linux-kernel - Re: [PATCH] minmax: clamp more efficiently by avoiding extra comparison

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Message-ID: <CAHmME9osJiKg8-o-OdfCPS6t_fZ=zgGKZdy0CgybaK2NDv8XLA@mail.gmail.com>
Date:   Fri, 23 Sep 2022 12:40:47 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] minmax: clamp more efficiently by avoiding extra comparison

Hi Andy,

On Fri, Sep 23, 2022 at 12:36 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Fri, Sep 23, 2022 at 12:06:21PM +0200, Jason A. Donenfeld wrote:
> > Currently the clamp algorithm does:
> >
> >       if (val > hi)
> >               val = hi;
> >       if (val < lo)
> >               val = lo;
> >
> > But since hi > lo by definition, this can be made more efficient with:
>
> It's strongly speaking, but we have to proof that, right?
> So, while I haven't checked the code, this change should also
> include (does it?) the corresponding compile-time checks (for
> constant arguments) in similar way how it's done for GENMASK().
>
> Otherwise I have no objections.

I think most cases are with compile time constants, but some cases are
with variables. What should we do in that case? Checking variables at
runtime incurs the same cost as the old code. I guess we could do this
fast thing for constants and the slower old thing for non-constants?
Or not do either, keep this commit as is, and just accept that if you
pass bogus bounds to clamp, you're going to end up with something
weird, which is already the case now so not a big deal?

Jason