M20k block configuration

I had never seen this attribute before, it seems it has been recently (and silently) added to the documentation, without proper description or examples. This attribute, whatever it actually does, is certainly not supported in Quartus v16.1, or likely even 17.0.

I would recommend seeking alternatives to reduce your Block RAM utilization. You should make sure to minimize the number of accesses to your local arrays. If you are unrolling loops, make sure the accesses inside of the loop are consecutive so that they can be coalesced into one larger access. You can also consider reducing the number of access ports to your local buffers using the numreadports or numwriteports attributes, which forces the compiler to share ports. Of course this will come at the cost of lowered performance. Since each Block RAM has a limited number of ports, merging different arrays into the same Block RAM will very likely require port sharing, which will also result in lowered performance.