Re: [PATCH 2/2] Add a new warning option -Wstrict-flex-arrays.

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

> On Nov 22, 2022, at 9:10 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> 
> 
>> On Nov 22, 2022, at 3:16 AM, Richard Biener <rguenther@suse.de> wrote:
>> 
>> On Mon, 21 Nov 2022, Qing Zhao wrote:
>> 
>>> 
>>> 
>>>> On Nov 18, 2022, at 11:31 AM, Kees Cook <keescook@chromium.org> wrote:
>>>> 
>>>> On Fri, Nov 18, 2022 at 03:19:07PM +0000, Qing Zhao wrote:
>>>>> Hi, Richard,
>>>>> 
>>>>> Honestly, it?s very hard for me to decide what?s the best way to handle the interaction 
>>>>> between -fstrict-flex-array=M and -Warray-bounds=N. 
>>>>> 
>>>>> Ideally,  -fstrict-flex-array=M should completely control the behavior of -Warray-bounds.
>>>>> If possible, I prefer this solution.
>>>>> 
>>>>> However, -Warray-bounds is included in -Wall, and has been used extensively for a long time.
>>>>> It?s not safe to change its default behavior. 
>>>> 
>>>> I prefer that -fstrict-flex-arrays controls -Warray-bounds. That
>>>> it is in -Wall is _good_ for this reason. :) No one is going to add
>>>> -fstrict-flex-arrays (at any level) without understanding what it does
>>>> and wanting those effects on -Warray-bounds.
>>> 
>>> 
>>> The major difficulties to let -fstrict-flex-arrays controlling -Warray-bounds was discussed in the following threads:
>>> 
>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604133.html
>>> 
>>> Please take a look at the discussion and let me know your opinion.
>> 
>> My opinion is now, after re-considering and with seeing your new 
>> patch, that -Warray-bounds=2 should be changed to only add
>> "the intermediate results of pointer arithmetic that may yield out of 
>> bounds values" and that what it considers a flex array should now
>> be controlled by -fstrict-flex-arrays only.
>> 
>> That is, I think, the only thing that's not confusing to users even
>> if that implies a change from previous behavior that we should
>> document by clarifying the -Warray-bounds documentation as well as
>> by adding an entry to the Caveats section of gcc-13/changes.html
>> 
>> That also means that =2 will get _less_ warnings with GCC 13 when
>> the user doesn't use -fstrict-flex-arrays as well.
> 
> Okay.  So, this is for -Warray-bounds=2.
> 
> For -Warray-bounds=1 -fstrict-flex-array=N, if N > 1, should -fstrict-flex-array=N control -Warray-bounds=1?

More thinking on this. (I might misunderstand a little bit in the previous email)

If I understand correctly now, what you proposed was:

1. The level of -Warray-bounds will NOT control how a trailing array is considered as a flex array member anymore. 
2. Only the level of -fstrict-flex-arrays will control this;
3. Keep the current default  behavior of -Warray-bounds on treating trailing arrays as flex array member (treating all [0],[1], and [] as flexible array members). 
4. Updating the documentation for -Warray-bounds by clarifying this change, and also as an entry to the Caveats section on such change on -Warray-bounds.

If the above is correct, Yes, I like this change. Both the user interface and the internal implementation will be simplified and cleaner. 

Let me know if you see any issue with my above understanding.

Thanks a lot.

Qing

> 
> Qing
> 
>> 
>> Richard.
>> 
>> -- 
>> 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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