From 152261fac8720fc61d1a84626d40ab5edeca5049 Mon Sep 17 00:00:00 2001
From: Anton Blanchard <anton@linux.ibm.com>
Date: Sun, 15 Sep 2019 18:03:48 +1000
Subject: [PATCH] Remove cycle in writeback

The pipeline had a cycle in writeback. Writeback is pretty
simple and unlikely to be a bottleneck, so lets remove it.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>
---
 writeback.vhdl | 75 +++++++++++++++-----------------------------------
 1 file changed, 22 insertions(+), 53 deletions(-)

diff --git a/writeback.vhdl b/writeback.vhdl
index 3544fec..82f5e5e 100644
--- a/writeback.vhdl
+++ b/writeback.vhdl
@@ -21,34 +21,12 @@ entity writeback is
 end entity writeback;
 
 architecture behaviour of writeback is
-    type reg_internal_type is record
-        complete : std_ulogic;
-    end record;
-    type reg_type is record
-        w : WritebackToRegisterFileType;
-        c : WritebackToCrFileType;
-    end record;
-    signal r, rin         : reg_type;
-    signal r_int, rin_int : reg_internal_type;
 begin
-    writeback_0: process(clk)
-    begin
-        if rising_edge(clk) then
-            r <= rin;
-            r_int <= rin_int;
-        end if;
-    end process;
-
     writeback_1: process(all)
-        variable x     : std_ulogic_vector(0 downto 0);
-        variable y     : std_ulogic_vector(0 downto 0);
-        variable z     : std_ulogic_vector(0 downto 0);
-        variable v     : reg_type;
-        variable v_int : reg_internal_type;
+        variable x : std_ulogic_vector(0 downto 0);
+        variable y : std_ulogic_vector(0 downto 0);
+        variable z : std_ulogic_vector(0 downto 0);
     begin
-        v := r;
-        v_int := r_int;
-
         x := "" & e_in.valid;
         y := "" & l_in.valid;
         z := "" & m_in.valid;
@@ -61,51 +39,42 @@ begin
 
         assert not(e_in.write_cr_enable = '1' and m_in.write_cr_enable = '1');
 
-        v.w := WritebackToRegisterFileInit;
-        v.c := WritebackToCrFileInit;
+        w_out <= WritebackToRegisterFileInit;
+        c_out <= WritebackToCrFileInit;
 
-        v_int.complete := '0';
+        complete_out <= '0';
         if e_in.valid = '1' or l_in.valid = '1' or m_in.valid = '1' then
-            v_int.complete := '1';
+            complete_out <= '1';
         end if;
 
         if e_in.write_enable = '1' then
-            v.w.write_reg := e_in.write_reg;
-            v.w.write_data := e_in.write_data;
-            v.w.write_enable := '1';
+            w_out.write_reg <= e_in.write_reg;
+            w_out.write_data <= e_in.write_data;
+            w_out.write_enable <= '1';
         end if;
 
         if e_in.write_cr_enable = '1' then
-            v.c.write_cr_enable := '1';
-            v.c.write_cr_mask := e_in.write_cr_mask;
-            v.c.write_cr_data := e_in.write_cr_data;
+            c_out.write_cr_enable <= '1';
+            c_out.write_cr_mask <= e_in.write_cr_mask;
+            c_out.write_cr_data <= e_in.write_cr_data;
         end if;
 
         if l_in.write_enable = '1' then
-            v.w.write_reg := l_in.write_reg;
-            v.w.write_data := l_in.write_data;
-            v.w.write_enable := '1';
+            w_out.write_reg <= l_in.write_reg;
+            w_out.write_data <= l_in.write_data;
+            w_out.write_enable <= '1';
         end if;
 
         if m_in.write_reg_enable = '1' then
-            v.w.write_enable := '1';
-            v.w.write_reg := m_in.write_reg_nr;
-            v.w.write_data := m_in.write_reg_data;
+            w_out.write_enable <= '1';
+            w_out.write_reg <= m_in.write_reg_nr;
+            w_out.write_data <= m_in.write_reg_data;
         end if;
 
         if m_in.write_cr_enable = '1' then
-            v.c.write_cr_enable := '1';
-            v.c.write_cr_mask := m_in.write_cr_mask;
-            v.c.write_cr_data := m_in.write_cr_data;
+            c_out.write_cr_enable <= '1';
+            c_out.write_cr_mask <= m_in.write_cr_mask;
+            c_out.write_cr_data <= m_in.write_cr_data;
         end if;
-
-        -- Update registers
-        rin <= v;
-        rin_int <= v_int;
-
-        -- Update outputs
-        complete_out <= r_int.complete;
-        w_out <= r.w;
-        c_out <= r.c;
     end process;
 end;