Re: cast truncates bits from constant value (8000000000000000 becomes 0)

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 1 Dec 2006, Geert Uytterhoeven wrote:
>
> On Tue, 21 Nov 2006, Geoff Levand wrote:
> > +enum ps3_vendor_id {
> > +	PS3_VENDOR_ID_NONE = 0,
> > +	PS3_VENDOR_ID_SONY = 0x8000000000000000UL,
> > +};
> 
> I've just ran `make C=1' (PPC in 64-bit mode, and sparse is called with -m64),
> and noticed that sparse (cloned from
> git://git.kernel.org/pub/scm/devel/sparse/sparse.git a few minutes ago)
> complains about the second value with:
> 
> | warning: cast truncates bits from constant value (8000000000000000 becomes 0)
> 
> Section 6.7.2.2.4 of C99 says:
> 
> | Each enumerated type shall be compatible with char, a signed integer type, or
> | an unsigned integer type. The choice of type is implementation-defined, but
> | shall be capable of representing the values of all the members of the
> | enumeration.
> 
> The code snippet
> 
> | u64 x = PS3_VENDOR_ID_SONY;
> | printk("PS3_VENDOR_ID_SONY = %lu\n", x);
> 
> does print the expected (i.e. non-zero) result.
> 
> Hence this looks like a bug in sparse.

It's really a bug in gcc, but it's documented, so it's a "feature".

Gcc allows large enums, but does so in such a strange manner that it's 
totally hopeless to catch problems. Also, putting a value that is larger 
than "unsigned int" into an enum is really setting yourself up for bugs 
and not even guaranteed to work for standard C, so sparse takes a dim view 
of it and just says that enums are limited in size to "int" or "unsigned 
int".

You can either ignore that warning or just use a #define. 

			Linus