[mythtv-users] AMD Athlon 64 CPUs and MythTV Compiling

Anthony Vito anthony.vito at gmail.com
Tue Dec 21 16:59:14 UTC 2004

> But you do get the advantage of an extra eight registers, which makes
> a huge difference to most code as the x86 architechure is very short on
> registers which gives optimisers a very hard time.

No, you got it backwards. Small amounts of registers make an
optimizing compilers job easy. That was nearly the entire reason for
CISC... simple code using a few registers, optimization on the metal.
It doesn't always work, and optimizing compilers can better fit the
code for specific applications. Because of this trend, and the
possible benefits, AMD has added 8 GPRs when in 64bit mode. Make no
mistake compilers targeting x86-64 are more complicated.

 If you are on a 32bit OS the processor is in 32bit mode, and you
don't get any more GPRs. You get extra hidden registers, but that
happens on the metal, the compiler doesn't have to do a thing. This is
one of the key reasons why the AMD64 chips perform so well in 32bit
mode, because of these extra hidden registers. In 64bit mode the
compiler does gain access to an extra 8 GPRs, which makes the
compilers job a bit more difficult, but not too bad. Still nothing
like the register painting that goes on with a powerPC chip... here's
the reference


Anthony Vito
anthony.vito at gmail.com

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