Alias DPC to the real program counter, small savings overall

hdl/hazard3_core.v

@@ -224,6 +224,10 @@ wire                 x_jump_not_except;
 wire                 x_mmode_execution;
 wire                 x_trap_wfi;
 
+wire [W_ADDR-1:0]    debug_dpc_wdata;
+wire                 debug_dpc_wen;
+wire [W_ADDR-1:0]    debug_dpc_rdata;
+
 hazard3_decode #(
 `include "hazard3_config_inst.vh"
 ) decode_u (
@@ -243,6 +247,10 @@ hazard3_decode #(
 	.m_mode               (x_mmode_execution),
 	.trap_wfi             (x_trap_wfi),
 
+	.debug_dpc_wdata      (debug_dpc_wdata),
+	.debug_dpc_wen        (debug_dpc_wen),
+	.debug_dpc_rdata      (debug_dpc_rdata),
+
 	.d_starved            (d_starved),
 	.x_stall              (x_stall),
 	.f_jump_now           (f_jump_now),
@@ -964,6 +972,10 @@ hazard3_csr #(
 	.dbg_data0_wdata            (dbg_data0_wdata),
 	.dbg_data0_wen              (dbg_data0_wen),
 
+	.debug_dpc_wdata            (debug_dpc_wdata),
+	.debug_dpc_wen              (debug_dpc_wen),
+	.debug_dpc_rdata            (debug_dpc_rdata),
+
 	// CSR access port
 	// *en_soon are early access strobes which are not a function of bus stall.
 	// Can generate access faults (hence traps), but do not actually perform access.

hdl/hazard3_csr.v

@@ -36,6 +36,10 @@ module hazard3_csr #(
 	output wire [W_DATA-1:0]  dbg_data0_wdata,
 	output wire               dbg_data0_wen,
 
+	output wire [W_ADDR-1:0]  debug_dpc_wdata,
+	output wire               debug_dpc_wen,
+	input  wire [W_ADDR-1:0]  debug_dpc_rdata,
+
 	// Read port is combinatorial.
 	// Write port is synchronous, and write effects will be observed on the next clock cycle.
 	// The *_soon strobes are versions which the core does not gate with its stall signal.
@@ -510,20 +514,14 @@ always @ (posedge clk or negedge rst_n) begin
 	end
 end
 
-reg [XLEN-1:0] dpc;
-
-always @ (posedge clk or negedge rst_n) begin
-	if (!rst_n) begin
-		dpc <= X0;
-	end else if (DEBUG_SUPPORT) begin
-		if (enter_debug_mode)
-			dpc <= mepc_in;
-		else if (debug_mode && wen && addr == DPC)
-			// 1 or 2 LSBs are hardwired to 0, depending on IALIGN.
-			dpc <= wdata_update & (~X0 << 2 - EXTENSION_C);
-	end
-end
+// DPC is mapped to the real PC in the decode module. We set it to the
+// exception return address when entering Debug Mode, and whilst in Debug
+// Mode we can write to it as though it were a CSR. Note only IALIGN'd values
+// can be written to dpc.
+assign debug_dpc_wdata = (debug_mode ? wdata_update       : mepc_in) & (~X0 << 2 - EXTENSION_C);
+assign debug_dpc_wen   =  debug_mode ? wen && addr == DPC : enter_debug_mode;
 
+// Debug Module's data0 register is mapped into the core's CSR space:
 assign dbg_data0_wdata = wdata;
 assign dbg_data0_wen = debug_mode && wen && addr == DMDATA0;
 
@@ -1059,7 +1057,7 @@ always @ (*) begin
 
 	DPC: if (DEBUG_SUPPORT) begin
 		decode_match = match_drw;
-		rdata = dpc;
+		rdata = debug_dpc_rdata;
 	end
 
 	DMDATA0: if (DEBUG_SUPPORT) begin
@@ -1290,8 +1288,8 @@ wire [3:0] mcause_irq_num =	irq_active ? standard_irq_num : 4'd0;
 wire [3:0] vector_sel =	!exception_req_any && irq_vector_enable ? mcause_irq_num : 4'd0;
 
 assign trap_addr =
-	except == EXCEPT_MRET ? mepc :
-	pending_dbg_resume    ? dpc  : mtvec | {26'h0, vector_sel, 2'h0};
+	except == EXCEPT_MRET ? mepc             :
+	pending_dbg_resume    ? debug_dpc_rdata  : mtvec | {26'h0, vector_sel, 2'h0};
 
 // Check for exception-like or IRQ-like trap entry; any debug mode entry takes
 // priority over any regular trap.

hdl/hazard3_decode.v

@@ -25,6 +25,10 @@ module hazard3_decode #(
 	input  wire                 m_mode,
 	input  wire                 trap_wfi,
 
+	input  wire [W_ADDR-1:0]    debug_dpc_wdata,
+	input  wire                 debug_dpc_wen,
+	output wire [W_ADDR-1:0]    debug_dpc_rdata,
+
 	output wire                 d_starved,
 	input  wire                 x_stall,
 	input  wire                 f_jump_now,
@@ -131,6 +135,7 @@ end
 reg  [W_ADDR-1:0] pc;
 wire [W_ADDR-1:0] pc_seq_next = pc + (d_instr_is_32bit ? 32'h4 : 32'h2);
 assign d_pc = pc;
+assign debug_dpc_rdata = pc;
 
 // Frontend should mark the whole instruction, and nothing but the
 // instruction, as a predicted branch. This goes wrong when we execute the
@@ -146,7 +151,11 @@ always @ (posedge clk or negedge rst_n) begin
 	if (!rst_n) begin
 		pc <= RESET_VECTOR;
 	end else begin
-		if ((f_jump_now && !assert_cir_lock) || (cir_lock_prev && deassert_cir_lock)) begin
+		if (debug_dpc_wen) begin
+			pc <= debug_dpc_wdata;
+		end else if (debug_mode) begin
+			pc <= pc;
+		end else if ((f_jump_now && !assert_cir_lock) || (cir_lock_prev && deassert_cir_lock)) begin
 			pc <= f_jump_target;
 		end else if (!d_stall && !cir_lock) begin
 			// If this instruction is a predicted-taken branch (and has not