frontend: disable branch prediction, mkI

Signed-off-by: Alban Gruin <alban.gruin@irit.fr>

src/ariane.sv

@@ -281,6 +281,7 @@ module ariane import ariane_pkg::*; #(
     .fetch_entry_ready_i ( fetch_ready_id_if             ),
     .has_mem_access_o    ( has_mem_access_if_verif       ),
     .branch_speculation_o( bsp_if_perf                   ),
+    .enable_bp_i         ( 1'b0                          ),
     .*
   );
 

src/frontend/frontend.sv

@@ -46,7 +46,9 @@ module frontend import ariane_pkg::*; #(
   input  logic               fetch_entry_ready_i, // ID acknowledged this instruction
 
   output logic               has_mem_access_o,
-  output logic               branch_speculation_o
+  output logic               branch_speculation_o,
+
+  input  logic               enable_bp_i
 );
     // Instruction Cache Registers, from I$
     logic [FETCH_WIDTH-1:0] icache_data_q;
@@ -179,7 +181,7 @@ module frontend import ariane_pkg::*; #(
           4'b0001: begin
             ras_pop = 1'b0;
             ras_push = 1'b0;
-            if (btb_prediction_shifted[i].valid) begin
+            if (btb_prediction_shifted[i].valid & enable_bp_i) begin
               predict_address = btb_prediction_shifted[i].target_address;
               cf_type[i] = ariane_pkg::JumpR;
             end
@@ -195,26 +197,30 @@ module frontend import ariane_pkg::*; #(
           // return
           4'b0100: begin
             // make sure to only alter the RAS if we actually consumed the instruction
-            ras_pop = ras_predict.valid & instr_queue_consumed[i];
             ras_push = 1'b0;
-            predict_address = ras_predict.ra;
-            cf_type[i] = ariane_pkg::Return;
+            ras_pop = ras_predict.valid & instr_queue_consumed[i];
+            if (enable_bp_i) begin
+              predict_address = ras_predict.ra;
+              cf_type[i] = ariane_pkg::Return;
+            end
           end
           // branch prediction
           4'b1000: begin
             ras_pop = 1'b0;
             ras_push = 1'b0;
-            // if we have a valid dynamic prediction use it
-            if (bht_prediction_shifted[i].valid) begin
-              taken_rvi_cf[i] = rvi_branch[i] & bht_prediction_shifted[i].taken;
-              taken_rvc_cf[i] = rvc_branch[i] & bht_prediction_shifted[i].taken;
-            // otherwise default to static prediction
-            end else begin
-              // set if immediate is negative - static prediction
-              taken_rvi_cf[i] = rvi_branch[i] & rvi_imm[i][riscv::VLEN-1];
-              taken_rvc_cf[i] = rvc_branch[i] & rvc_imm[i][riscv::VLEN-1];
+            if (enable_bp_i) begin
+              // if we have a valid dynamic prediction use it
+              if (bht_prediction_shifted[i].valid) begin
+                taken_rvi_cf[i] = rvi_branch[i] & bht_prediction_shifted[i].taken;
+                taken_rvc_cf[i] = rvc_branch[i] & bht_prediction_shifted[i].taken;
+              // otherwise default to static prediction
+              end else begin
+                // set if immediate is negative - static prediction
+                taken_rvi_cf[i] = rvi_branch[i] & rvi_imm[i][riscv::VLEN-1];
+                taken_rvc_cf[i] = rvc_branch[i] & rvc_imm[i][riscv::VLEN-1];
+              end
+              if (taken_rvi_cf[i] || taken_rvc_cf[i]) cf_type[i] = ariane_pkg::Branch;
             end
-            if (taken_rvi_cf[i] || taken_rvc_cf[i]) cf_type[i] = ariane_pkg::Branch;
           end
           default:;
             // default: $error("Decoded more than one control flow");
@@ -227,7 +233,9 @@ module frontend import ariane_pkg::*; #(
           end
           // calculate the jump target address
           if (taken_rvc_cf[i] || taken_rvi_cf[i]) begin
-            predict_address = addr[i] + (taken_rvc_cf[i] ? rvc_imm[i] : rvi_imm[i]);
+            if (rvi_jump[i] || rvc_jump[i] || enable_bp_i) begin
+              predict_address = addr[i] + (taken_rvc_cf[i] ? rvc_imm[i] : rvi_imm[i]);
+             end
           end
       end
     end