Add FPGA Optimized Register File Version

Add a register file, optimized for synthesis on FPGAs supporting
distributed RAM.

Principle:

The baseline implementation implements the register file as an array of
flip-flops and implements large multiplexers for read- and write-
accesses. On FPGAs, we have a more efficient implementation for data
storage: By using distributed RAM for memory storage, we can store up
to 64 bits in just one LUT (depending on the memory layout and FPGA
device). In addition, distributed RAM comes with integrated address
decoders. The register file features one distributed RAM block per
implemented sync write port, each with the parametrized number of
async read ports. The read access is arbitrated depending on which
block was last written to. For this purpose an additional array of
*NUM_WORDS* registers is maintained keeping track of write accesses.

Since both FFs and multiplexers are an expensive structure on FPGA
technology, the achieved savings are considerable. The register file
is used for the FPU and general purpose register files.

Concrete Savings: (Xilinx Kintex-7, xc7k325tffg900-2)

```
            LUT    FF      LUTRAM
---------------------------------
baseline:   40499  22799   0
optimized:  36350  18806   440
---------------------------------
Diff        -4149  -3993   +440
            -10.2% -17.5%
```

Signed-off-by: ganoam <gnoam@live.com>

Bender.yml

@@ -30,7 +30,6 @@ sources:
     # Stand-alone source files
     - src/ariane.sv
     - src/serdiv.sv
-    - src/ariane_regfile_ff.sv
     - src/amo_buffer.sv
     - src/id_stage.sv
     - src/branch_unit.sv
@@ -208,6 +207,12 @@ sources:
         - src/util/instruction_tracer_defines.svh
         - src/util/instruction_trace_item.svh
         - src/util/exception_trace_item.svh
+    - target: fpga
+      files:
+      - fpga/src/ariane_regfile_fpga.sv
+    - target: not(fpga)
+      files:
+        - src/ariane_regfile_ff.sv
     - target: all(fpga, xilinx)
       files:
       - fpga/src/ariane_peripherals_xilinx.sv

Makefile

@@ -585,6 +585,7 @@ fpga_filter += $(addprefix $(root-dir), src/util/ex_trace_item.sv)
 fpga_filter += $(addprefix $(root-dir), src/util/instr_trace_item.sv)
 fpga_filter += $(addprefix $(root-dir), src/util/instr_tracer_if.sv)
 fpga_filter += $(addprefix $(root-dir), src/util/instr_tracer.sv)
+fpga_filter += $(addprefix $(root-dir), src/ariane_regfile_ff.sv)
 
 fpga: $(ariane_pkg) $(util) $(src) $(fpga_src) $(uart_src)
 	@echo "[FPGA] Generate sources"

fpga/src/ariane_regfile_fpga.sv

+// Copyright 2018 ETH Zurich and University of Bologna.
+// Copyright and related rights are licensed under the Solderpad Hardware
+// License, Version 0.51 (the "License"); you may not use this file except in
+// compliance with the License.  You may obtain a copy of the License at
+// http://solderpad.org/licenses/SHL-0.51. Unless required by applicable law
+// or agreed to in writing, software, hardware and materials distributed under
+// this License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+//
+// Engineer:       Francesco Conti - f.conti@unibo.it
+//
+// Additional contributions by:
+//                 Markus Wegmann - markus.wegmann@technokrat.ch
+//                 Noam Gallmann - gnoam@live.com
+//
+// Design Name:    RISC-V register file
+// Project Name:   zero-riscy
+// Language:       SystemVerilog
+//
+// Description:    This register file is optimized for implementation on
+//                 FPGAs. The register file features one distributed RAM block per implemented
+//                 sync-write port, each with a parametrized number of async-read ports.
+//                 Read-accesses are multiplexed from the relevant block depending on which block
+//                 was last written to. For that purpose an additional array of registers is
+//                 maintained keeping track of write acesses.
+//
+
+module ariane_regfile #(
+  parameter int unsigned DATA_WIDTH     = 32,
+  parameter int unsigned NR_READ_PORTS  = 2,
+  parameter int unsigned NR_WRITE_PORTS = 2,
+  parameter bit          ZERO_REG_ZERO  = 0
+)(
+  // clock and reset
+  input  logic                                      clk_i,
+  input  logic                                      rst_ni,
+  // disable clock gates for testing
+  input  logic                                      test_en_i,
+  // read port
+  input  logic [NR_READ_PORTS-1:0][4:0]             raddr_i,
+  output logic [NR_READ_PORTS-1:0][DATA_WIDTH-1:0]  rdata_o,
+  // write port
+  input  logic [NR_WRITE_PORTS-1:0][4:0]            waddr_i,
+  input  logic [NR_WRITE_PORTS-1:0][DATA_WIDTH-1:0] wdata_i,
+  input  logic [NR_WRITE_PORTS-1:0]                 we_i
+);
+
+  localparam ADDR_WIDTH = 5;
+  localparam NUM_WORDS  = 2**ADDR_WIDTH;
+  localparam LOG_NR_WRITE_PORTS = NR_WRITE_PORTS == 1 ? 1 : $clog2(NR_WRITE_PORTS);
+
+  // Distributed RAM usually supports one write port per block - duplicate for each write port.
+  logic [NUM_WORDS-1:0][DATA_WIDTH-1:0]     mem [NR_WRITE_PORTS];
+
+  logic [NR_WRITE_PORTS-1:0][NUM_WORDS-1:0]     we_dec;
+  logic [NUM_WORDS-1:0][LOG_NR_WRITE_PORTS-1:0] mem_block_sel;
+  logic [NUM_WORDS-1:0][LOG_NR_WRITE_PORTS-1:0] mem_block_sel_q;
+
+  // write adress decoder (for block selector)
+  always_comb begin
+    for (int unsigned j = 0; j < NR_WRITE_PORTS; j++) begin
+      for (int unsigned i = 0; i < NUM_WORDS; i++) begin
+        if (waddr_i[j] == i) begin
+          we_dec[j][i] = we_i[j];
+        end else begin
+          we_dec[j][i] = 1'b0;
+        end
+      end
+    end
+  end
+
+  // update block selector:
+  // signal mem_block_sel records where the current valid value is stored.
+  // if multiple ports try to write to the same address simultaneously, the port with the highest
+  // index has priority.
+  always_comb begin
+    mem_block_sel = mem_block_sel_q;
+    for (int i = 0; i<NUM_WORDS; i++) begin
+      for (int j = 0; j<NR_WRITE_PORTS; j++) begin
+        if (we_dec[j][i] == 1'b1) begin
+          mem_block_sel[i] = LOG_NR_WRITE_PORTS'(j);
+        end
+      end
+    end
+  end
+
+  // block selector flops
+  always_ff @(posedge clk_i or negedge rst_ni) begin
+    if (!rst_ni) begin
+      mem_block_sel_q <= '0;
+    end else begin
+      mem_block_sel_q <= mem_block_sel;
+    end
+  end
+
+  // distributed RAM blocks
+  logic [NR_READ_PORTS-1:0] [DATA_WIDTH-1:0] mem_read [NR_WRITE_PORTS];
+  for (genvar j=0; j<NR_WRITE_PORTS; j++) begin : regfile_ram_block
+    always_ff @(posedge clk_i) begin
+      if (we_i[j]) begin
+        mem[j][waddr_i[j]] <= wdata_i[j];
+      end
+    end
+    for (genvar k=0; k<NR_READ_PORTS; k++) begin : block_read
+      assign mem_read[j][k] = mem[j][raddr_i[k]];
+    end
+  end
+
+  // output MUX
+  logic [NR_READ_PORTS-1:0][LOG_NR_WRITE_PORTS-1:0] block_addr;
+  for (genvar k = 0; k < NR_READ_PORTS; k++) begin : regfile_read_port
+    assign block_addr[k] = mem_block_sel_q[raddr_i[k]];
+    assign rdata_o[k] =
+        (ZERO_REG_ZERO && raddr_i[k] == '0 ) ? '0 : mem_read[block_addr[k]][raddr_i[k]];
+  end
+
+endmodule