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From: umh@vax5.cit.cornell.edu
Newsgroups: comp.arch
Subject: Is double->int slow?
Message-ID: <1991Jun6.172905.5304@vax5.cit.cornell.edu>
Date: 6 Jun 91 21:29:05 GMT
Distribution: comp
Organization: CIT, Cornell University
Lines: 16

I have a set of routines that perform a lot of double-> int conversions. These
are things like I interpolate some real number, then truncate that to use as an
index into an array of pixel values. Anyway profiling tells me that 20% of my
time is spent in these casts of a double to an int. This seems to me very high-
the interpolation and other fiddling are a large part of my code. 

So my point is: is the conversion of a float or double to an int, an inherently
expensive process- ie more than 1 or 2 clocks? Is it normally done in the FPU,
or is it done in software? Or does the C casting of double to int do something
strange over and above the straight conversion- some sort of consistency checks
or such that eat up ti?

For what it's worthm I'm working on an RS/6000, but I'd be interested to hear
about how this is handled under any modern architecture.

Maynard Handley