[Beowulf] Question about amd64 architecture and floating point operations

[Richard Walsh]

Ivan Paganini wrote:
> Hello everybody at beowulf. Sorry about the _really_ newbie question, 
> but after doing some tests and researching a little, a question arose 
> when fooling around with amd64 (more precisely, an amd64 Athlon 4200 
> X2) and gcc and sun studio 11. The architecture has 64 bits integer 
> registers and 128 bits floating point registers, but my test programs 
> in C just gave me the same precision that I got with an old athlon 
> 2400 xp (32bits), that is, long double go only to 1x10^ 4961, even 
> with the -m64 flag. I always imagined that I would get the double 
> precision without the long double declaration (or, maybe, 40bits 
> precision). What am I missing here? Is the compiler (gcc 4.1, sun 
> studio express 11), the operating system (ubuntu 64bits edgy), or just 
> an error in my logic?
>
Ivan,

A common confusion ... x86_64 changes nothing about the precision of 
floats or doubles in
C or Fortran.  They remain the same format as does the basic inner 
workings of the functional
units.  The 64-ness refers to the added memory addressibility provided 
by the larger address
registers.  This potential itself is not fully utilized as I believe 
only 40-bits are used (the socket
F series may have bumped this up to 48-bits).

So computed epsilons on on either platform should be the same.  Long 
doubles, which would
need to be supported in software, would be need to get more.

rbw

PS  In his note, Mark Hahn makes reference to the different extended 
precisions inside
       the processor of the SIMD floating-point units versus the 
old-style floating point.
       Results are truncated to 64-bits when stored to memory, but a 
path through the SIMD
       units could produce slightly different results in "vectorized" 
code from the non-vector
       old-style functional unit.

>
> -- 
> -----------------------------------------------------------
> Ivan S. P. Marin
> Laboratório de Física Computacional
> lfc.ifsc.usp.br <http://lfc.ifsc.usp.br>
> Instituto de Física de São Carlos - USP
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>
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-- 

Richard B. Walsh

"The world is given to me only once, not one existing and one
 perceived. The subject and object are but one."

Erwin Schroedinger

Project Manager
Network Computing Services, Inc.
Army High Performance Computing Research Center (AHPCRC)
rbw at ahpcrc.org  |  612.337.3467

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