Message-Id: <199903210023.TAA05029@delorie.com>
Comments: Authenticated sender is <mert0407 AT sable DOT ox DOT ac DOT uk>
From: "George Foot" <george DOT foot AT merton DOT ox DOT ac DOT uk>
To: djgpp AT delorie DOT com
Date: Sun, 21 Mar 1999 00:20:52 +0000
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Subject: Re: Fixed Point math
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Reply-To: djgpp AT delorie DOT com

horst DOT kraemer AT snafu DOT de (Horst Kraemer) wrote:
>On Thu, 18 Mar 1999 15:00:59 -0800, Kagenin <kagenin AT devnull DOT com>
>wrote:
>
>> John Carbrey wrote:
>> > 
>> > A friend of mine has informed me that floating point math is faster than
>> > fixed point math in pentiums.
>> > 
>> > He told me that I should use floats not fixed data types.
> 
>> No.  On almost all chips, integer math is faster than floating point. 
>> Plus, you can't use comparison operators on floats and doubles, as well
>> as you increase float underflow errors.
>
>
>You may not be aware of the fact that a floating point multiplication
>is faster than an 32 bit integer division already on a 486. (40 vs. 16
>cycles). and usually faster than a 32 bit integer multiplication.
>
>
>Moreover in fixed point math you have the same rounding errors than in
>floating point math.
>
>fixed point is "out" on Pentiums.

My two pence:  Use the right tool for the right job.  Fixed 
point numbers have a constant precision no matter how large the 
number is.  The precision of a floating point number is 
proportional to its magnitude.  Because of this, you do *not* 
have the same rounding problems in the two systems.  With 
floating point numbers, for instance, adding two non-zero 
numbers doesn't necessarily make a different number.  If you're 
comparing for equality you have to be careful about the 
epsilon.  Fixed and floating point systems are really good for 
different things; use whichever best suits the task you're 
doing.

I don't generally think it's a good idea to use the wrong tool
for a job just because you think it'll be faster than the
right tool.  You don't use floating point numbers when all you
need is integers, do you?

-- 
George