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

Chen Gong
2013-10-17T07:53:22Z

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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 <joe@perches.com>
> To: Chen Gong <gong.chen@linux.intel.com>
> Cc: tony.luck@intel.com, bp@alien8.de, naveen.n.rao@linux.vnet.ibm.com,
> m.chehab@samsung.com, arozansk@redhat.com, linux-acpi@vger.kernel.org,
> linux-kernel@vger.kernel.org, Thomas Winischhofer
> <thomas@winischhofer.net>, Jean-Christophe Plagniol-Villard
> <plagnioj@jcrosoft.com>, Tomi Valkeinen <tomi.valkeinen@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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