Actual benefits of amd64 over i386

Scott Long scottl at
Thu May 12 21:39:05 PDT 2005

Michael Hopkins wrote:
> Hi all
> I have been suffering quite a bit of frustration recently with many ports
> that I need being 'i386 only' and no straightforward fixes.  Maybe amd64
> could be described as 'tier 1.5'  ;o)
> Can I ask for honest opinions here on what speed (or other?) benefits I
> should get by running amd64 over i386?  I will never be using more than
> about 1 G of memory on this particular machine so memory limit is not an
> issue.  Maybe I should just rebuild the machine as i386?
> It's an Athlon64 3400+ on a Gigabyte motherboard - used mainly for
> cross-compiling to Linux and mingw32, NFS file sharing, some X11 over the
> network with my local Mac OS X machine as X server and using my own C/C++
> maths code (which must run as fast as possible).
> Michael

The theoretical advantage of amd64 is that there are more registers and
wider registers.  This means that more effecient code can be generated
by the compiler, especially math code.  There is also the benefit of
being able to address more than 4GB of RAM without requiring partial
tricks like PAE.  However, 64-bit addresses and pointers means that the 
cache gets consumed less efficiently.  So, it's a tradeoff, and much of 
it probably relies on how well the compiler generates code.  Of course,
the large memory handling is a definite win, but if your application
doesn't need that then the benefits are less clear.  Still, the Athlon64
and Opteron CPUs are quite speedy.


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