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

[Martin Sebor <msebor@gmail.com>]

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).  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