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

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Message-ID: <Yy3MgMhYDFYnLAV/@smile.fi.intel.com>
Date:   Fri, 23 Sep 2022 18:10:56 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Jason A. Donenfeld" <Jason@...c4.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

On Fri, Sep 23, 2022 at 12:40:47PM +0200, Jason A. Donenfeld wrote:
> 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?

I'm talking only for the cases where we _can_ check. For variables it's
probably tricky to do at compile time if possible at all.

-- 
With Best Regards,
Andy Shevchenko