New article: AMD's Jaguar Microarchitecture

Article: AMD's Jaguar Microarchitecture
By: NoSpammer (no.delete@this.spam.com), April 4, 2014 3:15 pm
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on April 4, 2014 2:00 pm wrote:
> The good news is that denormals *are* rare. The fact that geekbench happened
> to hit it was just a fiasco, but that benchmark has other problems, so..
>
> That said, they can and do happen. I remember some (helicopter flying?) game in the late 90's
> (maybe early 2000's) that caused tons and tons of denormal exceptions, and as a result we did
> really badly at it at Transmeta because we had thought that they never happen on real code.
>
> It probably wasn't intentional there either, but just some random case where something got sufficiently
> close to zero just by happenstance, and the developers had never noticed nor cared.

Well, of course numerical rotation of matrices back and forth gets into denormal territory rather quickly. With 3D linear algebra you either hate denormals when they slow you down or love them when they give you better accuracy (let's say doing 3D collisions). In practice you need to force matrix elements below some epsilon to zero and renormalize (getting back into an axis-orthogonal plane).

With general linear algebra I've seen cases where disabling denormals would bring bad results, but adding a little bit of random noise to the inputs would produce good enough results faster.

So I'd say we need denormals but they must be used intelligently by software people.
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