Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Fri, 23 Sep 2022 12:48:48 +0200 | Subject | Re: [PATCH] minmax: clamp more efficiently by avoiding extra comparison |
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Hey again,
On Fri, Sep 23, 2022 at 12:40 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > Hi Andy, > > On Fri, Sep 23, 2022 at 12:36 PM Andy Shevchenko > <andriy.shevchenko@linux.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?
Actually, yea, I think we should keep this commit as-is and not add additional checking becauseeeee not only is hi>lo by definition, but both for the old code and for the new code, the result of lo>hi is total nonsense:
Assuming hi > lo, these snippets all yield the same result:
if (val > hi) val = hi; if (val < lo) val = lo;
if (val > hi) val = hi; else if (val < lo) val = lo;
if (val < lo) val = lo; if (val > hi) val = hi;
if (val < lo) val = lo; else if (val > hi) val = hi;
Assuming lo > hi, and the first condition triggers, these snippets all yield different results, all of which are undefined nonsense:
if (val > hi) val = hi; if (val < lo) val = lo; --> val is lo
if (val > hi) val = hi; else if (val < lo) val = lo; --> val is hi
if (val < lo) val = lo; if (val > hi) val = hi; --> val is hi
if (val < lo) val = lo; else if (val > hi) val = hi; --> val is lo
Jason
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