Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

[Nick Desaulniers]

On Fri, Apr 10, 2020 at 5:33 AM Russell King - ARM Linux admin
<linux@xxxxxxxxxxxxxxx> wrote:
>
> For older CPUs, it doesn't matter what the latest ARM ARM says, the
> appropriate version of the ARM ARM is the one relevant for the CPU
> architecture.  This is a mistake frequently made, and it's been pointed
> out by Arm Ltd in the past (before ARMv6 even came on the scene) that
> keeping older revisions is necessary if you want to be interested in
> the older architectures.

As if it never existed *waves hands*.  Interesting.  Does ARM still
distribute these older reference manuals? Do you keep copies of the
older revisions?

>
> However, there's an additional complication here: DEC's license from
> Arm Ltd back in the days of StrongARM allowed them to make changes to
> the architecture - that was passed over to Intel when they bought that
> part of DEC.  Consequently, these "non-Arm vendor" cores contain
> extensions that are not part of the ARM ARM.  iWMMXT is one such
> example, which first appeared in the Intel PXA270 SoC (an ARMv5
> derived CPU).
>
> In fact, several of the features found in later versions of the ARM
> architecture came from DEC and Intel enhancements.
>
> If your compiler/assembler only implements what is in the latest ARM
> ARM, then it is not going to be suitable for these older CPUs and
> alternate vendor "ARM compatible" CPUs.

This is a neat piece of history, thanks for sharing.
-- 
Thanks,
~Nick Desaulniers