Groups > comp.lang.java.programmer > #30413

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

From	Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid>
Newsgroups	comp.lang.java.programmer
Subject	Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)"
Date	2014-11-25 18:39 -0600
Organization	A noiseless patient Spider
Message-ID	<m537fj$40t$1@dont-email.me> (permalink)
References	<slrnm7770k.uoi.avl@login.logic.tuwien.ac.at> <m52urc$qmn$1@gwaiyur.mb-net.net> <m530qp$fbj$1@dont-email.me>

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On 11/25/2014 4:45 PM, Eric Sosman wrote:
>      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?

In two's complement arithmetic, signed and unsigned multiplication are 
exactly identical. Replacing a multiplication with a series of 
shifts-and-adds is therefore completely legal for both signed and 
unsigned multiplies. It's division where the conversion between division 
and shifts is extremely laden with pitfalls.

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

Thread

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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