On Wed, Oct 16, 2013 at 11:58:56PM -0700, Joe Perches wrote: > Date: Wed, 16 Oct 2013 23:58:56 -0700 > From: Joe Perches <j...@perches.com> > To: Chen Gong <gong.c...@linux.intel.com> > Cc: tony.l...@intel.com, b...@alien8.de, naveen.n....@linux.vnet.ibm.com, > m.che...@samsung.com, aroza...@redhat.com, linux-a...@vger.kernel.org, > linux-kernel@vger.kernel.org, Thomas Winischhofer > <tho...@winischhofer.net>, Jean-Christophe Plagniol-Villard > <plagn...@jcrosoft.com>, Tomi Valkeinen <tomi.valkei...@ti.com> > Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro > X-Mailer: Evolution 3.6.4-0ubuntu1 > > 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. >
Yes, even we have following codes __builtin_constant_p still can return 1,
so long as the value of variable can be identified.
int len = sizeof(int);
if (__builtin_constant_p(len)) {
do_1;
} else {
do_0;
}
but the point is we can use GENMASK like GENMASK(end_bit, start_bit) but
we don't know the value of end_bit/start_bit at compile-time.
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