ibex_fetch_fifo.sv

// Copyright lowRISC contributors.
// Copyright 2018 ETH Zurich and University of Bologna, see also CREDITS.md.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

/**
 * Fetch Fifo for 32 bit memory interface
 *
 * input port: send address and data to the FIFO
 * clear_i clears the FIFO for the following cycle, including any new request
 */

`include "prim_assert.sv"

module ibex_fetch_fifo #(
  parameter int unsigned NUM_REQS = 2
) (
    input  logic        clk_i,
    input  logic        rst_ni,

    // control signals
    input  logic        clear_i,          // clears the contents of the FIFO

    // input port
    input  logic        in_valid_i,
    output logic        in_ready_o,
    input  logic [31:0] in_addr_i,
    input  logic [31:0] in_rdata_i,
    input  logic        in_err_i,

    // output port
    output logic        out_valid_o,
    input  logic        out_ready_i,
    output logic [31:0] out_addr_o,
    output logic [31:0] out_rdata_o,
    output logic        out_err_o,
    output logic        out_err_plus2_o
);

  // To gain extra performance DEPTH should be increased, this is due to some inefficiencies in the
  // way the fetch fifo operates see issue #574 for more details
  localparam int unsigned DEPTH = NUM_REQS+1;

  // index 0 is used for output
  logic [DEPTH-1:0] [31:0]  rdata_d,   rdata_q;
  logic [DEPTH-1:0]         err_d,     err_q;
  logic [DEPTH-1:0]         valid_d,   valid_q;
  logic [DEPTH-1:0]         lowest_free_entry;
  logic [DEPTH-1:0]         valid_pushed, valid_popped;
  logic [DEPTH-1:0]         entry_en;

  logic                     pop_fifo;
  logic             [31:0]  rdata, rdata_unaligned;
  logic                     err,   err_unaligned, err_plus2;
  logic                     valid, valid_unaligned;

  logic                     aligned_is_compressed, unaligned_is_compressed;

  logic                     addr_incr_two;
  logic [31:1]              instr_addr_d, instr_addr_q;
  logic                     instr_addr_en;
  logic                     unused_addr_in;

  /////////////////
  // Output port //
  /////////////////

  assign rdata = valid_q[0] ? rdata_q[0] : in_rdata_i;
  assign err   = valid_q[0] ? err_q[0]   : in_err_i;
  assign valid = valid_q[0] | in_valid_i;

  // The FIFO contains word aligned memory fetches, but the instructions contained in each entry
  // might be half-word aligned (due to compressed instructions)
  // e.g.
  //              | 31               16 | 15               0 |
  // FIFO entry 0 | Instr 1 [15:0]      | Instr 0 [15:0]     |
  // FIFO entry 1 | Instr 2 [15:0]      | Instr 1 [31:16]    |
  //
  // The FIFO also has a direct bypass path, so a complete instruction might be made up of data
  // from the FIFO and new incoming data.
  //

  // Construct the output data for an unaligned instruction
  assign rdata_unaligned = valid_q[1] ? {rdata_q[1][15:0], rdata[31:16]} :
                                        {in_rdata_i[15:0], rdata[31:16]};

  // If entry[1] is valid, an error can come from entry[0] or entry[1], unless the
  // instruction in entry[0] is compressed (entry[1] is a new instruction)
  // If entry[1] is not valid, and entry[0] is, an error can come from entry[0] or the incoming
  // data, unless the instruction in entry[0] is compressed
  // If entry[0] is not valid, the error must come from the incoming data
  assign err_unaligned   = valid_q[1] ? ((err_q[1] & ~unaligned_is_compressed) | err_q[0]) :
                                        ((valid_q[0] & err_q[0]) |
                                         (in_err_i & (~valid_q[0] | ~unaligned_is_compressed)));

  // Record when an error is caused by the second half of an unaligned 32bit instruction.
  // Only needs to be correct when unaligned and if err_unaligned is set
  assign err_plus2       = valid_q[1] ? (err_q[1] & ~err_q[0]) :
                                        (in_err_i & valid_q[0] & ~err_q[0]);

  // An uncompressed unaligned instruction is only valid if both parts are available
  assign valid_unaligned = valid_q[1] ? 1'b1 :
                                        (valid_q[0] & in_valid_i);

  assign unaligned_is_compressed    = rdata[17:16] != 2'b11;
  assign aligned_is_compressed      = rdata[ 1: 0] != 2'b11;

  ////////////////////////////////////////
  // Instruction aligner (if unaligned) //
  ////////////////////////////////////////

  always_comb begin
    if (out_addr_o[1]) begin
      // unaligned case
      out_rdata_o     = rdata_unaligned;
      out_err_o       = err_unaligned;
      out_err_plus2_o = err_plus2;

      if (unaligned_is_compressed) begin
        out_valid_o = valid;
      end else begin
        out_valid_o = valid_unaligned;
      end
    end else begin
      // aligned case
      out_rdata_o     = rdata;
      out_err_o       = err;
      out_err_plus2_o = 1'b0;
      out_valid_o     = valid;
    end
  end

  /////////////////////////
  // Instruction address //
  /////////////////////////

  // Update the address on branches and every time an instruction is driven
  assign instr_addr_en = clear_i | (out_ready_i & out_valid_o);

  // Increment the address by two every time a compressed instruction is popped
  assign addr_incr_two = instr_addr_q[1] ? unaligned_is_compressed :
                                           aligned_is_compressed;

  assign instr_addr_d = clear_i ? in_addr_i[31:1] :
                                  (instr_addr_q[31:1] +
                                   // Increment address by 4 or 2
                                   {29'd0,~addr_incr_two,addr_incr_two});

  always_ff @(posedge clk_i) begin
    if (instr_addr_en) begin
      instr_addr_q <= instr_addr_d;
    end
  end

  assign out_addr_o[31:1] = instr_addr_q[31:1];
  assign out_addr_o[0]    = 1'b0;

  // The LSB of the address is unused, since all addresses are halfword aligned
  assign unused_addr_in = in_addr_i[0];

  ////////////////
  // input port //
  ////////////////

  // Accept data as long as our FIFO has space to accept the maximum number of outstanding
  // requests. Note that the prefetch buffer does not count how many requests are actually
  // outstanding, so space must be reserved for the maximum number.
  assign in_ready_o = ~valid_q[DEPTH-NUM_REQS];

  /////////////////////
  // FIFO management //
  /////////////////////

  // Since an entry can contain unaligned instructions, popping an entry can leave the entry valid
  assign pop_fifo = out_ready_i & out_valid_o & (~aligned_is_compressed | out_addr_o[1]);

  for (genvar i = 0; i < (DEPTH - 1); i++) begin : g_fifo_next
    // Calculate lowest free entry (write pointer)
    if (i == 0) begin : g_ent0
      assign lowest_free_entry[i] = ~valid_q[i];
    end else begin : g_ent_others
      assign lowest_free_entry[i] = ~valid_q[i] & valid_q[i-1];
    end

    // An entry is set when an incoming request chooses the lowest available entry
    assign valid_pushed[i] = (in_valid_i & lowest_free_entry[i]) |
                             valid_q[i];
    // Popping the FIFO shifts all entries down
    assign valid_popped[i] = pop_fifo ? valid_pushed[i+1] : valid_pushed[i];
    // All entries are wiped out on a clear
    assign valid_d[i] = valid_popped[i] & ~clear_i;

    // data flops are enabled if there is new data to shift into it, or
    assign entry_en[i] = (valid_pushed[i+1] & pop_fifo) |
                         // a new request is incoming and this is the lowest free entry
                         (in_valid_i & lowest_free_entry[i] & ~pop_fifo);

    // take the next entry or the incoming data
    assign rdata_d[i]  = valid_q[i+1] ? rdata_q[i+1] : in_rdata_i;
    assign err_d  [i]  = valid_q[i+1] ? err_q  [i+1] : in_err_i;
  end
  // The top entry is similar but with simpler muxing
  assign lowest_free_entry[DEPTH-1] = ~valid_q[DEPTH-1] & valid_q[DEPTH-2];
  assign valid_pushed     [DEPTH-1] = valid_q[DEPTH-1] | (in_valid_i & lowest_free_entry[DEPTH-1]);
  assign valid_popped     [DEPTH-1] = pop_fifo ? 1'b0 : valid_pushed[DEPTH-1];
  assign valid_d [DEPTH-1]          = valid_popped[DEPTH-1] & ~clear_i;
  assign entry_en[DEPTH-1]          = in_valid_i & lowest_free_entry[DEPTH-1];
  assign rdata_d [DEPTH-1]          = in_rdata_i;
  assign err_d   [DEPTH-1]          = in_err_i;

  ////////////////////
  // FIFO registers //
  ////////////////////

  always_ff @(posedge clk_i or negedge rst_ni) begin
    if (!rst_ni) begin
      valid_q <= '0;
    end else begin
      valid_q <= valid_d;
    end
  end

  for (genvar i = 0; i < DEPTH; i++) begin : g_fifo_regs
    always_ff @(posedge clk_i) begin
      if (entry_en[i]) begin
        rdata_q[i]   <= rdata_d[i];
        err_q[i]     <= err_d[i];
      end
    end
  end

  ////////////////
  // Assertions //
  ////////////////

  // Must not push and pop simultaneously when FIFO full.
  `ASSERT(IbexFetchFifoPushPopFull,
      (in_valid_i && pop_fifo) |-> (!valid_q[DEPTH-1] || clear_i))

  // Must not push to FIFO when full.
  `ASSERT(IbexFetchFifoPushFull,
      (in_valid_i) |-> (!valid_q[DEPTH-1] || clear_i))

endmodule