Re: Altivec 'bug' or limitation?
Re: Altivec 'bug' or limitation?
- Subject: Re: Altivec 'bug' or limitation?
- From: Robert Purves <email@hidden>
- Date: Sun, 30 Jan 2005 15:34:41 +1300
Scott Horn wrote:
This is a curiosity question. I've no experience in distributed or
high-precision software development. I just noticed a funny thing
when playing around with xgrid at home.
In Xgrid Preview 2, with two CPUs, a 800mhz G3 and a 1.25ghz G4.
Running mandelbrot, manual zoom, the following coords:
X: -0.7499982136839042
Y: -0.008411978256855454
Width: 4.903769581863252e-07
With 'Use Altivec if available' is checked I get 7 squares with
horizontal colored lines
Question: Is this possibly a bug in the altivec libraries or
hardware, or in the mandelbrot app, or is it simply a mathematical
precision limitation of altivec?
It's a precision thing. AltiVec only has 32-bit float precision, which
works OK for low-magnification fractal images. At high magnification,
the coordinates are not represented accurately enough.
A 'production' fractal program (as contrasted with a programmer's toy)
has therefore to switch from AltiVec to scalar double precision above a
certain magnification (typically several thousand). Double in turn
runs out of steam at about 10^11 x.
My Mandelbrot program (a free 160 KB download at
<http://fred.tekotago.ac.nz/~rp/>) then switches to double-double
arithmetic that allows it to work (perfectly, but also painfully
slowly) up to 10^28 x. At low mag, the combination of AltiVec and dual
processor support makes it quite lively, because 16 pixels are computed
simultaneously.
Robert P.
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