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

From: Geoff Levand <[email-masked]>
Date: Fri, 01 Dec 2006 08:51:05 -0800

Linus Torvalds wrote:
> 
> 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. 

One of the gcc maintainers (Andrew Pinski) told me to set it up that
way, so I figured it was safe.

-Geoff