On Wed, Aug 24, 2005 at 05:09:04PM -0400, Alan Stange wrote:
> The older 32bit RISC processors do have 64 bit registers, ALUs and
> datapaths, and they are marketed toward high end scientific computing,
> and you're claiming that such a processor is slower than one which has
> the addition of 64 bit pointers added to it?
No. I'm claiming that you are talking about a hybrid 64/32 processor,
and that this isn't fair to declare that 64-bit arithmetic units don't
provide benefit for 64-bit math. :-)
> As an example, an UltraSparc running a 32 bit kernel+application will
> have the same double precision floating point performance as one
> running a 64bit kernel+application (except for the additional FP
> registers in the 64bit API). For a function like daxpy, it's the exact
> same hardware running the exact same instructions! So why do you think
> the performance would be different?
Double precision floating point isn't 64-bit integer arithmetic. I think
this is all a little besides the point. If you point is that the SPARC
was designed well - I agree with you.
I won't agree that a SPARC with 64-bit registers should be considered
a 32-bit machine. The AMD 64-bit machines come in two forms as well -
the ones that allow you to use the 64-bit integer registers (not
floating point! those are already 80-bit!), and the ones that allow
you to address more memory. I wouldn't consider either to be a 32-bit
CPU, although they will allow 32-bit applications to run fine.
> I believe the IBM Power processors also upped everything to double
> precision internally because of some details of the "multiply-add fused"
> instructions. It's been a few years since I taught H&P to CS
> undergrads, but I'm fairly sure the details are all the same for MIPS
> processors as well.
Smart design, that obscures the difference - but doesn't make the
difference a myth. If it's already there, then it's already there,
and we can't talk as if it isn't.
Cheers,
mark
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