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

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

> On Oct 22, 2022, at 12:54 PM, Martin Sebor <msebor@gmail.com> 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).
> 
>> 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).  

From the doc: (and I also checked the source code)

 -Warray-bounds=2
This warning level also warns about out of bounds accesses to trailing
struct members of one-element array types (@pxref{Zero Length}) and about
the intermediate results of pointer arithmetic that may yield out of bounds
values.  This warning level may give a larger number of false positives and
is deactivated by default.

As I understand, -Warray-bounds=1 (i.e., -Warray-bounds) will report out-of-bounds access to trailing arrays with two or more elements, and treat trailing arrays with 0 or 1 as FAMs;
-Warray-bounds=2 will report out-of-bounds access to trailing arrays with 0 or 1elements in addition to -Warray-bounds =1. 

Is the above understanding correct?


> 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).

If I understand correctly, Current Level 2 already include warning about past-the-end accesses to trailing one-element arrays (and also 0-length arrays).  

Qing

> 
> Martin