[gmx-developers] stuff in double precision
lindahl at cbr.su.se
Mon Feb 5 17:18:04 CET 2007
On Feb 5, 2007, at 5:08 PM, Berk Hess wrote:
> This difference can be avoided by having all energy terms in double
> precision. In general this is for free, but it might need recoding
> of the assembly loops.
> We should consider making most/all variables double precision,
> except for the natoms sized arrays.
One problem I can see is that single/double conversions are not free,
and you would do extra operations to sum in double (SSE is 2-way for
double 4-way for single).
A possible intermediary approach could be to introduce a handful of
extra single precision variables that we zero in the outer loop, and
after each inner loop we convert the sum to double and add it to a
double precision summation.
I've started to go more in the direction of "double unless it's an
array" recently, but for the innermost loop it would mean sacrificing
In principle I could also recode the loops to enable a solution with
"double for all summation variables", but then we might want to have
that as a third option besides all-double and all-single. Before
doing that for ASM I'd like to see hard result that it solves the
problem when done for the C loops, though :-)
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