No warning for unreachable code

[Segher Boessenkool]

> I'm suggesting that there's no big difference between unsigned char and
> unsigned int (and unsigned long...) in this case, and, therefore
> compiler's behaviour should be consistent.

But there is a difference.

When "x" is an unsigned int, the expression "x < 0" is equivalent to
	(unsigned int) x < (unsigned int) 0
which can never be true, whatever "x" is, and results in
	warning: comparison of unsigned expression < 0 is always false

When "x" is an unsigned char however, it reads
	(int) x < (int) 0
which only cannot happen because of the particular values "x" can take,
and the warning is
	warning: comparison is always false due to limited range of data type

Sounds sane to me, and both warning messages are clear.

> And in this case I personally
> would prefer a warning in both cases with -Wall.

Then please file a PR in bugzilla so your request won't get lost.

It might be that the warning isn't in -Wall because it has too many
false positives.  This is just a guess though.

> And this wasn't a purely theoretical observation. There is a driver in 
> the
> Linux kernel, that does exactly this: assigns function return code to 
> an
> unsigned int variable, and then checks for "<0"... And the compiler
> happily throws the check away. Yes, I will submit a patch, but if 
> there is
> no strong reason against, maybe it would make sense to warn in these
> cases.

I can try to shift the blame and say the kernel should be built with
-Wextra .  But then people will shout at me again, so I won't.


Segher