Re: [PATCH] cmake: Default to PNG_ARM_NEON=off for arm targets

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On Fri, 14 Oct 2022, John Bowler wrote:

> There are four different byte-size instruction sets on the various ARM chips
> and they can all be supported in the same core; 8-bit(Java), 16-bit(Thumb),
> 32-bit(original ARM), 64-bit(AArch64).  Switching can be done at run-time;
> last time I tried it (building a 16-bit Linux system) it was possible to
> intermingle 32-bit and 16-bit functions; the CPU instruction mode was
> switched in a trampoline inserted before the actual function implementation.

FWIW, aarch64 doesn't have 64 bit instructions, the instructions are still 
32 bit each. I don't have experience with the java stuff, but thumb and 
arm can easily be intermixed in the same binary/process, indeed via 
trampolines (and even without that, with the instruction mode being 
encoded in the lowest bit of code addresses). But I don't think it's 
possible to mix aarch64 and 32 bit arm in the same binary/process (at 
least without resorting to syscalls to switch modes).

// Martin

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