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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"The world is given to me only once, not one existing and one
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Erwin Schroedinger
Project Manager
Network Computing Services, Inc.
Army High Performance Computing Research Center (AHPCRC)
[EMAIL PROTECTED] | 612.337.3467
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