linux-kernel - Re: [RFC PATCH 2/2] treewide, bits: use ffs

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Message-ID: <alpine.DEB.2.21.2601110235310.30566@angie.orcam.me.uk>
Date: Sun, 11 Jan 2026 03:15:22 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: David Laight <david.laight.linux@...il.com>
cc: Yury Norov <ynorov@...dia.com>, Petr Tesarik <ptesarik@...e.com>, 
    Yury Norov <yury.norov@...il.com>, 
    Rasmus Villemoes <linux@...musvillemoes.dk>, 
    Richard Henderson <richard.henderson@...aro.org>, 
    Matt Turner <mattst88@...il.com>, Magnus Lindholm <linmag7@...il.com>, 
    Vineet Gupta <vgupta@...nel.org>, 
    Geert Uytterhoeven <geert@...ux-m68k.org>, 
    Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
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    Michael Ellerman <mpe@...erman.id.au>, Heiko Carstens <hca@...ux.ibm.com>, 
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    Alexander Gordeev <agordeev@...ux.ibm.com>, 
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    Patrik Jakobsson <patrik.r.jakobsson@...il.com>, 
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    Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, 
    Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
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    Andrew Morton <akpm@...ux-foundation.org>, 
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    Luiz Augusto von Dentz <luiz.dentz@...il.com>, 
    Pablo Neira Ayuso <pablo@...filter.org>, Florian Westphal <fw@...len.de>, 
    linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] treewide, bits: use ffs_val() where it is
 open-coded

On Sat, 10 Jan 2026, David Laight wrote:

> > > > diff --git a/arch/mips/dec/ecc-berr.c b/arch/mips/dec/ecc-berr.c
> > > > index 1eb356fdd8323..8934b8b1cf375 100644
> > > > --- a/arch/mips/dec/ecc-berr.c
> > > > +++ b/arch/mips/dec/ecc-berr.c
> > > > @@ -153,7 +153,7 @@ static int dec_ecc_be_backend(struct pt_regs *regs, int is_fixup, int invoker)
> > > >  			/* Ack now, now we've rewritten (or not). */
> > > >  			dec_ecc_be_ack();
> > > >  
> > > > -			if (syn && syn == (syn & -syn)) {
> > > > +			if (syn && syn == ffs_val(syn)) {  
> > > 
> > > It opencodes is_power_of_2().
> 
> Badly...

 It's 4 MIPS instructions and exactly the same machine code produced as 
compiler output from `is_power_of_2'.  If you know of a better alternative 
I'll be happy to get enlightened, however it's academic anyway, as it's 
the ECC memory error handler.  If you get here, then you have a bigger 
issue than a couple of extra instructions executed per kernel log entry 
produced.  If you indeed can get rid of them in the first place, that is.

  Maciej