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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
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