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Date: Sun, 06 Nov 2011 21:52:30 +0100
From: Dimitry Andric <dim@FreeBSD.org>
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To: Alexander Best <arundel@freebsd.org>
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Cc: freebsd-toolchain@freebsd.org
Subject: Re: [poc] buildkernel + clang + -Werror
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On 2011-11-06 21:33, Alexander Best wrote:
... 
> the problem is, something like
> 
> uint x;
> 
> if (x < 0) ...
> 
> clang will warn about this, yet it is 100% valid code so my vote would be to
> make such an error into a warning.

Sorry, but checking something unsigned to be smaller than zero is bogus,
or at the least superfluous, and it's perfectly sane to warn about this,
especially since the compiler is not going to emit code for it at all.

The only time when this sort of check could be relevant, is when you are
using a typedef'd type, and you have no way to be sure if the type is
signed or unsigned.  But then you will be in for trouble anyway...


...
> the same with 
> 
> int x;
> 
> x = x;
> 
> i believe in both cases clang is too picky.

This is a often-used idiom for attempting to 'shut up' a compiler when
it (quite legitimately) complains about unused variables or arguments.

However, a better idiom is:

  (void) x;

but even this can lead some compilers or analysis tools to complain
about "statements that don't do anything".  A better way is to provide
UNUSED_VARIABLE and UNUSED_ARGUMENT macros, and handle shutting up the
compiler in its specific way there.

Of course, the best solution is to just eliminate the unused variables
or arguments.  If that is not possible, for example due to a function
signature that cannot be changed because of API considerations, you can
always use __unused attributes.