ARMv8 backward compatibility with ARMv7 (Snapdragon 820 vs Cortex-A15)

Question

I see that ARMv8 is merely an extension of ARMv7 architecture and all code compiled on ARMv7 should run on ARMv8. I am interested in the backward compatibility of ARMv8 to ARMv7. Will code that was compiled on ARMv8 run on ARMv7?

I have a particular exact case of interest: I would like to run the comma.ai's Openpilot visiond binary which was compiled for the OnePlus 3 smartphone (Qualcomm MSM8996 Snapdragon 820 CPU) on the Nvidia Jetson TK1 (NVIDIA Cortex-A15 CPU). Will the visiond run on Jetson?

EDIT: There may be more in question than CPU compatibility since visiond probably heavily uses GPU on that phone. Will probably depend whether they use some standard parallelization ways (OpenCL, NEON etc.) or have some custom code for Snapdragons GPU. Even with OpenCL the chance of compatibility is probably quite low on different HW.

Answer 1

I believe that aarch32 userland is fully or very highly backwards compatible with ARMv7, i.e. userland programs compiled for ARMv7 should just work in AArch32, but I couldn't find a precise quote in the ARM manual.

aarch32 does have new instructions added over ARMv7 however, most of them seem to be functionality that ARMv8 added and the designers decided to expose on aarch32. Therefore, aarch32 is not forward compatible with ARMv7, i.e., programs compiled for aarch32 might not run on ARMv7.

I'm not sure about system land. See also: Does ARMv8 AArch32 mode has backward compatible with armv4 , armv5 or armv6?