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

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Message-ID: <20201121204626.tw3jribsz326e3ag@ltop.local>
Date:   Sat, 21 Nov 2020 21:46:26 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Jakub Jelinek <jakub@...hat.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 09:29:54PM +0100, Jakub Jelinek wrote:
> 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 Just for info, the description is outdated and Sparse is just fine with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 Why is the description outdated?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.

I was only talking about the part "Use this where *sparse* expects a true
constant expression, e.g. for array 75 indices." and wanted to say that 
__builtin_clzll() with a constant argument is now also expanded to a
constant, like GCC does (it wasn't the case before 2017 and I think
it was the main reason why const_ilog2() is written as it is).

-- Luc