> However, I think an array of ALU's actually could be quite useful for some applications over an FPGA.
http://www.adapteva.com/announcements/epiphany-v-a-1024-core...
http://www.greenarraychips.com/home/documents/greg/PB001-100...
Nothing stops you from treating LUT outputs in groups like corse grain system though. FPGA manufactures could make chip with a different routeting topology that works really well for certain applications.
However, we could be making lots of devices that fit certain data flow patterns better. By doing so makes the devices simpler and faster.
Routing is pretty important. Its just current FPGAs are built with quite flexible interconnect.
If you want see really limited programble interconnect look at some old PLDs that you program by blowing fuses.
Honestly, a LUT can be pretty efficient structure for what it does. The biggest advantage to coarse grained structures is their they are much faster since the internal construction can use optimal routing.
The biggest issue with FGPA's is the programmable routing/connections. Ideally each LUT would form a complete graph. However, the number of wires grows at the approx rate n(n-2)/2 where N is the number of LUTs. So instead the structure is more hierarchical. Still the majority of silicon on an FPGA is still just used for routing.
However, I think an array of ALU's actually could be quite useful for some applications over an FPGA.