Set nonnull attribute to ptr_info_def based on VRP

Richard Biener richard.guenther@gmail.com
Fri Oct 7 10:03:00 GMT 2016


On Fri, Oct 7, 2016 at 2:53 AM, kugan <kugan.vivekanandarajah@linaro.org> wrote:
> Hi Richard,
>
> Thanks for the review.
>
>
> On 09/08/16 18:58, Richard Biener wrote:
>>
>> On Tue, Aug 9, 2016 at 12:58 AM, kugan
>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>
>>> Hi Jakub,
>>>
>>> Thanks for the review.
>>>
>>> On 08/08/16 16:40, Jakub Jelinek wrote:
>>>>
>>>>
>>>> On Mon, Aug 08, 2016 at 01:36:51PM +1000, kugan wrote:
>>>>>
>>>>>
>>>>> diff --git a/gcc/tree-ssanames.h b/gcc/tree-ssanames.h
>>>>> index c81b1a1..6e34433 100644
>>>>> --- a/gcc/tree-ssanames.h
>>>>> +++ b/gcc/tree-ssanames.h
>>>>> @@ -43,6 +43,9 @@ struct GTY(()) ptr_info_def
>>>>>       above alignment.  Access only through the same helper functions
>>>>> as
>>>>> align
>>>>>       above.  */
>>>>>    unsigned int misalign;
>>>>> +  /* When this pointer is knonw to be nnonnull this would be true
>>>>> otherwise
>>>>> +     false.  */
>>>>> +  bool  nonnull_p;
>>>>>  };
>>>>
>>>>
>>>>
>>>> Why do you need this?  Doesn't the pt.null bit represent that already?
>>>
>>>
>>>
>>> It looks like I can use this. As in gcc/tree-ssa-alias.h:
>>>
>>>   /* Nonzero if the points-to set includes 'nothing', the points-to set
>>>      includes memory at address NULL.  */
>>>   unsigned int null : 1;
>>>
>>> But in gcc/tree-ssa-alias.c, ptrs_compare_unequal has the following
>>> comment
>>> which says:
>>>
>>> /* ???  We'd like to handle ptr1 != NULL and ptr1 != ptr2
>>>      but those require pt.null to be conservatively correct.  */
>>>
>>> Does that means it can be wrong at times? I haven't looked it in detail
>>> yet
>>> but if it is, it would be a problem.
>>
>>
>> Yes, this bit is not correctly computed for all cases (well, it works
>> for trivial
>> ones but for example misses weaks and maybe some other corner-cases).
>>
>> Currently there are no consumers of this bit so the incorrectness
>> doesn't matter.
>>
>> I suggest you simply use that bit but set it conservatively from PTA (to
>> true)
>> for now and adjust it from VRP only.
>>
>> I do have some patches for fixinig some of the .nonnull_p issues in PTA
>> but
>> they come at a cost of more constraints.
>
>
> Here is a version of the patch that does this. I have a follow up patch to
> use this in ipa-vrp. I will send it for review once this is OK.
>
> Bootstrapped and regression tested on x86_64-linux-gnu with no new
> regressions. Is this OK for trunk?

@@ -6359,7 +6361,7 @@ find_what_var_points_to (tree fndecl, varinfo_t orig_vi)
       if (vi->is_artificial_var)
        {
          if (vi->id == nothing_id)
-           pt->null = 1;
+           ;

please leave this here.

 pt_solution_singleton_p (struct pt_solution *pt, unsigned *uid)
 {
   if (pt->anything || pt->nonlocal || pt->escaped || pt->ipa_escaped
-      || pt->null || pt->vars == NULL
+      || pt->vars == NULL
       || !bitmap_single_bit_set_p (pt->vars))
     return false;

humm... this is a correctness problem I guess.  A latent one currently
depending on who relies on it.
There is a single use of the above predicate which looks for the
points-to solution of a __builtin_alloca call.  We should somehow
arrange for its solution to not include ->null.  Or, simpler, change
the predicates name to pt_solution_singleton_or_null_p () as
the use does not care for pt->null.

+void
+set_ptr_nonnull (tree name)
+{
+  gcc_assert (POINTER_TYPE_P (TREE_TYPE (name)));
+  struct ptr_info_def *pi = get_ptr_info (name);
+  pi->pt.null = 0;

 = false;

+}

There is the question on how pt.null semantics should be
with respect to pt.anything, pt.escaped and pt.nonlocal.
I think get_ptr_nonnull needs to return false if any
of those are set.

Richard.

>
> Thanks,
> Kugan
>
> gcc/ChangeLog:
>
> 2016-10-07  Kugan Vivekanandarajah  <kuganv@linaro.org>
>
>         * tree-ssa-alias.c (dump_points_to_solution): Dump non null as it
>           is set to null conservatively.
>         * tree-ssa-structalias.c (find_what_var_points_to): Set to null
>           conservatively.
>         (pt_solution_singleton_p): Dont check null.
>         * tree-ssanames.c (set_ptr_nonnull): New.
>         (get_ptr_nonnull): Likewise.
>         * tree-vrp.c (vrp_finalize): Set ptr that are nonnull.
>         (evrp_dom_walker::before_dom_children): Likewise.
>
>>
>> Richard.
>>
>>>> Also, formatting and spelling:
>>>> s/knonw/known/
>>>> s/nnon/non/
>>>> s/bool  /bool /
>>>
>>>
>>>
>>> I will change this.
>>>
>>> Thanks,
>>> Kugan
>>>>
>>>>
>>>>
>>>>         Jakub
>>>>
>>>
>



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