Re: [PATCH] look harder for MEM_REF operand equality to avoid -Wstringop-truncation (PR 84561)

[Martin Sebor <msebor@gmail.com>]

On 08/31/2018 04:07 AM, Richard Biener wrote:
> On Thu, Aug 30, 2018 at 7:39 PM Martin Sebor <msebor@gmail.com> wrote:
>>
>> On 08/30/2018 11:22 AM, Richard Biener wrote:
>>> On August 30, 2018 6:54:21 PM GMT+02:00, Martin Sebor <msebor@gmail.com> wrote:
>>>> On 08/30/2018 02:35 AM, Richard Biener wrote:
>>>>> On Thu, Aug 30, 2018 at 2:12 AM Martin Sebor <msebor@gmail.com>
>>>> wrote:
>>>>>>
>>>>>> The attached patch adds code to work harder to determine whether
>>>>>> the destination of an assignment involving MEM_REF is the same
>>>>>> as the destination of a prior strncpy call.  The included test
>>>>>> case demonstrates when this situation comes up.  During ccp,
>>>>>> dstbase and lhsbase returned by get_addr_base_and_unit_offset()
>>>>>> end up looking like this:
>>>>>
>>>>> "During CCP" means exactly when?  The CCP lattice tracks copies
>>>>> so CCP should already know that _1 == _8.  I suppose during
>>>>> substitute_and_fold then?  But that replaces uses before folding
>>>>> the stmt.
>>>>
>>>> Yes, when ccp_finalize() performs the final substitution during
>>>> substitute_and_fold().
>>>
>>> But then you shouldn't need the loop but at most look at the pointer SSA Def to get at the non-invariant ADDR_EXPR.
>>
>> I don't follow.   Are you suggesting to compare
>> SSA_NAME_DEF_STMT (dstbase) to SSA_NAME_DEF_STMT (lhsbase) for
>> equality?  They're not equal.
>
> No.
>
>> The first loop iterates once and retrieves
>>
>>    1.  _8 = &pb_3(D)->a;
>>
>> The second loop iterates three times and retrieves:
>>
>>    1.  _1 = _9
>>    2.  _9 = _8
>>    3.  _8 = &pb_3(D)->a;
>>
>> How do I get from _1 to &pb_3(D)->a without iterating?  Or are
>> you saying to still iterate but compare the SSA_NAME_DEF_STMT?
>
> I say you should retrieve _8 = &pb_3(D)->a immediately since the
> copies should be
> propagated out at this stage.

The warning is issued as the strncpy call is being folded (during
the dom walk in substitute_and_fold_engine::substitute_and_fold)
but before the subsequent statements have been folded (during
the subsequent loop to eliminate statements).  So at the point
of the strncpy folding the three assignments above are still
there.

I can't think of a good way to solve this problem that's not
overly intrusive.  Unless you have some suggestions for how
to deal with it, is the patch okay as is?

Martin