Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

Joe Perches
2013-10-17T06:59:01Z

On Thu, 2013-10-17 at 02:30 -0400, Chen Gong wrote:
> On Wed, Oct 16, 2013 at 07:59:09PM -0700, Joe Perches wrote:
[]
> > Maybe add a
> >
> > BUILD_BUG_ON(__builtin_constant_p(l) && __builtin_constant_p(h) && \
> > (h) < (l))
> >
> No, if so, users can't use variables for this macro.

__builtin_constant_p checks for constants

Built-in Function: int __builtin_constant_p (exp)
You can use the built-in function __builtin_constant_p to
determine if a value is known to be constant at compile-time and
hence that GCC can perform constant-folding on expressions
involving that value. The argument of the function is the value
to test. The function returns the integer 1 if the argument is
known to be a compile-time constant and 0 if it is not known to
be a compile-time constant. A return of 0 does not indicate that
the value is not a constant, but merely that GCC cannot prove it
is a constant with the specified value of the -O option.