It seems AMD wants to get rid of GCN support, so to get people to move to RDNA.
CDNA is GCN actually. And ROCm doesn't support RDNA yet.
Honestly, the problem is that GPUs assembly languages keep changing. NVidia solved this problem by inventing PTX, a recompilable, portable assembly language that recompiles every time a new GPU comes out.
Intel solves this problem on the SIMD-side by simply always supporting older assembly language statements. Intel CPUs still boot in realmode IIRC (8086 compatible), and have a complicated bootup process to enable 16-bit 80286, then 32-bit 80386, and finally 64-bit modes.
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I don't know if this is really that big a deal IMO. ROCm 4.5 will continue to work for the Polaris 580x. Your code will continue to run as written. In fact, most people on Polaris saw a bug on ROCm 3.8+, so I'm pretty sure most Polaris-users were sticking with ROCm 3.3 instead. "Officially" cutting support on 4.5 isn't a big deal since most ROCm / Polaris users are already just staying on 3.3 ya know?