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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 14 Jan 2020 13:49:03 -0800
Message-ID: <CAHk-=wjX-c9YpPhbQ073UPnTvELNQCN49vqK1yY7JGuHSn5-ew@mail.gmail.com>
Subject: Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user
To:     Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>
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On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:
>
> On 1/14/20 12:42 PM, Arnd Bergmann wrote:
> >
> > What's wrong with the generic version on little-endian? Any
> > chance you can find a way to make it work as well for you as
> > this copy?
>
> find_zero() by default doesn't use pop count instructions.

Don't you think the generic find_zero() is likely just as fast as the
pop count instruction? On 32-bit, I think it's like a shift and a mask
and a couple of additions.

The 64-bit case has a multiply that is likely expensive unless you
have a good multiplication unit (but what 64-bit architecture
doesn't?), but the generic 32-bit LE code should already be pretty
close to optimal, and it might not be worth it to worry about it.

(The big-endian case is very different, and architectures really can
do much better. But LE allows for bit tricks using the carry chain)

             Linus