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

[Martin Sebor <msebor@gmail.com>]

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.

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?

Martin

>
> Richard.
>
>> Martin
>>
>>>
>>> So I'm confused.
>>>
>>>>
>>>>    _8 = &pb_3(D)->a;
>>>>    _9 = _8;
>>>>    _1 = _9;
>>>>    strncpy (MEM_REF (&pb_3(D)->a), ...);
>>>>    MEM[(struct S *)_1].a[n_7] = 0;
>>>>
>>>> so the loops follow the simple assignments until we get at
>>>> the ADDR_EXPR assigned to _8 which is the same as the strncpy
>>>> destination.
>>>>
>>>> Tested on x86_64-linux.
>>>>
>>>> Martin
>