GCN 1.2 changed the opcodes from GCN 1.0. If the machine code doesn't line up anymore, is it really fare to call it the same ISA?
In the case of RDNA: its all 32-wide SIMD instead of 64-wide SIMD. That dramatic difference completely destroys the bpermute / permute / DPP assembly instructions (https://gpuopen.com/learn/amd-gcn-assembly-cross-lane-operat...), which have gone from 64-way permutes (in GCN) into 32-way permutes (in RDNA).
GCN 1.2 was already looking pretty different from GCN 1.0, I'd say RDNA absolutely deserves the name "new ISA".
After all, GCN 1.2 is about as similar to GCN 1.0 as 8080 was to 8086 (same assembly language and registers, but new opcodes). I think most people are willing to call 8080 and 8086 different ISAs.
RDNA is extremely different at a base level because of that 32x SIMD vs 64x SIMD.
> Graphics Core Next (GCN). This design would last for nearly 8 years […] still in use today as the integrated GPU
And in desktop GPUs.
The goddamn power of marketing! Even tech writers seem to assume that "RDNA" is some kind of revolutionary from-scratch change. It's still the same ISA!! Look at the drivers and compilers. It's simply the regular evolution, just with a rebrand.
(upd: even later in this article they do say it's "a significant reworking of GCN"… well, why did the first mention sound kinda like it wasn't acknowledging this?)