Most FPGAs have converged on 18 bit wide multiplier blocks. If you ask for a 64 bit multiplier, the router will automatically chain together four multiplier blocks and add them together in a single cycle, which is really going to hurt your maximum clock speed (fmax).
VexRiscv is aware of this unofficial standard, and asks for four 16x64 multiplies and adds the result together on the next cycle. This produces a much better fmax on FPGAs, but if you were targeting an ASIC, you would be better off asking for a 64-bit multiplier, or not trying for a single-cycle multiply.
Most modern CPUs tend to target a 3 cycle pipelined multiplication, which means 22-bit wide multipliers. Doing this on an FPGA each 22-bit multiplication would require two 18-bit multiplier blocks, for a total of six multipliers, wasting more resources.
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In general, "FPGA friendly" means optimizing your design to take advantage of the things which are cheap on FPGAs, like the 18-bit wide multipliers and the block ram. Such designs tend to run faster on FPGAs and use less resources, but it's wasteful to synthesize them to ASICs.
VexRiscv is aware of this unofficial standard, and asks for four 16x64 multiplies and adds the result together on the next cycle. This produces a much better fmax on FPGAs, but if you were targeting an ASIC, you would be better off asking for a 64-bit multiplier, or not trying for a single-cycle multiply.
Most modern CPUs tend to target a 3 cycle pipelined multiplication, which means 22-bit wide multipliers. Doing this on an FPGA each 22-bit multiplication would require two 18-bit multiplier blocks, for a total of six multipliers, wasting more resources.
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In general, "FPGA friendly" means optimizing your design to take advantage of the things which are cheap on FPGAs, like the 18-bit wide multipliers and the block ram. Such designs tend to run faster on FPGAs and use less resources, but it's wasteful to synthesize them to ASICs.