linux-kernel - Re: [PATCH] ilog2: Improve ilog2 for constant arguments

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Message-ID: <20201121202310.lx2iqqebal3fmmmo@ltop.local>
Date:   Sat, 21 Nov 2020 21:23:10 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
        Jakub Jelinek <jakub@...hat.com>, rdunlap@...radead.org
Subject: Re: [PATCH] ilog2: Improve ilog2 for constant arguments

On Fri, Nov 20, 2020 at 01:51:54PM +0100, Peter Zijlstra wrote:
> 
> Other option would be to change the const_ilog2 macro, though as the
> description says it is meant to be used also in C constant expressions,
> and while GCC will fold it to constant with constant argument even in
> those, perhaps it is better to avoid using extensions in that case.

Just for info, the description is outdated and Sparse is just fine with
__builtin_clzll() and friends in constant expressions (since Feb 2017)
 
-- Luc