linux-kernel - Re: [PATCH] log2: make is_power_of_2() integer constant expression when possible

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Message-Id: <20190301122659.1776b678fadcc16c1fb2ab61@linux-foundation.org>
Date:   Fri, 1 Mar 2019 12:26:59 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH] log2: make is_power_of_2() integer constant expression
 when possible

On Fri,  1 Mar 2019 14:52:07 +0200 Jani Nikula <jani.nikula@...el.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?