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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 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));
>
> 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?

I tend to agree: it is an anachronism.
It makes absolutely no sense with respect to performance, especially in 
case of Java or other JIT languages, where an important goal is that the 
JIT optimizer /understands/ your code to do platform specific 
optimizations. And I am pretty sure that multiplication by a constant is 
understood by the JIT.

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

Exactly. And if you put hand optimized code for a single platform into 
the JIT it is likely to fail sadly on other platforms. Especially if the 
target platform is not optimized for the rarely used shift operations 
and does not have a barrel shifter.

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.


Marcel

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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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