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Re: types for VR_VARYING




On 8/15/19 7:23 AM, Richard Biener wrote:
On Thu, Aug 15, 2019 at 12:40 PM Aldy Hernandez <aldyh@redhat.com> wrote:

On 8/14/19 1:37 PM, Jeff Law wrote:
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.

I think that if name1 was !supports_type_p, we will have never called
union/intersect.  We will have bailed at some point earlier.  However, I
do see your point about being consistent.


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.

Done.



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.

Hmmm, the problem with setting VR_VARYING for unsupported types is that
we have no min/max to use.  Even though min/max will not be used in any
calculation, it's nice to have it set so type() will work consistently.
May I suggest this generic approach while we disassociate the lattice
and ranges from value_range_base (or remove vrp altogether :))?

void
value_range_base::set_varying (tree type)
{
    m_kind = VR_VARYING;
    if (supports_type_p (type))
      {
        m_min = vrp_val_min (type, true);
        m_max = vrp_val_max (type, true);
      }
    else
      {
        /* We can't do anything range-wise with these types.  Build
          something for which type() will work as a temporary measure
          until lattices and value_range_base are disassociated.  */
        m_min = m_max = build1 (NOP_EXPR, type, void_type_node);
      }
}

This way, no changes happen throughout the code, varying remains
varying, type() works everywhere, and we don't have to dig into all
value_range users to skip unsupported types.

This should work, as no one is going to call min() / max() on an
unsupported type, since they're just being used for the lattice.

Then use error_mark_node or NULL_TREE?!

I was using something for which type() could work, to always be consistent. But perhaps no one should be asking a type() for something we don't support range-wise ??.

I would prefer not to use NULL_TREE, so the assertion/checking routines can always expect something and not have to special case anything. Error_mark_node would work, if no one ever asks for type().

Thanks for your feedback.
Aldy


Sorry, couldn't resist to chime in when seeing the NOP_EXPR build.

Last time.  Promised.  I'm resisting to comment on the supports_type_p
abdomination.

Richard.

Aldy


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