I would prefer feature-based.

        TARGET_HARD_FLOAT represents the presence of FPUs.

        TARGET_FPRS represents the presence of FP register set because
one variant used GPRs for FP operations.

        E500 then added another variant with double-precision FP in the
GPRs. 

        PowerPC is an architecture and the mess of FP variants clearly is
not a architecture allowing compatibility.

David

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