[PING #2] [PATCH] look harder for MEM_REF operand equality to avoid -Wstringop-truncation (PR 84561)

[Martin Sebor <msebor@gmail.com>]

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.
>>
>