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

[Jeff Law <law at redhat dot com>]

On 9/15/18 2:14 AM, Bernd Edlinger wrote:
> On 9/14/18, Martin Sebor wrote:
>> As I said above, this happens during the dom walk in the ccp
>> pass:
>>
>>   substitute_and_fold_dom_walker walker (CDI_DOMINATORS, this);
>>   walker.walk (ENTRY_BLOCK_PTR_FOR_FN (cfun));
>>
>> The warning is issued during the walker.walk() call as
>> strncpy is being folded into memcpy.  The prior assignments are
>> only propagated later, when the next statement after the strncpy
>> call is reached.  It happens in
>> substitute_and_fold_dom_walker::before_dom_children(). So during
>> the strncpy folding we see the next statement as:
>>
>>   MEM[(struct S *)_1].a[n_7] = 0;
>>
>> After the strncpy call is transformed to memcpy, the assignment
>> above is transformed to
>>
>>   MEM[(struct S *)_8].a[3] = 0;
>>
>>
>>>     If they're only discovered as copies within the pass where you're trying
>>>     to issue the diagnostic, then you'd want to see if the pass has any
>>>     internal structures that tell you about equivalences.
>>
>>
>> I don't know if this is possible.  I don't see any APIs in
>> tree-ssa-propagate.h that would let me query the internal data
>> somehow to find out during folding (when the warning is issued).
> 
> 
> Well,
> 
> if I see this right, the CCP is doing tree transformations
> while from the folding of strncpy the predicate maybe_diag_stxncpy_trunc
> is called, and sees inconsistent information, in the tree,
> and therefore it issues a warning.
> 
> I understand that walking the references is fragile at least
> in this state.
> 
> But why not just prevent warnings when this is called from CCP?
> 
> 
> Like this?
> 
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?
No.  That's just hacking around the real problem.

jeff