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

[Richard Biener <richard.guenther@gmail.com>]

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. 

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