linux-kernel - Re: [PATCH 1/5] ARM: findbit: document ARMv5 bit offset calculation

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Message-ID: <CAHk-=wi63Sw3vNJ86gzg1Tdr=_xGwGyj+mH-eT0UgaAfGAHX+A@mail.gmail.com>
Date:   Fri, 28 Oct 2022 10:05:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>
Cc:     Yury Norov <yury.norov@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/5] ARM: findbit: document ARMv5 bit offset calculation

On Fri, Oct 28, 2022 at 9:47 AM Russell King (Oracle)
<rmk+kernel@...linux.org.uk> wrote:
>
> Document the ARMv5 bit offset calculation code.

Hmm. Don't the generic bitop functions end up using this? We do have a
comment in the code that says

 * On ARMv5 and above, the gcc built-ins may rely on the clz instruction
 * and produce optimal inlined code in all cases. On ARMv7 it is even
 * better by also using the rbit instruction.

but that 'may' makes me wonder...

IOW, what is it in the hand-written code that doesn't get done by the
generic code these days?

                     Linus