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From: joel@kaprap.Solbourne.COM (Joel Boney)
Newsgroups: comp.arch
Subject: Re: DeNorms (was Alignment on RS/6000)
Keywords: denorms
Message-ID: <1990Nov28.154221.26355@Solbourne.COM>
Date: 28 Nov 90 15:42:21 GMT
References: <897@sibyl.eleceng.ua.OZ> <13342@encore.Encore.COM> <46866@apple.Apple.COM>
Sender: news@Solbourne.COM
Organization: Solbourne Computer, Inc.
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   In article <46866@apple.Apple.COM> baum@apple.UUCP (Allen Baum) writes:
   >[]
       >>In article <13342@encore.Encore.COM> jkenton@pinocchio.encore.com (Jeff Kenton) writes:
       >>
       >>This is probably true for NaN's and Infinities, but Denorms occur all the
       >>time.  printf() often generates them when dealing with floats, and the
       >>math library routines do also.
   >
   >I still don't understand the assertion that Denorms occur all the time.
   >Just because printf() knows how to handle them doesn't mean that it
   >happens very often. Likewise the math routines. What is your evidence
   >than it happens "often" (whatever that means- 1% is incredibly often by
   >my definition in this case)

Statistically denormalized numbers almost never occur (surely less than 1%).
That doesn't mean they are not useful. They make writing robust floating 
point codes much easier. However, if your algorithm is producing lots of
denorms then it probably is time to reconsider your algorithm. The library
routines may generate intermediate denorms from time to time, but good
library routines will not generate very many.

It is fine (and certainly in the spirit of the IEEE standard) to implement 
some rare functions such as generating and computing with denorms via 
a combination of hardware and software. Most of the RISC vendors do this now
and the performance degradation is not noticeable for any normal codes (SPECmarks
for example). I would much rather the chip implementators use their silicon
for things like more cache than for handling things like denorms in hardware.