bp: add BHT with private history (#2793)

This PR adds a new two-level BHT predictor with private history. The new BPType parameters allow choosing between the original BHT and the new one.

Co-authored-by: Gianmarco Ottavi <ottavig91@gmail.com>

Bender.yml

@@ -122,6 +122,7 @@ sources:
       # Frontend (i.e., fetch, decode, dispatch)
       - core/frontend/btb.sv
       - core/frontend/bht.sv
+      - core/frontend/bht2lvl.sv
       - core/frontend/ras.sv
       - core/frontend/instr_scan.sv
       - core/frontend/instr_queue.sv

Flist.ariane

@@ -72,6 +72,7 @@ core/decoder.sv
 core/ex_stage.sv
 core/frontend/btb.sv
 core/frontend/bht.sv
+core/frontend/bht2lvl.sv
 core/frontend/ras.sv
 core/frontend/instr_scan.sv
 core/frontend/instr_queue.sv

core/Flist.cva6

@@ -144,6 +144,7 @@ ${CVA6_REPO_DIR}/core/cva6_fifo_v3.sv
 // What is "frontend"?
 ${CVA6_REPO_DIR}/core/frontend/btb.sv
 ${CVA6_REPO_DIR}/core/frontend/bht.sv
+${CVA6_REPO_DIR}/core/frontend/bht2lvl.sv
 ${CVA6_REPO_DIR}/core/frontend/ras.sv
 ${CVA6_REPO_DIR}/core/frontend/instr_scan.sv
 ${CVA6_REPO_DIR}/core/frontend/instr_queue.sv

core/frontend/bht2lvl.sv

@@ -0,0 +1,133 @@
+// Copyright 2025 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.
+//
+// Original author: Gianmarco Ottavi, University of Bologna
+// Description: Private history BHT
+
+module bht2lvl #(
+    parameter config_pkg::cva6_cfg_t CVA6Cfg = config_pkg::cva6_cfg_empty,
+    parameter type bht_update_t = logic
+) (
+    input  logic                                                      clk_i,
+    input  logic                                                      rst_ni,
+    input  logic                                                      flush_i,
+    input  logic                        [           CVA6Cfg.VLEN-1:0] vpc_i,
+    input  bht_update_t                                               bht_update_i,
+    // we potentially need INSTR_PER_FETCH predictions/cycle
+    output ariane_pkg::bht_prediction_t [CVA6Cfg.INSTR_PER_FETCH-1:0] bht_prediction_o
+);
+
+  // the last bit is always zero, we don't need it for indexing
+  localparam OFFSET = CVA6Cfg.RVC == 1'b1 ? 1 : 2;
+  // re-shape the branch history table
+  localparam NR_ROWS = CVA6Cfg.BHTEntries / CVA6Cfg.INSTR_PER_FETCH;
+  // number of bits needed to index the row
+  localparam ROW_ADDR_BITS = $clog2(CVA6Cfg.INSTR_PER_FETCH);
+  localparam ROW_INDEX_BITS = CVA6Cfg.RVC == 1'b1 ? $clog2(CVA6Cfg.INSTR_PER_FETCH) : 1;
+  // number of bits we should use for prediction
+  localparam PREDICTION_BITS = $clog2(NR_ROWS) + OFFSET + ROW_ADDR_BITS;
+
+  struct packed {
+    logic valid;
+    logic [CVA6Cfg.BHTHist-1:0] hist;
+    logic [2**CVA6Cfg.BHTHist-1:0][1:0] saturation_counter;
+  }
+      bht_d[NR_ROWS-1:0][CVA6Cfg.INSTR_PER_FETCH-1:0],
+      bht_q[NR_ROWS-1:0][CVA6Cfg.INSTR_PER_FETCH-1:0];
+
+
+  logic [$clog2(NR_ROWS)-1:0] index, update_pc;
+  logic [CVA6Cfg.BHTHist-1:0] update_hist;
+  logic [ ROW_INDEX_BITS-1:0] update_row_index;
+
+  assign index       = vpc_i[PREDICTION_BITS-1:ROW_ADDR_BITS+OFFSET];
+  assign update_pc   = bht_update_i.pc[PREDICTION_BITS-1:ROW_ADDR_BITS+OFFSET];
+  assign update_hist = bht_q[update_pc][update_row_index].hist;
+
+  if (CVA6Cfg.RVC) begin : gen_update_row_index
+    assign update_row_index = bht_update_i.pc[ROW_ADDR_BITS+OFFSET-1:OFFSET];
+  end else begin
+    assign update_row_index = '0;
+  end
+
+
+  logic [1:0] saturation_counter;
+
+  // Get the current history of the entry
+  logic [CVA6Cfg.INSTR_PER_FETCH-1:0][CVA6Cfg.BHTHist-1:0] read_history;
+  for (genvar i = 0; i < CVA6Cfg.INSTR_PER_FETCH; i++) begin
+    assign read_history[i] = bht_q[index][i].hist;
+  end
+
+  // prediction assignment
+  for (genvar i = 0; i < CVA6Cfg.INSTR_PER_FETCH; i++) begin : gen_bht_output
+    assign bht_prediction_o[i].valid = bht_q[index][i].valid;
+    assign bht_prediction_o[i].taken = bht_q[index][i].saturation_counter[read_history[i]][1] == 1'b1;
+  end
+
+  always_comb begin : update_bht
+    bht_d = bht_q;
+    saturation_counter = bht_q[update_pc][update_row_index].saturation_counter[update_hist];
+
+    if (bht_update_i.valid) begin
+      bht_d[update_pc][update_row_index].valid = 1'b1;
+
+      if (saturation_counter == 2'b11) begin
+        // we can safely decrease it
+        if (!bht_update_i.taken)
+          bht_d[update_pc][update_row_index].saturation_counter[update_hist] = saturation_counter - 1;
+        // then check if it saturated in the negative regime e.g.: branch not taken
+      end else if (saturation_counter == 2'b00) begin
+        // we can safely increase it
+        if (bht_update_i.taken)
+          bht_d[update_pc][update_row_index].saturation_counter[update_hist] = saturation_counter + 1;
+      end else begin  // otherwise we are not in any boundaries and can decrease or increase it
+        if (bht_update_i.taken)
+          bht_d[update_pc][update_row_index].saturation_counter[update_hist] = saturation_counter + 1;
+        else
+          bht_d[update_pc][update_row_index].saturation_counter[update_hist] = saturation_counter - 1;
+      end
+
+      bht_d[update_pc][update_row_index].hist = {
+        update_hist[CVA6Cfg.BHTHist-2:0], bht_update_i.taken
+      };
+    end
+  end
+
+  always_ff @(posedge clk_i or negedge rst_ni) begin
+    if (!rst_ni) begin
+      for (int unsigned i = 0; i < NR_ROWS; i++) begin
+        for (int j = 0; j < CVA6Cfg.INSTR_PER_FETCH; j++) begin
+          bht_q[i][j] <= '0;
+          for (int k = 0; k < 2 ** CVA6Cfg.BHTHist; k++) begin
+            bht_q[i][j].saturation_counter[k] <= 2'b10;
+          end
+        end
+      end
+    end else begin
+      // evict all entries
+      if (flush_i) begin
+        for (int i = 0; i < NR_ROWS; i++) begin
+          for (int j = 0; j < CVA6Cfg.INSTR_PER_FETCH; j++) begin
+            bht_q[i][j].valid <= 1'b0;
+            bht_q[i][j].hist  <= '0;
+            for (int k = 0; k < 2 ** CVA6Cfg.BHTHist; k++) begin
+              bht_q[i][j].saturation_counter[k] <= 2'b10;
+            end
+          end
+        end
+      end else begin
+        bht_q <= bht_d;
+      end
+    end
+  end
+
+
+endmodule

core/frontend/frontend.sv

@@ -510,7 +510,7 @@ module frontend
 
   if (CVA6Cfg.BHTEntries == 0) begin
     assign bht_prediction = '0;
-  end else begin : bht_gen
+  end else if (CVA6Cfg.BPType == config_pkg::BHT) begin : bht_gen
     bht #(
         .CVA6Cfg   (CVA6Cfg),
         .bht_update_t(bht_update_t),
@@ -524,6 +524,18 @@ module frontend
         .bht_update_i    (bht_update),
         .bht_prediction_o(bht_prediction)
     );
+  end else if (CVA6Cfg.BPType == config_pkg::PH_BHT) begin : bht2lvl_gen
+    bht2lvl #(
+        .CVA6Cfg     (CVA6Cfg),
+        .bht_update_t(bht_update_t)
+    ) i_bht (
+        .clk_i,
+        .rst_ni,
+        .flush_i         (flush_bp_i),
+        .vpc_i           (icache_vaddr_q),
+        .bht_update_i    (bht_update),
+        .bht_prediction_o(bht_prediction)
+    );
   end
 
   // we need to inspect up to CVA6Cfg.INSTR_PER_FETCH instructions for branches

core/include/build_config_pkg.sv

@@ -103,7 +103,9 @@ package build_config_pkg;
     cfg.ExceptionAddress = CVA6Cfg.ExceptionAddress;
     cfg.RASDepth = CVA6Cfg.RASDepth;
     cfg.BTBEntries = CVA6Cfg.BTBEntries;
+    cfg.BPType = CVA6Cfg.BPType;
     cfg.BHTEntries = CVA6Cfg.BHTEntries;
+    cfg.BHTHist = CVA6Cfg.BHTHist;
     cfg.DmBaseAddress = CVA6Cfg.DmBaseAddress;
     cfg.TvalEn = CVA6Cfg.TvalEn;
     cfg.DirectVecOnly = CVA6Cfg.DirectVecOnly;

core/include/config_pkg.sv

@@ -35,6 +35,12 @@ package config_pkg;
     HPDCACHE_WT_WB = 4
   } cache_type_t;
 
+  /// Branch predictor parameter
+  typedef enum logic {
+    BHT = 0,  // Bimodal predictor
+    PH_BHT = 1  // Private History Bimodal predictor
+  } bp_type_t;
+
   /// Data and Address length
   typedef enum logic [3:0] {
     ModeOff  = 0,
@@ -214,8 +220,12 @@ package config_pkg;
     int unsigned                 RASDepth;
     // Branch target buffer entries
     int unsigned                 BTBEntries;
+    // Branch predictor type
+    bp_type_t                    BPType;
     // Branch history entries
     int unsigned                 BHTEntries;
+    // Branch history bits
+    int unsigned                 BHTHist;
     // MMU instruction TLB entries
     int unsigned                 InstrTlbEntries;
     // MMU data TLB entries
@@ -299,7 +309,9 @@ package config_pkg;
     logic [63:0] ExceptionAddress;
     int unsigned RASDepth;
     int unsigned BTBEntries;
+    bp_type_t    BPType;
     int unsigned BHTEntries;
+    int unsigned BHTHist;
     int unsigned InstrTlbEntries;
     int unsigned DataTlbEntries;
     bit unsigned UseSharedTlb;

core/include/cv32a60x_config_pkg.sv

@@ -62,7 +62,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(2),
       BTBEntries: unsigned'(0),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(32),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(0),
       DirectVecOnly: bit'(1),

core/include/cv32a65x_config_pkg.sv

@@ -62,7 +62,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(2),
       BTBEntries: unsigned'(0),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(32),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(0),
       DirectVecOnly: bit'(1),

core/include/cv32a6_embedded_config_pkg_deprecated.sv

@@ -117,7 +117,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv32a6_ima_sv32_fpga_config_pkg.sv

@@ -115,7 +115,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: unsigned'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv32a6_imac_sv0_config_pkg.sv

@@ -115,7 +115,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: unsigned'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv32a6_imac_sv32_config_pkg.sv

@@ -114,7 +114,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv32a6_imafc_sv32_config_pkg.sv

@@ -115,7 +115,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a60ax_config_pkg.sv

@@ -69,7 +69,9 @@ localparam config_pkg::cva6_user_cfg_t cva6_cfg = '{
    ExceptionAddress: 64'h808,
    RASDepth: unsigned'(4),
    BTBEntries: unsigned'(16),
+   BPType: config_pkg::BHT,
    BHTEntries: unsigned'(64),
+   BHTHist: unsigned'(3),
    DmBaseAddress: 64'h0,
    TvalEn: bit'(1),
    DirectVecOnly: bit'(0),

core/include/cv64a6_imadfcv_sv39_polara_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdc_sv39_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdc_sv39_hpdcache_config_pkg.sv

@@ -125,7 +125,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdc_sv39_hpdcache_wb_config_pkg.sv

@@ -125,7 +125,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdc_sv39_openpiton_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdc_sv39_wb_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdch_sv39_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdch_sv39_wb_config_pkg.sv

@@ -118,7 +118,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_imafdcv_sv39_config_pkg.sv

@@ -120,7 +120,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(CVA6ConfigRASDepth),
       BTBEntries: unsigned'(CVA6ConfigBTBEntries),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(CVA6ConfigBHTEntries),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(CVA6ConfigTvalEn),
       DirectVecOnly: bit'(0),

core/include/cv64a6_mmu_config_pkg.sv

@@ -69,7 +69,9 @@ package cva6_config_pkg;
       ExceptionAddress: 64'h808,
       RASDepth: unsigned'(2),
       BTBEntries: unsigned'(0),
+      BPType: config_pkg::BHT,
       BHTEntries: unsigned'(32),
+      BHTHist: unsigned'(3),
       DmBaseAddress: 64'h0,
       TvalEn: bit'(0),
       DirectVecOnly: bit'(1),

docs/scripts/spec_builder.py

@@ -181,6 +181,7 @@ def main():
         file.append("../core/cva6.sv")
         file.append("../core/frontend/frontend.sv")
         file.append("../core/frontend/bht.sv")
+        file.append("../core/frontend/bht2lvl.sv")
         file.append("../core/frontend/btb.sv")
         file.append("../core/frontend/ras.sv")
         file.append("../core/frontend/instr_queue.sv")

src_files.yml

@@ -22,6 +22,7 @@ ariane:
     src/ex_stage.sv,
     src/frontend/btb.sv,
     src/frontend/bht.sv,
+    src/frontend/bht2lvl.sv,
     src/frontend/ras.sv,
     src/frontend/instr_scan.sv,
     src/frontend/frontend.sv,