR_VARYING seems like the right
state here.


> +
> +  if (!value_range_base::supports_type_p (TREE_TYPE (var)))
> +    {
> +      vr->set_undefined ();
> +      return vr;
Probably better as VR_VARYING here too.

> +	{
> +	  /* If we have an unsupported type (structs, void, etc), there
> +	     is nothing we'll be able to do with this entry.
> +	     Initialize it to UNDEFINED as a sanity measure, just in
> +	     case.  */
> +	  vr->set_undefined ();
Here too.

Jeff