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echo: rberrypi
to: DRUCK
from: DAVID HIGTON
date: 2019-07-02 14:19:00
subject: Re: Raspberry Pi 4 - fast

In message 
          druck  wrote:

>On 30/06/2019 21:02, Richard Kettlewell wrote:
>> For both Intel and ARM the 64-bit instruction sets double the number of
>> architectural GPRs. Particularly on Intel where the 32-bit situation is
>> pretty poor, that’s a big improvement for anyone writing in assembler.
>
>Don't expect general performance gains with 64bit ARM. It's only because
>x86 is such an atrocious outdated mess, that the AMD64 instruction set
>shows significant gains. On every other architecture supporting both 32
>bit and 64 bit code (Power, Sparc, MIPS) the 64 bit code is usually
>slower than the 32 bit on the same chip, due to the increased demand on
>the data cache from 64 bit pointers.
>
>With the current state of ARMv8 support on GCC, CPU and memory bound
>programs vary from a barely noticeable few percent drop, to 50% slower.
>So it only makes sense to move from running 32 bit to 64 bit code when
>moving to a chip with which supports 4GB or more of memory, and a much
>larger data cache.

Or when all new chips are coming out with 64 bit instruction sets and
it becomes clear that 32 bit instruction sets will no longer be supported
by new processors as from some date in the quite near future.

Which is where we are now.

David

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