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From:   Jani Nikula <jani.nikula@intel.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org,
        Chris Wilson <chris@chris-wilson.co.uk>
Subject: Re: [PATCH] log2: make is_power_of_2() integer constant expression when possible
In-Reply-To: <20190301122659.1776b678fadcc16c1fb2ab61@linux-foundation.org>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20190301125207.30973-1-jani.nikula@intel.com> <20190301122659.1776b678fadcc16c1fb2ab61@linux-foundation.org>
Date:   Mon, 04 Mar 2019 12:16:02 +0200
Message-ID: <875zszndwd.fsf@intel.com>
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On Fri, 01 Mar 2019, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Fri,  1 Mar 2019 14:52:07 +0200 Jani Nikula <jani.nikula@intel.com> wrote:
>
>> 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.
>
> hm, what code wants to do these things?
>
>> Make is_power_of_2() an integer constant expression when possible,
>> otherwise using the inline function to avoid multiple evaluation of the
>> parameter.
>
> Spose so.  But I fear that some gcc versions will get it right and
> others will mess it up.  While this patch is under test I think it
> would be best to also have at least one or two callsites which actually
> use the compile-time evaluation feature.  Possible?

I have some drm/i915 patches that have a local compile-time version of
IS_POWER_OF_2(). It'll take a while before they hit Linus' master. This
can wait.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center