[rtl] Switch multdiv_en to multdiv_sel where possible

Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>

rtl/ibex_ex_block.sv

@@ -60,7 +60,7 @@ module ibex_ex_block #(
   logic [33:0] alu_adder_result_ext;
   logic        alu_cmp_result, alu_is_equal_result;
   logic        multdiv_valid;
-  logic        multdiv_en;
+  logic        multdiv_sel;
   logic [31:0] alu_imd_val_d;
   logic        alu_imd_val_we;
   logic [33:0] multdiv_imd_val_d;
@@ -76,12 +76,12 @@ module ibex_ex_block #(
     from the multdiv_i module are eliminated
   */
   if (RV32M) begin : gen_multdiv_m
-    assign multdiv_en     = mult_en_i | div_en_i;
+    assign multdiv_sel = multdiv_sel_i;
   end else begin : gen_multdiv_no_m
-    assign multdiv_en     = 1'b0;
+    assign multdiv_sel = 1'b0;
   end
 
-  assign result_ex_o = multdiv_en ? multdiv_result : alu_result;
+  assign result_ex_o = multdiv_sel ? multdiv_result : alu_result;
 
   // branch handling
   assign branch_decision_o  = alu_cmp_result;
@@ -204,7 +204,9 @@ module ibex_ex_block #(
     );
   end
 
-  // ALU output valid in same cycle, multiplier/divider may require multiple cycles
-  assign ex_valid_o = multdiv_en ? multdiv_valid : !alu_imd_val_we;
+  // Multiplier/divider may require multiple cycles. The ALU output is valid in the same cycle
+  // unless the intermediate result register is being written (which indicates this isn't the
+  // final cycle of ALU operation).
+  assign ex_valid_o = multdiv_sel ? multdiv_valid : !alu_imd_val_we;
 
 endmodule

rtl/ibex_multdiv_fast.sv

@@ -226,7 +226,7 @@ module ibex_multdiv_fast #(
       if (!rst_ni) begin
         mult_state_q <= MULL;
       end else begin
-        if (mult_en_i) begin
+        if (mult_en_internal) begin
           mult_state_q <= mult_state_d;
         end
       end

rtl/ibex_multdiv_slow.sv

@@ -161,7 +161,7 @@ module ibex_multdiv_slow
       op_numerator_q   <= 32'h0;
       md_state_q       <= MD_IDLE;
     end else begin
-      if (~multdiv_hold) begin
+      if ((mult_en_i || div_en_i) && !multdiv_hold) begin
         multdiv_state_q  <= multdiv_state_d;
         op_b_shift_q     <= op_b_shift_d;
         op_a_shift_q     <= op_a_shift_d;