[Bug c/38470] value range propagation (VRP) would improve -Wsign-compare

["mkretz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38470

--- Comment #29 from Matthias Kretz (Vir) <mkretz at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #28)
> (In reply to Matthias Kretz (Vir) from comment #27)
> > Fair enough. But how can the compiler be certain that the developer realized
> > u and u % 100 is unsigned? Maybe when (s)he wrote the code the expectation
> > was for the RHS to be within [-99..99].
> 
> Indeed. (I never use % when its LHS can be either positive or negative, so
> that I didn't think about this case.)

FWIW, personally I could use a warning whenever I use % on variables that can
potentially be negative. Because my mental model of % N is that I get a result
in [0..N-1]. I tried to fix it but my head is stubborn. ;)

> Now, the cause of the bug in such a case would be that the user messed up
> with the signedness of u, not because he forgot about the promotion rule.
> This is something rather different.

Right. But if that's the case wouldn't a warning about the unexpected promotion
be useful? (which -Wsign-compare happens to provide)

To reduce the noise, I won't argue this point any further and let whoever
actually implements a fix for the PR decide. :)