frontend: disable branch prediction, mkII

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

src/frontend/frontend.sv

@@ -89,7 +89,9 @@ module frontend import ariane_pkg::*; #(
     // re-aligned instruction and address (coming from cache - combinationally)
     logic [INSTR_PER_FETCH-1:0][31:0] instr;
     logic [INSTR_PER_FETCH-1:0][riscv::VLEN-1:0] addr;
-    logic [INSTR_PER_FETCH-1:0]       instruction_valid;
+    logic [INSTR_PER_FETCH-1:0]       instruction_valid, instruction_really_valid;
+    logic [INSTR_PER_FETCH-1:0]       instr_really_valid;
+
     // BHT, BTB and RAS prediction
     bht_prediction_t [INSTR_PER_FETCH-1:0] bht_prediction;
     btb_prediction_t [INSTR_PER_FETCH-1:0] btb_prediction;
@@ -239,13 +241,28 @@ module frontend import ariane_pkg::*; #(
           end
       end
     end
+
+    logic has_speculative;
+
     // or reduce struct
     always_comb begin
       bp_valid = 1'b0;
+      has_speculative = 1'b0;
+      instruction_really_valid = '0;
+
       // BP cannot be valid if we have a return instruction and the RAS is not giving a valid address
       // Check that we encountered a control flow and that for a return the RAS 
       // contains a valid prediction.
-      for (int i = 0; i < INSTR_PER_FETCH; i++) bp_valid |= ((cf_type[i] != NoCF & cf_type[i] != Return) | ((cf_type[i] == Return) & ras_predict.valid));
+      for (int unsigned i = 0; i < INSTR_PER_FETCH; i++) begin
+         bp_valid |= ((cf_type[i] != NoCF & cf_type[i] != Return) | ((cf_type[i] == Return) & ras_predict.valid));
+         has_speculative |= (!(cf_type[i] inside {NoCF, Return, Jump}) | ((cf_type[i] == Return) & ras_predict.valid));
+
+         if (enable_bp_i) begin
+           instruction_really_valid[i] = instruction_valid[i];
+         end else begin
+           instruction_really_valid[i] = instruction_really_valid[i] && !(has_speculative);
+         end
+      end
     end
     assign is_mispredict = resolved_branch_i.valid & resolved_branch_i.is_mispredict;
 
@@ -434,7 +451,7 @@ module frontend import ariane_pkg::*; #(
       .exception_addr_i    ( icache_vaddr_q       ),
       .predict_address_i   ( predict_address      ),
       .cf_type_i           ( cf_type              ),
-      .valid_i             ( instruction_valid    ), // from re-aligner
+      .valid_i             ( instruction_really_valid ), // from re-aligner
       .consumed_o          ( instr_queue_consumed ),
       .ready_o             ( instr_queue_ready    ),
       .replay_o            ( replay               ),