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Mail Archives: djgpp/1997/02/02/13:51:08

From: mharris AT blackwidow DOT saultc DOT on DOT ca
Date: Sun, 2 Feb 1997 13:43:33 -0500 (EST)
Reply-To: mharris AT blackwidow DOT saultc DOT on DOT ca
To: Benjamin D Chambers <chambersb AT juno DOT com>
cc: djgpp AT delorie DOT com
Subject: Re: fixed point math: NEED HELP QUICK!
In-Reply-To: <19970201.123135.4847.3.chambersb@juno.com>
Message-ID: <Pine.LNX.3.95.970202133943.5878k-100000@capslock.com>
Organization: Total disorganization.
MIME-Version: 1.0

On Sat, 1 Feb 1997, Benjamin D Chambers wrote:

> >Write a program which demonstrates that using shifts/adds is more
> >efficient than using mul/divs for the majority of integer
> >multipliers/divisors between -32768 and +32767 (signed) and 0 and
> >65535 unsigned.
> Well, I'll give it a try :)
> Just for the record, I spent a few minutes on that example working out
> the ASM code, but for this I'll just do the mul's and shifts
> incrementally in ASM and do them again in C (the optimizer should switch
> them to shifts/muls).  If I have to, I WILL NOT write out 65536 shifting
> routines to prove my point - I don't have that kind of time ;)  But, I'll
> post the code when I get the results...

Great!  The result is sure to be a useful program for us manual
optimizers.  I think it will boil down to the number of 1 bits in the
multiplier however.

> >If you can convince me that most (read that 90%) mul/div
> >operations are slower than the equiv shift add, then you
> >win.  The prize will be your choice of; an unopened box
> >full of 360k disks or 25 Atari 2600 cartridges.
> Please, no - I've got too much junk around already (I finally got around
> to dumping my boxes of 5 1/4 inch diskettes that I haven't touched for 2
> years :)

Ok, the prize will be...  Hmm.  I've got no idea.  A copy of the
latest DJGPP with sources?  How's that?

> >I want to see the source code, and also profiled output
> >showing that shifts are faster than muls as stated above. 
> >(Instead of profiled output you can make the program keep 
> >it's own statistics and output them to a file.)
> >
> >Processor aim is 386, 486, and Pentium.
> I only have access to a 486, maybe somebody can do the others.
I can test the program on a 386 (I'm the lucky owner of a DX40!)
Also I've got a 286-8Mhz and a 486/66.

> >Good luck.  I'm packaging up those cartridges.  :o)
> >
> >Hey, can you do square roots in less than 10 cycles too?
> I wish :)  The current method I use when I don't have a calculator handy
> is rather inaccurate with larger numbers, and wouldn't translate very
> well into machine code.

Hmm.  I was really hoping to get rid of those carts...
Oh well.  :o)


Mike A. Harris        |             http://blackwidow.saultc.on.ca/~mharris
Computer Consultant   |    My webpage has moved and my address has changed.
My dynamic address: http://blackwidow.saultc.on.ca/~mharris/ip-address.html
mailto:mharris AT blackwidow DOT saultc DOT on DOT ca

Question: Will Linus trash the Linux project when HURD comes out? I hope not.

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