Re: 64bit processing - Why?
Re: 64bit processing - Why?
- Subject: Re: 64bit processing - Why?
- From: Ian Kemmish <email@hidden>
- Date: Fri, 31 Jul 2009 07:43:59 +0100
Rats, I'm breaking my own convictions never to post to threads like
this.
On 30 Jul 2009, at 22:34, Justin Carlson
<email@hidden>,
talking about promoting intermediate floating results, wrote:
While this is common on many architectures, it is not (correct me if I
am wrong along the way) standard in AU plugin developers' most common
deployment architecture: Intel, 32 bit, OS X. For this runtime, SSE
instructions+registers are *default*.
Actually, the 8087 and its descendants were about the only
architecture to have done this. Everyone else followed the VAX route
of not promoting floats, and eventually what the VAX did became (with
a few tweaks) the IEE standard.
The reason most modern Intel OSs use the scalar variants of SSE for
scalar floating point in preference to the FPU is that the former are
IEEE compliant, whereas the latter are not. Why do people prefer to
use the floating point unit which offers less intermediate precision?
Because IEEE compliant units allow to to better model and predict the
propagation of errors. For example, with an 8087 FPU, an optimising
compiler would frequently produce different answers to one which
didn't optimise, simply because it homed variables less often... (and
yes, I have seen this materially affect the reproducibility of
customer bug reports). Not good!
As far as I can see, this hu-u-u-uge argument boils down to two things:
1) If you want to end up with 24 bits, then having a few guard bits to
pass between AUs would be nice.
I think we can all agree with that, to some extent.
2) I want more than the 8 guard bits which 32 bit integer samples
would give me.
I don't think we can all agree with that. How many AUs in a chain
would you need for 8 guard bits to become a necessity?
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Ian Kemmish 18 Durham Close, Biggleswade, Beds
SG18 8HZ
email@hidden Tel: +44 1767 601361 Mob: +44 7952
854387
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