Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.fsmpi.rwth-aachen.de!eternal-september.org!feeder.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail
From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: just for fun: JSL: "q*100" hand-optimized to "(q<<6)+(q<<5)+(q<<2)"
Date: Mon, 24 Nov 2014 16:31:45 -0500
Organization: A noiseless patient Spider
Lines: 32
Message-ID: <m5083q$c7g$1@dont-email.me>
References: <slrnm7770k.uoi.avl@login.logic.tuwien.ac.at>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Mon, 24 Nov 2014 21:31:38 +0000 (UTC)
Injection-Info: mx02.eternal-september.org; posting-host="bd4e1cba8d8592bec5d5ed9e4537fbfd"; logging-data="12528"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+l7vowoHKKYR5NIrPA3IW3"
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0
In-Reply-To: <slrnm7770k.uoi.avl@login.logic.tuwien.ac.at>
Cancel-Lock: sha1:wOUQ04slQLng2XmqwZAf2i54Pgw=
Xref: csiph.com comp.lang.java.programmer:30401

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