linux-kernel - Re: [PATCH 0/4] log2: make is_power_of

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Message-Id: <20230330125041.83b0f39fa3a4ec1a42dfd95f@linux-foundation.org>
Date:   Thu, 30 Mar 2023 12:50:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        Christian König <christian.koenig@....com>,
        David Gow <davidgow@...gle.com>
Subject: Re: [PATCH 0/4] log2: make is_power_of_2() more generic

On Thu, 30 Mar 2023 13:42:39 +0300 Jani Nikula <jani.nikula@...el.com> wrote:

> is_power_of_2() only works for types <= sizeof(unsigned long) and it's
> also not a constant expression. There are a number of places in kernel
> where is_power_of_2() is called on u64, which fails on 32-bit
> builds. Try to remedy that. While at it, make it a constant expression
> when possible.

Yes, the current `is_power_of_2(unsigned long n)' isn't very general.

But wouldn't all these issues be addressed by simply doing

#define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))

?

(With suitable tweaks to avoid evaluating `n' more than once)