Re: using linux instead of osf

Toon Moene (toon@moene.indiv.nluug.nl)
Tue, 26 Nov 96 23:18:19 +0100

David Mosberger-Tang wrote:

[ Answering the following: ]

> I ran some code to check these functions before I made my
> statement and when I used the sinf and cosf functions the test
> program I was using showed no improvement. So does this mean that
> my libm is is broken?

> Always possible, but very unlikely. You can look at
> s_sinf.o and k_sinf.o in libm using objdump -d. You'll
> find that all fp ops are done in single precision (muls,
> lds, etc) so, fundamentally, the routines are certainly
> not broken (at least not in the way you suggest ;-).

Naively, one would expect that, to compute the sine of some
argument in single precision would require *less* fp operations than
to get it correct up to double precision.

E.g., because almost all calculations _we_ perform are single
precision, on my NeXT I could always get away with a simple 200 line
library of exp, log, etc. - coded by someone of our local nuclear
physics lab - that simply cut some corners here and there. Those
routines were, on average, twice as fast as the ones provided by
NeXT.

> On the Alpha, there is only two reasons why single
> precision might give improve execution time:

> (a) Your code is memory bandwidth limited.

Yep, saw exactly that when we got an early Alpha on loan in January
'93 - the execution time difference between the same model, same
input, when running 32-bit calculations or [the same number of]
64-bit ones was of the order of 60 %, i.e. it took 1.6 times as much
time to pull the (twice larger) data stream through.

Hope this helps,
Toon.

--
Toon Moene (toon@moene.indiv.nluug.nl)
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Phone: +31 346 214290; Fax: +31 346 214286
URL: http://www.knmi.nl/hirlam

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