Anne Archibald wrote: > On 1 April 2010 03:15, David Cournapeau <da...@silveregg.co.jp> wrote: >> Anne Archibald wrote: >> >>> Particularly given the comments in the boost source code, I'm leery of >>> this fix; who knows what an optimizing compiler will do with it? >> But the current code *is* wrong: it is not true that u == 1 implies u - >> 1 == 0 (and that (u-1) != 0 -> u != 1), because the spacing between two >> consecutive floats is much bigger at 1 than at 0. And the current code >> relies on this wrong assumption: at least with the correction, we test >> for what we care about. > > I don't think this is true for IEEE floats, at least in the case we're > interested in where u is approximately 1.
Yes, sorry, you're right. For log1p, we can use the msun code, it is claimed to be such as the error is bounded by 1 ulp, does not rely on ASM, and we already have all the necessary macros in npymath so that the code should be easy to integrate for single and double precision. I don't see code for the long double version, though. We can use the boost test data to see if we get something sensible there . > Who knows > which one gets used for which operations? -ffloat-store is supposed to > force the constant discarding of extra digits, but we certainly don't > want to apply that to all of numpy. I think relying on -ffloat-store is buggy anyway, and it would be a maintenance hell. It is useful to know when it makes a difference, though. cheers, David _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
