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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 1 Feb 2021 13:41:35 -0800
Message-ID: <CAKwvOdkZRdBzzW19sVAs+pX-7wWwN6AWrxUkkZwP8L4OT7SLfQ@mail.gmail.com>
Subject: Re: [PATCH v2] ext4: Enable code path when DX_DEBUG is set
To:     "Theodore Ts'o" <tytso@mit.edu>
Cc:     Vinicius Tinti <viniciustinti@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Mon, Feb 1, 2021 at 1:38 PM Theodore Ts'o <tytso@mit.edu> wrote:
>
> On Mon, Feb 01, 2021 at 01:16:19PM -0800, Nick Desaulniers wrote:
> > I agree; Vinicius, my recommendation for -Wunreachable-* with Clang
> > was to see whether dead code identified by this more aggressive
> > diagnostic (than -Wunused-function) was to ask maintainers whether
> > code identified by it was intentionally dead and if they would
> > consider removing it.  If they say "no," that's fine, and doesn't need
> > to be pushed.  It's not clear to maintainers that:
> > 1. this warning is not on by default
> > 2. we're not looking to pursue turning this on by default
> >
> > If maintainers want to keep the dead code, that's fine, let them and
> > move on to the next instance to see if that's interesting (or not).
>
> It should be noted that in Documenting/process/coding-style.rst, there
> is an expicit recommendation to code in a way that will result in dead
> code warnings:
>
>    Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
>    symbol into a C boolean expression, and use it in a normal C conditional:
>
>    .. code-block:: c
>
>         if (IS_ENABLED(CONFIG_SOMETHING)) {
>                 ...
>         }
>
>    The compiler will constant-fold the conditional away, and include or exclude
>    the block of code just as with an #ifdef, so this will not add any runtime
>    overhead.  However, this approach still allows the C compiler to see the code
>    inside the block, and check it for correctness (syntax, types, symbol
>    references, etc).  Thus, you still have to use an #ifdef if the code inside the
>    block references symbols that will not exist if the condition is not met.
>
> So our process documentation *explicitly* recommends against using
> #ifdef CONFIG_XXX ... #endif, and instead use something that will
> -Wunreachable-code-aggressive to cause the compiler to complain.

I agree.

>
> Hence, this is not a warning that we will *ever* be able to enable
> unconditionally ---

I agree.

> so why work hard to remove such warnings from the
> code?  If the goal is to see if we can detect real bugs using this

Because not every instance of -Wunreachable-code-aggressive may be that pattern.

> technique, well and good.  If the data shows that this warning
> actually is useful in finding bugs, then manybe we can figure out a
> way that we can explicitly hint to the compiler that in *this* case,
> the maintainer actually knew what they were doing.
>
> But if an examination of the warnings shows that
> -Wunreachable-code-aggressive isn't actually finding any real bugs,
> then perhaps it's not worth it.

I agree. Hence the examination of instances found by Vinicius.
-- 
Thanks,
~Nick Desaulniers