Re: No warning for unreachable code

[Guennadi Liakhovetski <g dot liakhovetski at gmx dot de>]

Hi Segher,

On Wed, 4 Jun 2008, Segher Boessenkool wrote:

> > I just discovered a behaviour in gcc-4, which I don't understand: code
> > 
> > 	unsigned int x;
> > 
> > 	if (x < 0)
> > 		do_something();
> > 
> > compiled with -Wall doesn't produce a warning, and the call to
> > do_something() is silently dropped, whereas if x is of type unsigned char,
> > I get as expected
> > 
> > warning: comparison is always false due to limited range of data type
> > 
> > Is there a reason for such behaviour or is this a bug?
> 
> With -Wextra I get with both 4.2.4 and current TOT:
> 
> x.c: In function 'f_int':
> x.c:5: warning: comparison of unsigned expression < 0 is always false
> x.c: In function 'f_char':
> x.c:11: warning: comparison is always false due to limited range of data type
> 
> With just -Wall, 4.2.4 gives only the warning you mention, and TOT
> gives nothing.
> 
> Are you suggesting that both warnings should be in -Wall?  Or are
> you telling us that you should be using -Wextra ;-)

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

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.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer