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

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Date:	Thu, 17 Oct 2013 03:38:24 -0400
From:	Chen Gong <gong.chen@...ux.intel.com>
To:	Joe Perches <joe@...ches.com>
Cc:	tony.luck@...el.com, bp@...en8.de, naveen.n.rao@...ux.vnet.ibm.com,
	m.chehab@...sung.com, arozansk@...hat.com,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Winischhofer <thomas@...ischhofer.net>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

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@...ches.com>
> To: Chen Gong <gong.chen@...ux.intel.com>
> Cc: tony.luck@...el.com, bp@...en8.de, naveen.n.rao@...ux.vnet.ibm.com,
>  m.chehab@...sung.com, arozansk@...hat.com, linux-acpi@...r.kernel.org,
>  linux-kernel@...r.kernel.org, Thomas Winischhofer
>  <thomas@...ischhofer.net>, Jean-Christophe Plagniol-Villard
>  <plagnioj@...osoft.com>, Tomi Valkeinen <tomi.valkeinen@...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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