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

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 09/17/18 19:35, Richard Biener wrote:
> On September 17, 2018 7:24:16 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>> 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.
> 
> The real problem is emitting diagnostics from folding code.
> 

Yes, I am also very concerned about that.

So if this is a design bug, then it is probably impossible
to fix at the implementation, and we should fix the design.

It is unfortunate, that this means regressions on the existing
(and probably also proposed, future) test cases.


Bernd.