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

[Michael Ellerman <michael@ellerman.id.au>]

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On Fri, 2006-12-01 at 15:55 +0100, Geert Uytterhoeven wrote:
> On Fri, 1 Dec 2006, Al Viro wrote:
> > On Fri, Dec 01, 2006 at 03:25:08PM +0100, 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.
> > 
> > FWIW, that code is *not* valid C99; note that all enumeration members must
> > fit the range of int (see 6.7.2.2.2).  What you quote speaks about the
> 
> You're right. I missed that one.
> 
> > IOW, you are using a gccism in an area where gcc is bloody inconsistent
> > in the set of bugs it shows in different versions.
> 
> Hmmm...
> 
> > Generally safe way is to split the anonymous huge enum members into
> > single-element enums and pray that gcc will at least stay consistent
> > in handling of those.
> 
> Or fall back to #defines, which is what we were trying to avoid in the first
> place (i.e. group related values together in enums)...

The enum achieves very little in this case IMHO, it's just a more
verbose way of #defining - and it doesn't even have reliable semantics.

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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