[cfe-commits] PATCH: Warn on left shifts which overflow, even when not due to the RHS

[Abramo Bagnara]

Il 21/02/2011 05:21, Chandler Carruth ha scritto:
> This patch (originally by Oleg Slezberg) implements a warning that would
> have caught several major bugs we found, so I'm pretty excited about it.
> The fundamental premise is that the user writes code as follows:
> 
> int64_t i = 10 << 30;
> 
> The user thinks this is a nice 10 gig constant for their disk size. The
> compiler thinks this is -2 gigs. Oops.
> 
> As it turns out, this is technically undefined behavior, and so it seems
> doubly good to warn about it when we see it. The warning has a special
> case though. It still provides a warning for the expression (1 << 31)
> because it's undefined behavior to do this, but that warning is by
> default ignored. If the value is converted to an unsigned integer, it
> will have the intended value as all the bits did in fact get set. It
> seems much less important to warn about this case. The result looks
> something like:

I don't know if this is relevant for your design, but it should be noted
that according to C++ standard (and differently from C99 standard) the
excess left shift of signed integers is not undefined behaviour, but it
is implementation dependent (see C++03 5.8p2), i.e. as the shift should
be implemented as a logical shift the result depends from integer
representation (C++ allows 1's complement, 2's complement and signed
magnitude).