Simplify default assignments to enum types

This commit simplifies the assignment of literals to enum types in
default cases by:
- defining or using existing enum values for all-zero values,
- feeding a single `1'bX` into the type cast instead of exact width
  (the tools are fine with that).

rtl/ibex_controller.sv

@@ -163,10 +163,10 @@ module ibex_controller (
 
     csr_save_cause_o       = 1'b0;
 
-    exc_cause_o            = exc_cause_e'({$bits(exc_cause_e){1'b0}});
+    exc_cause_o            = EXC_CAUSE_INSN_ADDR_MISA; // = 6'h00
     exc_pc_mux_o           = EXC_PC_IRQ;
 
-    csr_cause_o            = exc_cause_e'({$bits(exc_cause_e){1'b0}});
+    csr_cause_o            = EXC_CAUSE_INSN_ADDR_MISA; // = 6'h00
 
     pc_mux_o               = PC_BOOT;
     pc_set_o               = 1'b0;
@@ -513,7 +513,7 @@ module ibex_controller (
 
       default: begin
         instr_req_o = 1'b0;
-        ctrl_fsm_ns = ctrl_fsm_e'({$bits(ctrl_fsm_e){1'bX}});
+        ctrl_fsm_ns = ctrl_fsm_e'(1'bX);
       end
     endcase
   end

rtl/ibex_cs_registers.sv

@@ -239,7 +239,7 @@ module ibex_cs_registers #(
         mstatus_n = '{
           mie:  csr_wdata_int[`MSTATUS_MIE_BITS],
           mpie: csr_wdata_int[`MSTATUS_MPIE_BITS],
-          mpp:  priv_lvl_e'(PRIV_LVL_M)
+          mpp:  PRIV_LVL_M
         };
       end
       // mepc: exception program counter
@@ -371,8 +371,8 @@ module ibex_cs_registers #(
 
       depc_q      <= '0;
       dcsr_q     <= '{
-        xdebugver: x_debug_ver_e'({$bits(x_debug_ver_e){1'b0}}),
-        cause:     dbg_cause_e'({$bits(dbg_cause_e){1'b0}}),
+        xdebugver: XDEBUGVER_NO,   // 4'h0
+        cause:     DBG_CAUSE_NONE, // 3'h0
         prv:       PRIV_LVL_M,
         default:   '0
       };

rtl/ibex_defines.sv

@@ -112,8 +112,8 @@ typedef enum logic[1:0] {
 
 // Constants for the dcsr.xdebugver fields
 typedef enum logic[3:0] {
-   XDEBUGVER_NO  = 4'd0, // no external debug support
-   XDEBUGVER_STD = 4'd4, // external debug according to RISC-V debug spec
+   XDEBUGVER_NO     = 4'd0, // no external debug support
+   XDEBUGVER_STD    = 4'd4, // external debug according to RISC-V debug spec
    XDEBUGVER_NONSTD = 4'd15 // debug not conforming to RISC-V debug spec
 } x_debug_ver_e;
 
@@ -185,6 +185,7 @@ typedef enum logic [2:0] {
 
 // Exception cause
 typedef enum logic [5:0] {
+  EXC_CAUSE_INSN_ADDR_MISA     = 6'h00,
   EXC_CAUSE_ILLEGAL_INSN       = 6'h02,
   EXC_CAUSE_BREAKPOINT         = 6'h03,
   EXC_CAUSE_LOAD_ACCESS_FAULT  = 6'h05,
@@ -209,6 +210,7 @@ typedef enum logic [7:0] {
 
 // Debug cause
 typedef enum logic [2:0] {
+  DBG_CAUSE_NONE    = 3'h0,
   DBG_CAUSE_EBREAK  = 3'h1,
   DBG_CAUSE_TRIGGER = 3'h2,
   DBG_CAUSE_HALTREQ = 3'h3,

rtl/ibex_int_controller.sv

@@ -87,7 +87,7 @@ module ibex_int_controller (
       end
 
       default: begin
-        exc_ctrl_ns = exc_ctrl_e'({$bits(exc_ctrl_e){1'bX}});
+        exc_ctrl_ns = exc_ctrl_e'(1'bX);
       end
     endcase
   end

rtl/ibex_load_store_unit.sv

@@ -423,7 +423,7 @@ module ibex_load_store_unit (
       end //~ WAIT_RVALID
 
       default: begin
-        ls_fsm_ns = ls_fsm_e'({$bits(ls_fsm_e){1'bX}});
+        ls_fsm_ns = ls_fsm_e'(1'bX);
       end
     endcase
   end

rtl/ibex_multdiv_fast.sv

@@ -247,7 +247,7 @@ module ibex_multdiv_fast (
       end
 
       default: begin
-        divcurr_state_n = div_fsm_e'({$bits(div_fsm_e){1'bX}});
+        divcurr_state_n = div_fsm_e'(1'bX);
       end
     endcase // divcurr_state_q
   end
@@ -327,7 +327,7 @@ module ibex_multdiv_fast (
         mult_is_ready = 1'b1;
       end
       default: begin
-        mult_state_n = mult_fsm_e'({$bits(mult_fsm_e){1'bX}});
+        mult_state_n = mult_fsm_e'(1'bX);
       end
     endcase // mult_state_q
   end

rtl/ibex_multdiv_slow.sv

@@ -284,7 +284,7 @@ module ibex_multdiv_slow (
         end
 
         default: begin
-          curr_state_d = div_fsm_e'({$bits(div_fsm_e){1'bX}});
+          curr_state_d = div_fsm_e'(1'bX);
         end
         endcase // curr_state_q
       end

rtl/ibex_prefetch_buffer.sv

@@ -200,7 +200,7 @@ module ibex_prefetch_buffer (
       end
 
       default: begin
-        NS = prefetch_fsm_e'({$bits(prefetch_fsm_e){1'bX}});
+        NS = prefetch_fsm_e'(1'bX);
       end
     endcase
   end