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Altivec 'bug' or limitation?



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, and two squares with the (apparently) correct rendering.  I surmise that the two correct squares were calculated by the G3 and the seven incorrect ones by the G4.  

To test this I turn off Altivec and run 'Show Image' again.  This time all nine squares return apparently correct results.

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?
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