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Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)"

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On 11/24/2014 3:58 PM, Andreas Leitgeb wrote:
> Out of idle curiosity I stepped into StringBuilder.append(int),
> and reached non-public Integer.getChars(int,int,char[]).
>
> "    // really: r = i - (q * 100);
> "    r = i - ((q << 6) + (q << 5) + (q << 2));
>
> Is this some relict from early early days, or are 5 operations
> (3 "<<" and 2 "+") still faster than one multiplication on
> modern cpus?   Or did just no one ever care to revise it?

     Beats me.  Have you measured a difference?  (Or tried to;
micro-measurements of Java performance are notoriously hard to
do well.)

> PS: even if it *is* still faster, I'd expect the JIT/hotspot
>    to do that optimization on its own...

     Since the immediately preceding line is

	q = i / 100;

... and since `i' is known to be positive, I'd expect an immediately
following `r = i % 100;' to be recognizable by most optimizers as
"Divide once, getting quotient and remainder, use both outputs."

     Andreas, I think you've spotted a certifiable Oddity.  I will
instruct the Secretary to make a notation in the record books.

-- 
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM

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just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Andreas Leitgeb <avl@auth.logic.tuwien.ac.at> - 2014-11-24 20:58 +0000
  Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-11-24 16:31 -0500
    Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Andreas Leitgeb <avl@auth.logic.tuwien.ac.at> - 2014-11-26 12:39 +0000
  Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Marcel Mueller <news.5.maazl@spamgourmet.org> - 2014-11-25 23:11 +0100
    Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-11-25 17:45 -0500
      Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2014-11-25 18:39 -0600
  Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2014-11-25 18:30 -0600
    Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Andreas Leitgeb <avl@auth.logic.tuwien.ac.at> - 2014-11-26 08:53 +0000
      Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2014-11-26 08:10 -0600
        Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joerg Meier <joergmmeier@arcor.de> - 2014-11-26 17:30 +0100
    Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joerg Meier <joergmmeier@arcor.de> - 2014-11-26 17:25 +0100
      Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2014-11-27 12:17 -0600
  Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2014-11-27 16:39 -0800
    Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)" Andreas Leitgeb <avl@auth.logic.tuwien.ac.at> - 2014-11-29 15:17 +0000

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