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
