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

[Qing Zhao]

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?

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, 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.

thanks.

Qing