Why is 64 bit faster. RE: [Beowulf] Win64 Clusters!!!!!!!!!!!!

[Andrew Shewmaker]

On 4/12/07, Mark Hahn <hahn at mcmaster.ca> wrote:
> > 1.) Why is a 64 bit cpu faster?  I had assumed the main benefit was the
> > memory that could be addressed, obviously a bad assumption.
>
> being able to address more memory is indeed critical for some codes.
> certainly not all; in fact, the larger pointers hurt some codes.
> 64b mode also enables a lot more registers, of which the compiler can
> make very good use.  and some codes like the extra register width.

Real World Technologies has some good threads on what makes x86-64
better than x86.

People have mentioned the extra registers you get when going to
x86-64, but I don't
think anyone mentioned how that was related to the x87 unit.  x86-64's ABI
forces people off of the x87 unit (mostly), and that's a good thing
for several reasons.

http://www.realworldtech.com/forums/index.cfm?action=detail&id=68666&threadid=68664&roomid=2

Increasing the number of logical registers beyond a certain point can
have negative
consequences (increased write-back of intermediate values).  Register renaming
in hardware works pretty well.

http://www.realworldtech.com/forums/index.cfm?action=detail&id=75516&threadid=75235&roomid=2

For those worried about 64-bit pointers, they also talk about
"compressed pointers"
(aka indexes).

http://www.realworldtech.com/forums/index.cfm?action=detail&id=75624&threadid=75463&roomid=2

And why 64-bit pointers don't necessarily hurt performance that much even if
you don't compress them.

http://www.realworldtech.com/forums/index.cfm?action=detail&id=72864&threadid=72839&roomid=2

One nice benefit of x86-64 is that it allowed the Linux kernel
developers to clean up.

http://www.realworldtech.com/forums/index.cfm?action=detail&id=64142&threadid=64137&roomid=2

There's tons of other discussion about how 32-bits of virtual space
isn't enough, how PAE
sucked, why simple flat address spaces are good, and why x86-64 made sense even
if only a small percentage of the market required 64-bits.

-- 
Andrew Shewmaker