linux-kernel - Re: [RFC] clang: 'unused-function' warning on static inline functions

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Date:   Tue, 6 Jun 2017 15:29:07 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Rientjes <rientjes@...gle.com>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Mark Brown <broonie@...nel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC] clang: 'unused-function' warning on static inline functions

On 06/06/2017 03:23 PM, Matthias Kaehlcke wrote:
> El Tue, Jun 06, 2017 at 09:32:35AM -0700 Linus Torvalds ha dit:
> 
>> On Tue, Jun 6, 2017 at 4:16 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>>
>>> Those should all be fairly easy to address, I'd vote for leaving the
>>> warning enabled
>>> in clang, and possibly asking the gcc maintainers to add a similar feature for
>>> warning about it.
>>
>> Hell no. That warning is pointless shit.
> 
> I tend to disagree, the warning is useful to detect truly unused
> static inline functions, which should be removed, rather than be
> carried around/maintained for often long periods of time.

One example is the patch sent for CFQ, which has macros for define
functions for setting/clearing bits on the queue:

#define CFQ_CFQQ_FNS(name)

for one bit, we never clear the bit after we set it, we only set it and
later test for it. Hence the clear variant of that function is unused.

Now I get a warning. The fix to that would be to define a new variant of
CFQ_CFQQ_FNS() that only declares the exact one I need for the version
that is never cleared.

Or the fix is to just ignore the bogus warning on an unused inline.  I
greatly prefer the latter.

The counter example is this one:

http://git.kernel.dk/cgit/linux-block/commit/?id=03ea8ad78cfb2910862c8dfcd2a627fc04097db2

where it is truly just dead junk. I'd rather just leave the dead junk
than have pointless warnings, if I have to choose one of the two
outcomes.

-- 
Jens Axboe