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

[Richard Biener <rguenther@suse.de>]

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.  Can you check whether
it's possible to distinguish -Warray-bounds from -Warray-bounds=N?  I'd
say that explicit -Warray-bounds=N should exactly get the documented
set of diagnostis, independent of -fstrict-flex-arrays=N.

> > 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;
HRB 36809 (AG Nuernberg)