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To:     Jani Nikula <jani.nikula@intel.com>, linux-kernel@vger.kernel.org
From:   Chris Wilson <chris@chris-wilson.co.uk>
In-Reply-To: <20190301125207.30973-1-jani.nikula@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, jani.nikula@intel.com
References: <20190301125207.30973-1-jani.nikula@intel.com>
Message-ID: <155144567811.6459.11821776880949183246@skylake-alporthouse-com>
User-Agent: alot/0.6
Subject: Re: [PATCH] log2: make is_power_of_2() integer constant expression when
 possible
Date:   Fri, 01 Mar 2019 13:07:58 +0000
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Quoting Jani Nikula (2019-03-01 12:52:07)
> While is_power_of_2() is an inline function and likely gets optimized
> for compile time constant arguments, it still doesn't produce an integer
> constant expression that could be used in, say, static data
> initialization or case labels.
> 
> Make is_power_of_2() an integer constant expression when possible,
> otherwise using the inline function to avoid multiple evaluation of the
> parameter.
> 
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

It does what it says on the tin and allows for

static const is_pot = is_power_of_two(32);

or in the actual case of interest,

BUILD_BUG_ON(is_power_of_two(x) ? test1(x) : test2(x)).

The only question remains can build_bug.h include ilog2.h and use this
macro instead of having its own copy? Or that may be overkill for a
single commonplace macro.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris