Mailinglist Archive: opensuse (686 mails)

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Re: [opensuse] 64 bit vs 32 bit RAM consumption
On 7/23/2013 11:37 AM, Carlos E. R. wrote:

With bigger registers you can move data twice as fast, because the load time
for 32 bits is
the same for 64 bits.

Huh, not quite.

If you have to add 2 and 3 it will take the same time on both archs. 64 bit
is not faster, it is larger. Compare a road truck of 6 wheels
to another of 12 wheels and double length.

Apples/Oranges....
Adding isn't just moving, and you are still taking about integer maths.

Moving blocks of text or strings around is a far more common task for a computer
than dealing with huge arrays. This email, that you are looking at this very
instant is a perfect example.

Moving the text body of the email from the the file to your
screen was probably handed over to a string move operation. If the compiler
writer did his work properly, the compiler will use 64bit registers to move
that string, resulting in fewer actual load/store operations, and those
operations (typically) operate in the same number of clocks. So fewer
moves happening at the same speed = much shorter execution time.

Like I mentioned, its taken years for the compiler writers to catch up,
so results have changed over time.


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