Related to Bug 1758 (and https://stackoverflow.com/questions/58366939/eigen3-boost-mpfr-segfault-when-alignment-is-disabled), it may actually not worth to use blocking algorithms for custom types, especially if sub-optimal cache access is less expensive than copying.
This not only applies to GEMM/GEMV products but also, e.g., triangular solvers.
Yes that's one of the many items I have had in my mind for years...
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