🚀 go-pugleaf

In article <9105171235.AA20139@ucbvax.Berkeley.EDU>, jbs@WATSON.IBM.COM writes:
> 
>          I agree that easier communication between the units would help
> in computing elementary functions.  However the gains would not be
> spectacular (I guess 10% or so) and elementary functions themselves don't
> seem all that important (anybody have numbers on this?).

In my experiance, even for floating point intensive scientific codes,
the only functions that really use any time are square root and
exponentiation.  Square root will often account for between 5% and 15% of
the cpu time (shading to the high side), and exponentiation between 1% and
5% (shading to the low side).  (Exponentiation would be higher except that
the vast majority of exponents are "2", and many optimizers convert x**2
into x*x.)

Now of course, a *few* codes make much more extensive use of things like
sine, cosine, etc.  But I have not found them to be "typical", even for
scientific codes (which, arguably, are already in and of themselves not
typical of codes in general).  Your mileage may vary.

The upshot of this is that if you took the transistors and the design
time that you used to implement these instructions, and instead devoted
them to, say, making floating point multiplication run 1 clock faster,
you would buy a much larger performance gain over a much wider range
of programs.


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Bron Campbell Nelson       | "The usual approach is to pick one
Silicon Graphics, Inc.     | of several revolting kludges."
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Mtn. View, CA  94039       |___________________________________
bron@sgi.com
These statements are my own, not those of Silicon Graphics.