Re: [RFC] how to handle the combination of -fstrict-flex-arrays + -Warray-bounds

[Qing Zhao <qing.zhao@oracle.com>]

> On Oct 24, 2022, at 3:30 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Sat, 22 Oct 2022, Martin Sebor wrote:
> 
>> On 10/21/22 09:29, Qing Zhao wrote:
>>> Hi,
>>> 
>>> (FAM below refers to Flexible Array Members):
>>> 
>>> I need inputs on  how to handle the combination of -fstrict-flex-arrays +
>>> -Warray-bounds.
>>> 
>>> Our initial goal is to update -Warray-bounds with multiple levels of
>>> -fstrict-flex-arrays=N
>>> to issue warnings according to the different levels of ?N?.
>>> However, after detailed study, I found that this goal was very hard to be
>>> achieved.
>>> 
>>> 1. -fstrict-flex-arrays and its levels
>>> 
>>> The new option -fstrict-flex-arrays has 4 levels:
>>> 
>>> level   trailing arrays
>>>         treated as FAM
>>> 
>>>   0     [],[0],[1],[n]          	the default without option
>>>   1     [],[0],[1]
>>>   2     [],[0]
>>>   3     []                      	the default when option specified
>>>   without value
>>> 
>>> 2. -Warray-bounds and its levels
>>> 
>>> The option -Warray-bounds currently has 2 levels:
>>> 
>>> level   trailing arrays
>>>         treated as FAM
>>> 
>>>   1     [],[0],[1]			 the default when option specified
>>>   without value
>>>   2     []                     	
>>> 
>>> i.e,
>>> When -Warray-bounds=1, it treats [],[0],[1] as FAM, the same level as
>>> -fstrict-flex-arrays=1;
>>> When -Warray-bounds=2, it only treat [] as FAM, the same level as
>>> -fstrict-flex-arrays=3;
>>> 
>>> 3. How to handle the combination of  -fstrict-flex-arrays and
>>> -Warray-bounds?
>>> 
>>> Question 1:  when -fstrict-flex-arrays does not present, the default is
>>> -strict-flex-arrays=0,
>>>                     which treats [],[0],[1],[n] as FAM, so should we update
>>>                     the default behavior
>>>                     of -Warray-bounds to treat any trailing array [n] as
>>>                     FAMs?
>>> 
>>> My immediate answer to Q1 is NO, we shouldn?t, that will be a big regression
>>> on -Warray-bounds, right?
>> 
>> Yes, it would disable -Warray-bounds in the cases where it warns
>> for past-the-end accesses to trailing arrays with two or more
>> elements.  Diagnosing those has historically (i.e., before recent
>> changes) been a design goal.
>> 
>>> 
>>> Question 2:  when -fstrict-flex-arrays=N1 and -Warray-bounds=N2 present at
>>> the same time,
>>>                      Which one has higher priority? N1 or N2?
>>> 
>>> -fstrict-flex-arrays=N1 controls how the compiler code generation treats the
>>> trailing arrays as FAMs, it seems
>>> reasonable to give higher priority to N1,
>> 
>> I tend to agree.  In other words, set N2' = min(N1, N2).
> 
> Yes.  Or do nothing and treat them independently.

I prefer treating them independently. 

If there is no multiple levels of -Warray-bounds, it’s safe and reasonable to control -Warray-bounds with 
different levels of -fstrict-flex-arrays=N.  However, the current -Warray-bounds already has multiple levels which
have been exposed to and been used by the end users. Changing their behavior will impact the end-users.


> Can you check whether
> it's possible to distinguish -Warray-bounds from -Warray-bounds=N?

The current difference between -Warray-bounds and -Warray-bounds=2 is:  -Warray-bounds=2 
will NOT treat 0-length arrays and 1-element arrays as FAMs. Therefore report out-of-bounds
 access to 0-lenght arrays or 1-element arrays.


>  I'd
> say that explicit -Warray-bounds=N should exactly get the documented
> set of diagnostis, independent of -fstrict-flex-arrays=N.

If we decide to make -fstrict-flex-arrays=N1 and -Warray-bounds=N2 independently.
How about -fstrict-flex-array=N and -Wstringop-overflow (-Wstringop-overread, etc)? 
Shall we control -Wstringop-overflow with -fstrict-flex-array=N?  Or treat them independently?

Qing
> 
>>> However, then should we completely disable the level of -Warray-bounds
>>> N2 under such situation?
>>> 
>>> I really don?t know what?s the best way to handle the conflict  between N1
>>> and N2.
>>> 
>>> Can we completely cancel the 2 levels of -Warray-bounds, and always honor
>>> the level of -fstrict-flex-arrays?
>>> 
>>> Any comments or suggestion will be helpful.
>> 
>> The recent -fstrict-flex-array changes aside, IIRC, there's only
>> a subtle distinction between the two -Warray-bounds levels (since
>> level 1 started warning on a number of instances that only level
>> 2 used to diagnose a few releases ago).  I think that subset of
>> level 2 could be merged into level 1 without increasing the rate
>> of false positives.  Then level 2 could be assigned a new set of
>> potential problems to detect (such as past-the-end accesses to
>> trailing one-element arrays).
>> 
>> Martin
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
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