Re: fixed point vs floating point

[stt@houdini.camb.inmet.com (Tucker Taft)]

Matthew Heaney (mheaney@ni.net) wrote:

: Matthew asks:

: <<Can you elaborate on that?  I've often observed that floating point is
: used when fixed point is clearly called for, ie a bearing or latitude.
: What are the specific reasons to avoid fixed point?>>

: Robert replies:

: <<Most obviously, on most architectures, fixed-point is drastically slower 
: than flowting-point.>>

I'm not sure to what Robert is referring here.  Fixed + fixed is
as efficient as the normal integer operation, as is Fixed - Fixed, 
Fixed * Integer, and Fixed / Integer.  The only operations that are 
potentially inefficient are Fixed * Fixed and Fixed / Fixed, neither of which
are particularly likely when the fixed-point type represents an angle ;-).

Even if you do use Fixed * Fixed or Fixed / Fixed, the inefficiency
has to do with sometimes having to shift the result or one of the
operands after/before performing the operation (presuming binary
smalls).  If the machine has efficient shifting, this is not a major 
overhead.  However, it may be that on some compilers, the fixed-fixed 
multiplication/division operations are handled out-of-line, and the 
procedure-call overhead is the primary added expense.

: ...
: Matthew Heaney
: Software Development Consultant
: <mailto:matthew_heaney@acm.org>
: (818) 985-1271

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-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA