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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 11/25/2014 5:11 PM, Marcel Mueller wrote:
> On 24.11.14 21.58, 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));
>>  [...]
> However, there might be another subtile difference. The above code does
> an /unsigned/ multiplication rather than a signed one. So it effectively
> can handle a wider range of input numbers. Don't know whether this
> counts in your context.

     As it happens, no: Thanks to surrounding code, it is known that
`65536 <= i && i <= Integer.MAX_VALUE && q == i / 100'.  No minus
signs nor overflow to worry about.

     I'm wondering, though, about the distinction you draw between
signed and unsigned multiplication.  Given that Java (unlike C, say)
demands two's complement arithmetic, what's the difference?  Is
there *any* integer `q' for which shift-and-add and times-100 produce
different outcomes?

-- 
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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