Re: A bug in vrp_meet?

[Jeff Law <law@redhat.com>]

On 3/4/19 4:45 AM, Richard Biener wrote:
> 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).
Grr. This all sounds very familiar.  I know I've tracked down a bug of
this  nature before and would like to review how it was fixed in light
of your comment about tree-ssa-forwprop's handling of the same
situation.  I just can't seem to find it..  Ugh.

I assume you're referring to the PLF stuff?


Jeff