Does GCC really recognize __attribute__ ((bitwise))?

Does GCC really recognize __attribute__ ((bitwise))?

[Eus]

Hi Ho!

When browsing the Linux kernel 2.6.21.5's source code, I noticed a variable
definition like:

typedef __u64 __bitwise __be64;

Being curious about __bitwise, I found it out to be:

#define __bitwise __bitwise__

that finally turned out to be:

#define __bitwise__ __attribute__((bitwise))

Since it was __attribute__, I tried to find its documentation in GCC texinfo
file. But, I couldn't find one. Even I have tried to find it out in the Internet.
But, I still couldn't find one.

Is it a custom defined attribute? If it is, is the way to do such thing
documented in GCC texinfo? If it is not, why does it not described in GCC texinfo
file?

Thank you very much.

Best regards,
Eus (FSF member #4445)

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Re: Does GCC really recognize __attribute__ ((bitwise))?

[Ian Lance Taylor]

Eus <eus@member.fsf.org> writes:

> When browsing the Linux kernel 2.6.21.5's source code, I noticed a variable
> definition like:
>
> typedef __u64 __bitwise __be64;
>
> Being curious about __bitwise, I found it out to be:
>
> #define __bitwise __bitwise__
>
> that finally turned out to be:
>
> #define __bitwise__ __attribute__((bitwise))
>
> Since it was __attribute__, I tried to find its documentation in GCC texinfo
> file. But, I couldn't find one. Even I have tried to find it out in the Internet.
> But, I still couldn't find one.

There is no "bitwise" attribute in standard gcc.

Looking at the Linux 2.6.24 sources, I see that it is defined as an
attribute only when __CHECKER__ is defined.  This has something to do
with sparse--see Documentation/sparse.txt.  I don't know the details.

Ian

Re: Does GCC really recognize __attribute__ ((bitwise))?

[Sam Ravnborg]

On Thu, Mar 27, 2008 at 10:39:22AM -0700, Ian Lance Taylor wrote:
> Eus <eus@member.fsf.org> writes:
> 
> > When browsing the Linux kernel 2.6.21.5's source code, I noticed a variable
> > definition like:
> >
> > typedef __u64 __bitwise __be64;
> >
> > Being curious about __bitwise, I found it out to be:
> >
> > #define __bitwise __bitwise__
> >
> > that finally turned out to be:
> >
> > #define __bitwise__ __attribute__((bitwise))
> >
> > Since it was __attribute__, I tried to find its documentation in GCC texinfo
> > file. But, I couldn't find one. Even I have tried to find it out in the Internet.
> > But, I still couldn't find one.
> 
> There is no "bitwise" attribute in standard gcc.
> 
> Looking at the Linux 2.6.24 sources, I see that it is defined as an
> attribute only when __CHECKER__ is defined.  This has something to do
> with sparse--see Documentation/sparse.txt.  I don't know the details.

Correct.
We have in the kernel a number of sparse specific extensions.
bitwise, __iomem and __user is the ones most widely used.
sparse uses this to do additional checks that gcc not has context to do.

The __user annotation is used to annotate pointers to data in userspace
which may not be mixed up with pointers in kernel space.

	Sam