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Date:   Thu, 22 Oct 2020 11:37:39 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, jakub@redhat.com,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64()
Message-ID: <20201022163739.GU2672@gate.crashing.org>
References: <348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu>
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On Thu, Oct 22, 2020 at 02:05:46PM +0000, Christophe Leroy wrote:
> fls() and fls64() are using __builtin_ctz() and _builtin_ctzll().
> On powerpc, those builtins trivially use ctlzw and ctlzd power
> instructions.
> 
> Allthough those instructions provide the expected result with
> input argument 0, __builtin_ctz() and __builtin_ctzll() are
> documented as undefined for value 0.

> When the input of fls(x) is a constant, just check x for nullity and
> return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction
> directly.

That looks good :-)

Acked-by: Segher Boessenkool <segher@kernel.crashing.org>


Segher