Re: A bug in vrp_meet?

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Mar 1, 2019 at 10:02 PM Qing Zhao <qing.zhao@oracle.com> wrote:
>
>
> On Mar 1, 2019, at 1:25 PM, Richard Biener <richard.guenther@gmail.com> wrote:
>
> On March 1, 2019 6:49:20 PM GMT+01:00, Qing Zhao <qing.zhao@oracle.com> wrote:
>
> Jeff,
>
> thanks a lot for the reply.
>
> this is really helpful.
>
> I double checked the dumped intermediate file for pass “dom3", and
> located the following for _152:
>
> ****BEFORE the pass “dom3”, there is no _152, the corresponding Block
> looks like:
>
> <bb 4> [local count: 12992277]:
> _98 = (int) ufcMSR_52(D);
> k_105 = (sword) ufcMSR_52(D);
> i_49 = _98 > 0 ? k_105 : 0;
>
> ***During the pass “doms”,  _152 is generated as following:
>
> Optimizing block #4
> ….
> Visiting statement:
> i_49 = _98 > 0 ? k_105 : 0;
> Meeting
> [0, 65535]
> and
> [0, 0]
> to
> [0, 65535]
> Intersecting
> [0, 65535]
> and
> [0, 65535]
> to
> [0, 65535]
> Optimizing statement i_49 = _98 > 0 ? k_105 : 0;
> Replaced 'k_105' with variable '_98'
> gimple_simplified to _152 = MAX_EXPR <_98, 0>;
> i_49 = _152;
> Folded to: i_49 = _152;
> LKUP STMT i_49 = _152
> ==== ASGN i_49 = _152
>
> then bb 4 becomes:
>
> <bb 4> [local count: 12992277]:
> _98 = (int) ufcMSR_52(D);
> k_105 = _98;
> _152 = MAX_EXPR <_98, 0>;
> i_49 = _152;
>
> and all the i_49 are replaced with _152.
>
> However, the value range info for _152 doesnot reflect the one for
> i_49, it keeps as UNDEFINED.
>
> is this the root problem?
>
>
> It looks like DOM fails to visit stmts generated by simplification. Can you open a bug report with a testcase?
>
>
> The problem is, It took me quite some time in order to come up with a small and independent testcase for this problem,
> a little bit change made the error disappear.
>
> do you have any suggestion on this?  or can you give me some hint on how to fix this in DOM?  then I can try the fix on my side?

I remember running into similar issues in the past where I tried to
extract temporary nonnull ranges from divisions.
I have there

@@ -1436,11 +1436,16 @@ dom_opt_dom_walker::before_dom_children
   m_avail_exprs_stack->pop_to_marker ();

   edge taken_edge = NULL;
-  for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
-    {
-      evrp_range_analyzer.record_ranges_from_stmt (gsi_stmt (gsi), false);
-      taken_edge = this->optimize_stmt (bb, gsi);
-    }
+  gsi = gsi_start_bb (bb);
+  if (!gsi_end_p (gsi))
+    while (1)
+      {
+       evrp_range_analyzer.record_def_ranges_from_stmt (gsi_stmt (gsi), false);
+       taken_edge = this->optimize_stmt (bb, &gsi);
+       if (gsi_end_p (gsi))
+         break;
+       evrp_range_analyzer.record_use_ranges_from_stmt (gsi_stmt (gsi));
+      }

   /* Now prepare to process dominated blocks.  */
   record_edge_info (bb);

OTOH the issue in your case is that fold emits new stmts before gsi but the
above loop will never look at them.  See tree-ssa-forwprop.c for code how
to deal with this (setting a pass-local flag on stmts visited and walking back
to unvisited, newly inserted ones).  The fold_stmt interface could in theory
also be extended to insert new stmts on a sequence passed to it so the
caller would be responsible for inserting them into the IL and could then
more easily revisit them (but that's a bigger task).

So, does the following help?

Index: gcc/tree-ssa-dom.c
===================================================================
--- gcc/tree-ssa-dom.c  (revision 269361)
+++ gcc/tree-ssa-dom.c  (working copy)
@@ -1482,8 +1482,25 @@ dom_opt_dom_walker::before_dom_children
   edge taken_edge = NULL;
   for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
     {
+      gimple_stmt_iterator pgsi = gsi;
+      gsi_prev (&pgsi);
       evrp_range_analyzer.record_ranges_from_stmt (gsi_stmt (gsi), false);
       taken_edge = this->optimize_stmt (bb, gsi);
+      gimple_stmt_iterator npgsi = gsi;
+      gsi_prev (&npgsi);
+      /* Walk new stmts eventually inserted by DOM.  gsi_stmt (gsi) itself
+        while it may be changed should not have gotten a new definition.  */
+      if (gsi_stmt (pgsi) != gsi_stmt (npgsi))
+       do
+         {
+           if (gsi_end_p (pgsi))
+             pgsi = gsi_start_bb (bb);
+           else
+             gsi_next (&pgsi);
+           evrp_range_analyzer.record_ranges_from_stmt (gsi_stmt (pgsi),
+                                                        false);
+         }
+       while (gsi_stmt (pgsi) != gsi_stmt (gsi));
     }

   /* Now prepare to process dominated blocks.  */


Richard.

> Thanks a lot.
>
> Qing
>
>
>
> Richard.
>
>