Re: Intel Core2Duo (T7400)

[Gabor Gombas <gombasg@sztaki.hu>]

On Thu, Nov 08, 2007 at 09:31:31AM -0500, Lennart Sorensen wrote:

> I don't consider it a real issue either, but it is still something.  I
> am not sure why sparc tends to run 32bit for most programs and only
> 64bit for select cases where it helps.  Certainly x86_64 seems to be
> better than i386 in just about all cases.

Taking the same code, going from 32-bit to 64-bit will cause a slowdown,
period. The only way to overcome that is if you can write better code in
64-bit mode than you could in 32-bit mode. There are apps that indeed
benefit from directly accessing more than 2G of address space and
therefore can use simpler algorithms in 64-bit mode, but they are rare
(at least on desktop). 

AMD knew all this and they also knew they have to counter-balance the
slowdown if they ever wanted 64-bit to became the norm, so they did a
smart trick and doubled the register set size in 64-bit mode. Since i386
is a very register-starved architecture, that move indeed helped a lot
by making it much easier for compilers to generate better code. So it's
not only "in 64-bit mode you can keep more variables in registers" but
also "it is easier to write good compilers for 64-bit mode".

AFAIK Sparc (and basycally any other 64-bit capable processor) offers
the same number of registers in both 32-bit and 64-bit mode, so there is
nothing that could balance the slowdown caused by going 64-bit. And even
if they wanted to use the same trick as AMD it would not help as Sparc
already has enough registers - adding more would give a much much
smaller performance gain than it did for x86_64.

> I hope they come out with a way faster improved CPU before then.

Hehe, they could introduce a new 32-bit mode that has the same number of
registers as the 64-bit mode has. OTOH marketing people would have a
really tough time to push down such a change on consumers' throats...

Gabor

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     MTA SZTAKI Computer and Automation Research Institute
                Hungarian Academy of Sciences
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