On Tuesday, June 24, 2003, at 03:19 AM, Peter Klaver wrote:
Hi everyone,
Now that we are officially in the 64-bit era, I have a question: how
much will existing binaries, that do mainly 64 bit floating point
number crunching, benefit from the new cpu without recompiling? I'm
running VASP, a DFT code, which is mainly double precision floating
point crunching. On current G4s, each 64 bit calculation has to be
'split up' into two calculations. And I have no idea when Absoft will
release a ppc970-aware compiler. So, if I go out and buy a G5 in
august, will these binaries still split 64-bit calculations into two,
or will they do them in one step, automatically getting a speedup
factor of two? Or to put it in other words: is the 'splitting-up'
cooked into the binary (meaning no benefit from 64 bit), or is it done
by the the cpu itself as needed (meaning immediate G5 benefit compared
to G4)?
Or am I, as someone not too good with this stuff, overlooking
something else completely?
--- If you're doing 64-bit FLOATING POINT number crunching, you won't
get any architectural benefit. (you'll get the benefit of faster clock
speeds, but nothing else will have changed).
--- I don't understand why you say "On current G4s, each 64 bit
calculation has to be 'split up' into two calculations.". The G4 (and
all its predecessors since the 68040) have been able to do 64-bit (IEEE
double precision floating point) calculations without splitting them
up. All the floating-point CPU registers have been 64-bits all along.
--- The 64 bits applies to:
1... the main registers (which are now 32 bits)
2... integer arithmetic (2^31 * 4 will no longer overflow)
3... the address and data paths.
--- So it will be able to address more memory (bigger address
registers) and do larger integer calculations and move data around in
larger chunks, but I haven't seen anything about changed floating point
performance (other than clock speeds).
----------------------------------------------------------------
Steve Bird
Culverson Software - Elegant software that is a pleasure to use.
www.Culverson.com (toll free) 1-877-676-8175
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