Re: Intel Core2Duo (T7400)
On 11/12/07, Gabor Gombas <gombasg@sztaki.hu> wrote:
> 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).
In my opinion certainly all the applications wrote in the past has no
designed to use 64 bits operations and many algorithms could have
beneffits of this, if in the same bus (64 bits) are you only use 32>=
bits operands there is no gain at all...
The plus is involved in move and operate data at double rate 64/32 if
the code is rarely doing this we have this slowdown, obviously thats
not true if its not a full 64 bits REAL bus...
And (this is only my knowledge from 8085 -> 8088/80188 old procesors)
the pointers to the data are in many cases relatives and not
necesarily should be so long, if this is not true now let me know, I
just presume that the nowadays procesors still have the ugly segment
registers...
>
> 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
>
> --
> ---------------------------------------------------------
> MTA SZTAKI Computer and Automation Research Institute
> Hungarian Academy of Sciences
> ---------------------------------------------------------
>
>
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