[PING #3] [PATCH] look harder for MEM_REF operand equality to avoid -Wstringop-truncation (PR 84561)

[Martin Sebor]

Ping: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01934.html

On 10/08/2018 03:40 PM, Martin Sebor wrote:
> Ping: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01934.html
>
> On 10/01/2018 03:30 PM, Martin Sebor wrote:
>> Ping: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01934.html
>>
>> We have discussed a number of different approaches to moving
>> the warning somewhere else but none is feasible in the limited
>> amount of time remaining in stage 1 of GCC 9.  I'd like to
>> avoid the false positive in GCC 9 by using the originally
>> submitted, simple approach and look into the suggested design
>> changes for GCC 10.
>>
>> On 09/21/2018 08:36 AM, Martin Sebor wrote:
>>> On 09/20/2018 03:06 AM, Richard Biener wrote:
>>>> On Wed, Sep 19, 2018 at 4:19 PM Martin Sebor <msebor@gmail.com> wrote:
>>>>>
>>>>> On 09/18/2018 10:23 PM, Jeff Law wrote:
>>>>>> On 9/18/18 1:46 PM, Martin Sebor wrote:
>>>>>>> On 09/18/2018 12:58 PM, Jeff Law wrote:
>>>>>>>> On 9/18/18 11:12 AM, Martin Sebor wrote:
>>>>>>>>
>>>>>>>>>> My bad.  Sigh. CCP doesn't track copies, just constants, so
>>>>>>>>>> there's not
>>>>>>>>>> going to be any data structure you can exploit.  And I don't
>>>>>>>>>> think
>>>>>>>>>> there's a value number you can use to determine the two objects
>>>>>>>>>> are the
>>>>>>>>>> same.
>>>>>>>>>>
>>>>>>>>>> Hmm, let's back up a bit, what is does the relevant part of the
>>>>>>>>>> IL look
>>>>>>>>>> like before CCP?  Is the real problem here that we have
>>>>>>>>>> unpropagated
>>>>>>>>>> copies lying around in the IL?  Hmm, more likely the IL looksl
>>>>>>>>>> ike:
>>>>>>>>>>
>>>>>>>>>>    _8 = &pb_3(D)->a;
>>>>>>>>>>    _9 = _8;
>>>>>>>>>>    _1 = _9;
>>>>>>>>>>    strncpy (MEM_REF (&pb_3(D)->a), ...);
>>>>>>>>>>    MEM[(struct S *)_1].a[n_7] = 0;
>>>>>>>>>
>>>>>>>>> Yes, that is what the folder sees while the strncpy call is
>>>>>>>>> being transformed/folded by ccp.  The MEM_REF is folded just
>>>>>>>>> after the strncpy call and that's when it's transformed into
>>>>>>>>>
>>>>>>>>>   MEM[(struct S *)_8].a[n_7] = 0;
>>>>>>>>>
>>>>>>>>> (The assignments to _1 and _9 don't get removed until after
>>>>>>>>> the dom walk finishes).
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> If we were to propagate the copies out we'd at best have:
>>>>>>>>>>
>>>>>>>>>>    _8 = &pb_3(D)->a;
>>>>>>>>>>    strncpy (MEM_REF (&pb_3(D)->a), ...);
>>>>>>>>>>    MEM[(struct S *)_8].a[n_7] = 0;
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Is that in a form you can handle?  Or would we also need to
>>>>>>>>>> forward
>>>>>>>>>> propagate the address computation into the use of _8?
>>>>>>>>>
>>>>>>>>> The above works as long as we look at the def_stmt of _8 in
>>>>>>>>> the MEM_REF (we currently don't).  That's also what the last
>>>>>>>>> iteration of the loop does.  In this case (with _8) it would
>>>>>>>>> be discovered in the first iteration, so the loop could be
>>>>>>>>> replaced by a simple if statement.
>>>>>>>>>
>>>>>>>>> But I'm not sure I understand the concern with the loop.  Is
>>>>>>>>> it that we are looping at all, i.e., the cost?  Or that ccp
>>>>>>>>> is doing something wrong or suboptimal? (Should have
>>>>>>>>> propagated the value of _8 earlier?)
>>>>>>>> I suspect it's more a concern that things like copies are typically
>>>>>>>> propagated away.   So their existence in the IL (and consequently
>>>>>>>> your
>>>>>>>> need to handle them) raises the question "has something else
>>>>>>>> failed to
>>>>>>>> do its job earlier".
>>>>>>>>
>>>>>>>> During which of the CCP passes is this happening?  Can we pull the
>>>>>>>> warning out of the folder (even if that means having a distinct
>>>>>>>> warning
>>>>>>>> pass over the IL?)
>>>>>>>
>>>>>>> It happens during the third run of the pass.
>>>>>>>
>>>>>>> The only way to do what you suggest that I could think of is
>>>>>>> to defer the strncpy to memcpy transformation until after
>>>>>>> the warning pass.  That was also my earlier suggestion: defer
>>>>>>> both it and the warning until the tree-ssa-strlen pass (where
>>>>>>> the warning is implemented to begin with -- the folder calls
>>>>>>> into it).
>>>>>> If it's happening that late (CCP3) in general, then ISTM we ought
>>>>>> to be
>>>>>> able to get the warning out of the folder.  We just have to pick the
>>>>>> right spot.
>>>>>>
>>>>>> warn_restrict runs before fold_all_builtins, but after dom/vrp so we
>>>>>> should have the IL in pretty good shape.  That seems like about the
>>>>>> right time.
>>>>>>
>>>>>> I wonder if we could generalize warn_restrict to be a more generic
>>>>>> warning pass over the IL and place it right before fold_builtins.
>>>>>
>>>>> The restrict pass doesn't know about string lengths so it can't
>>>>> handle all the warnings about string built-ins (the strlen pass
>>>>> now calls into it to issue some).  The strlen pass does so it
>>>>> could handle most if not all of them (the folder also calls
>>>>> into it to issue some warnings).  It would work even better if
>>>>> it were also integrated with the object size pass.
>>>>>
>>>>> We're already working on merging strlen with sprintf.  It seems
>>>>> to me that the strlen pass would benefit not only from that but
>>>>> also from integrating with object size and warn-restrict.  With
>>>>> that, -Wstringop-overflow could be moved from builtins.c into
>>>>> it as well (and also benefit not only from accurate string
>>>>> lengths but also from the more accurate object size info).
>>>>>
>>>>> What do you think about that?
>>>>
>>>> I think integrating the various "passes" (objectsize is also
>>>> as much a facility as a pass) generally makes sense given
>>>> it might end up improving all of them and reduce code duplication.
>>>
>>> Okay.  If Jeff agrees I'll see if I can make it happen for GCC
>>> 10.  Until then, does either of you have any suggestions for
>>> a different approach in this patch or is it acceptable with
>>> the loop as is?
>>>
>>> Martin
>>>
>>>>
>>>> Richard.
>>>>
>>>>>
>>>>> Martin
>>>>>
>>>>> PS I don't think I could do more than merger strlen and sprintf
>>>>> before stage 1 ends (if even that much) so this would be a longer
>>>>> term goal.
>>>
>>
>