On 02/07/2019 14:19, David Higton wrote:
> In message 
>            druck  wrote:
>> 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.

Only some licencees high end server class ARMv8A cores are 64 bit only,
where there is no need of backwards compatibility. One of ARM's general
purpose cores (A72) now only support 64 bit privileged modes, but 32 bit
userland is still supported for running legacy code. The Pi 4's A72
supports both 32 and 64 bit OSs and userlands.

Support for 32 bit ARM isn't going to disappear as (hopefully) 32bit x86
will do soon, as billions of 32 bit ARM chips are still be used for low
power applications, which wont gain anything from 64 bits.

---druck

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