linux-kernel - Re: [patch] compiler, clang: suppress warning for unused static inline functions

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Date:   Wed, 24 May 2017 14:22:29 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [patch] compiler, clang: suppress warning for unused static
 inline functions

El Wed, May 24, 2017 at 02:01:15PM -0700 David Rientjes ha dit:

> GCC explicitly does not warn for unused static inline functions for
> -Wunused-function.  The manual states:
> 
> 	Warn whenever a static function is declared but not defined or
> 	a non-inline static function is unused.
> 
> Clang does warn for static inline functions that are unused.
> 
> It turns out that suppressing the warnings avoids potentially complex
> #ifdef directives, which also reduces LOC.
> 
> Supress the warning for clang.
> 
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---

As expressed earlier in other threads, I don't think gcc's behavior is
preferable in this case. The warning on static inline functions (only
in .c files) allows to detect truly unused code. About 50% of the
warnings I have looked into so far fall into this category.

In my opinion it is more valuable to detect dead code than not having
a few more __maybe_unused attributes (there aren't really that many
instances, at least with x86 and arm64 defconfig). In most cases it is
not necessary to use #ifdef, it is an option which is preferred by
some maintainers. The reduced LOC is arguable, since dectecting dead
code allows to remove it.

I'm not a kernel maintainer, so it's not my decision whether this
warning should be silenced, my personal opinion is that it's benfits
outweigh the inconveniences of dealing with half-false positives,
generally caused by the heavy use of #ifdef by the kernel itself.