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

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Message-ID: <20201121202954.GW3788@tucnak>
Date:   Sat, 21 Nov 2020 21:29:54 +0100
From:   Jakub Jelinek <jakub@...hat.com>
To:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
        rdunlap@...radead.org
Subject: Re: [PATCH] ilog2: Improve ilog2 for constant arguments

On Sat, Nov 21, 2020 at 09:23:10PM +0100, Luc Van Oostenryck wrote:
> 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 Just for info, the description is outdated and Sparse is just fine with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)

Why is the description outdated?  It is still an extension that not every
compiler might fold in constant expressions.  And, the large expressions
aren't really a problem in constant expressions, they will be folded there
to constant or error.
The problem the patch was trying to solve is that the large expressions are
a problem at least for GCC in runtime code when guarded by
__builtin_constant_p, because __builtin_constant_p is folded quite late
(intentionally so, so that more constants can be propagated into it, e.g.
after inlining etc.), and the large expressions might confuse inliner
heuristics.

	Jakub