Re: Separate warning/error thresholds for -Wfoo=<n>

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Dec 6, 2022 at 4:53 PM Alexander Monakov via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Greetings, David, community,
>
> I'd like to get your input on how GCC command line interface should support
> making a "tiered" warning like -Warray-bounds={1,2} an error for "tier 1"
> where fewer false positives are expected, and a plain warning otherwise.
>
> There was a recent thread mentioning the current limitation [1]:
>
> > This also shows nicely why I don't like warnings with levels, what if I want
> > -Werror=array-bounds=2 + -Warray-bounds=1?
>
> Also in PR 48088 [2] there was a request to make it work for stack size usage:
>
> > Stumbled on this bug today. I tried to use it in more intricate way:
> >
> >     -Wframe-larger-than=4096 -Werror=frame-larger-than=32768
> >
> > which would only warn about any stack more than 4096+, but would fail on
> > 32768+.
> >
> > Does it make sense to implement desired behaviour?
> > I guess it's not many '>=number' style options in gcc.
>
> A problem with implementing DWIM semantics like above for -Wfoo=k -Werror=foo=n
> combination is that technically it changes its current meaning.
>
> If we don't want to risk that, an alternative is to introduce a new option
> for selecting error threshold for a tiered warning, for example:
>
>   -Warray-bounds=2 -Werror-level=array-bounds=1
>
> Implementation-wise, we would need to extend common.opt language to annotate
> which tier is more inclusive (generally smaller 'n' means fewer warnings, but
> for -Wstack-usage and -Wstrict-aliasing it's the other way around).

Implementation-wise one could do a similar trick as we have for
global_options vs. global_options_set - add a global_options_error copy (ick!)
(and global_options_error_set!?) and have the logic that decides whether
a warning is an error pick that set.  Of course it's the actual pass that looks
at flag_xyz which holds the magic number so that pass would need to be able
to see whether it needs to look at both numbers.

Btw, does '-Werror=array-bounds=2' imply that =1 kinds are diagnosed as
warning?  Does it enable -Warray-bounds=2?

I think the DWIM behavior isn't all that clear and probably depends on the
actual option we want to make it work.

Richard.

>
> Opinions? Does anybody envision problems with going the DWIM way?
>
> Thanks.
> Alexander
>
> [1] https://inbox.sourceware.org/gcc-patches/2552ab22-916f-d0fe-2c78-d482f6ad8412@lauterbach.com/
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48088#c5