[gmx-users] 64-bit vs. 32-bit
lindahl at sbc.su.se
Fri Sep 3 14:29:00 CEST 2004
On Sep 3, 2004, at 12:00 PM, Berk Hess wrote:
>> Is it really impossible to have the same gain when going to 64 bit as
>> see for non-assembly-optimized code? I am just curious on what is
>> this "barrier".
> 64-bit computation is always slower than 32-bit.
> An Athlon 64 can be (much) faster than an Athlon XP/MP,
> because of a better architecture.
> But with the Athlon 64 you can have direct access to double
> the amount of registers in 64-bit mode, not in 32-bit mode.
> So programs could run faster in 64-bit mode, not due to the 64 bits,
> but due to the amount of registers.
The main reason why I haven't bothered to release the 64-bit SSE loops
yet is that I've been busy benchmarking and trying to use all 16 SSE
Unfortunately I really don't gain anything from using 16 instead of 8
registers; this is partly because the loops are pretty streamlined
already (not primarily register constrained), but also because of the
hardware register renaming - internally the CPU has over 100 registers
that are allocated dynamically - the 64-bit Athlons just have a
slightly bigger virtual "window" towards the hardware registers
compared to 32-bit code.
Anyway, this is almost done, and then I'll add the AMD-64 loops to the
distribution - just don't expect any huge magical performance boost
compared to the 32-bit code running on AMD64!
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 2363 bytes
Desc: not available
More information about the gromacs.org_gmx-users