Re: [PING] [PATCH] consider successor blocks when avoiding -Wstringop-truncation (PR 84468)

[Jeff Law <law at redhat dot com>]

On 02/24/2018 05:11 PM, Martin Sebor wrote:
> Attached is an updated patch with a fix for a bad assumption
> exposed by building the linux kernel.
> 
> On 02/19/2018 07:50 PM, Martin Sebor wrote:
>> PR 84468 points out a false positive in -Wstringop-truncation
>> in code like this:
>>
>>   struct A { char a[4]; };
>>
>>   void f (struct A *p, const struct A *q)
>>   {
>>     if (p->a)
>>       strncpy (p->a, q->a, sizeof p->a - 1);   // warning here
>>
>>     p->a[3] = '\0';
>>   }
>>
>> The warning is due to the code checking only the same basic block
>> as the one with the strncpy call for an assignment to the destination
>> to avoid it, but failing to check the successor basic block if there
>> is no subsequent statement in the current block.  (Eliminating
>> the conditional is being tracked in PR 21474.)
>>
>> The attached test case adds logic to avoid this false positive.
>> I don't know under what circumstances there could be more than
>> one successor block here so I don't handle that case.
So this is feeling more and more like we need to go back to the ideas
behind checking the virtual operand chains.

The patch as-written does not properly handle the case where BB has
multiple outgoing edges.  For gcc-8 you could probably get away with
checking that you have precisely one outgoing edge without EDGE_ABNORMAL
set in its flags in addition to the checks you're already doing.

But again, it's feeling more and more like the right fix is to go back
and walk the virtual operands.

Jeff