core: Predict not-taken conditional branches using BTC

This adds a bit to the BTC to store whether the corresponding branch
instruction was taken last time it was encountered.  That lets us pass
a not-taken prediction down to decode1, which for backwards direct
branches inhibits it from redirecting fetch to the target of the
branch.  This increases coremark by about 2%.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

common.vhdl

@@ -196,6 +196,7 @@ package common is
 	stop_mark: std_ulogic;
         sequential: std_ulogic;
         predicted : std_ulogic;
+        pred_ntaken : std_ulogic;
 	nia: std_ulogic_vector(63 downto 0);
     end record;
 
@@ -207,6 +208,7 @@ package common is
 	insn: std_ulogic_vector(31 downto 0);
         big_endian: std_ulogic;
         next_predicted: std_ulogic;
+        next_pred_ntaken: std_ulogic;
     end record;
 
     type IcacheEventType is record

decode1.vhdl

@@ -740,6 +740,8 @@ begin
         bv.br_offset := br_offset;
         if f_in.next_predicted = '1' then
             v.br_pred := '1';
+        elsif f_in.next_pred_ntaken = '1' then
+            v.br_pred := '0';
         end if;
         bv.predict := v.br_pred and f_in.valid and not flush_in and not busy_out and not f_in.next_predicted;
         -- after a clock edge...

fetch1.vhdl

@@ -40,7 +40,8 @@ architecture behaviour of fetch1 is
     type reg_internal_t is record
         mode_32bit: std_ulogic;
         rd_is_niap4: std_ulogic;
-        predicted: std_ulogic;
+        predicted_taken: std_ulogic;
+        pred_not_taken: std_ulogic;
         predicted_nia: std_ulogic_vector(63 downto 0);
     end record;
     signal r, r_next : Fetch1ToIcacheType;
@@ -52,7 +53,7 @@ architecture behaviour of fetch1 is
     constant BTC_TAG_BITS : integer := 62 - BTC_ADDR_BITS;
     constant BTC_TARGET_BITS : integer := 62;
     constant BTC_SIZE : integer := 2 ** BTC_ADDR_BITS;
-    constant BTC_WIDTH : integer := BTC_TAG_BITS + BTC_TARGET_BITS;
+    constant BTC_WIDTH : integer := BTC_TAG_BITS + BTC_TARGET_BITS + 1;
     type btc_mem_type is array (0 to BTC_SIZE - 1) of std_ulogic_vector(BTC_WIDTH - 1 downto 0);
 
     signal btc_rd_data : std_ulogic_vector(BTC_WIDTH - 1 downto 0) := (others => '0');
@@ -83,8 +84,10 @@ begin
             end if;
             if advance_nia = '1' then
                 r.predicted <= r_next.predicted;
+                r.pred_ntaken <= r_next.pred_ntaken;
                 r.nia <= r_next.nia;
-                r_int.predicted <= r_next_int.predicted;
+                r_int.predicted_taken <= r_next_int.predicted_taken;
+                r_int.pred_not_taken <= r_next_int.pred_not_taken;
                 r_int.predicted_nia <= r_next_int.predicted_nia;
                 r_int.rd_is_niap4 <= r_next.sequential;
             end if;
@@ -107,13 +110,12 @@ begin
         signal btc_wr : std_ulogic;
         signal btc_wr_data : std_ulogic_vector(BTC_WIDTH - 1 downto 0);
         signal btc_wr_addr : std_ulogic_vector(BTC_ADDR_BITS - 1 downto 0);
-        signal btc_wr_v : std_ulogic;
     begin
-        btc_wr_data <= w_in.br_nia(63 downto BTC_ADDR_BITS + 2) &
+        btc_wr_data <= w_in.br_taken &
+                       w_in.br_nia(63 downto BTC_ADDR_BITS + 2) &
                        w_in.redirect_nia(63 downto 2);
         btc_wr_addr <= w_in.br_nia(BTC_ADDR_BITS + 1 downto 2);
         btc_wr <= w_in.br_last;
-        btc_wr_v <= w_in.br_taken;
 
         btc_ram : process(clk)
             variable raddr : unsigned(BTC_ADDR_BITS - 1 downto 0);
@@ -131,7 +133,7 @@ begin
                 if inval_btc = '1' or rst = '1' then
                     btc_valids <= (others => '0');
                 elsif btc_wr = '1' then
-                    btc_valids(to_integer(unsigned(btc_wr_addr))) <= btc_wr_v;
+                    btc_valids(to_integer(unsigned(btc_wr_addr))) <= '1';
                 end if;
             end if;
         end process;
@@ -145,7 +147,9 @@ begin
 	v_int := r_int;
         v.sequential := '0';
         v.predicted := '0';
-        v_int.predicted := '0';
+        v.pred_ntaken := '0';
+        v_int.predicted_taken := '0';
+        v_int.pred_not_taken := '0';
 
 	if rst = '1' then
 	    if alt_reset_in = '1' then
@@ -172,19 +176,21 @@ begin
             if r_int.mode_32bit = '1' then
                 v.nia(63 downto 32) := (others => '0');
             end if;
-        elsif r_int.predicted = '1' then
+        elsif r_int.predicted_taken = '1' then
             v.nia := r_int.predicted_nia;
             v.predicted := '1';
         else
             v.sequential := '1';
+            v.pred_ntaken := r_int.pred_not_taken;
             v.nia := std_ulogic_vector(unsigned(r.nia) + 4);
             if r_int.mode_32bit = '1' then
                 v.nia(63 downto 32) := x"00000000";
             end if;
             if btc_rd_valid = '1' and r_int.rd_is_niap4 = '1' and
-                btc_rd_data(BTC_WIDTH - 1 downto BTC_TARGET_BITS)
+                btc_rd_data(BTC_WIDTH - 2 downto BTC_TARGET_BITS)
                 = v.nia(BTC_TAG_BITS + BTC_ADDR_BITS + 1 downto BTC_ADDR_BITS + 2) then
-                v_int.predicted := '1';
+                v_int.predicted_taken := btc_rd_data(BTC_WIDTH - 1);
+                v_int.pred_not_taken := not btc_rd_data(BTC_WIDTH - 1);
             end if;
         end if;
         v_int.predicted_nia := btc_rd_data(BTC_TARGET_BITS - 1 downto 0) & "00";

icache.vhdl

@@ -577,6 +577,7 @@ begin
         i_out.fetch_failed <= r.fetch_failed;
         i_out.big_endian <= r.big_endian;
         i_out.next_predicted <= i_in.predicted;
+        i_out.next_pred_ntaken <= i_in.pred_ntaken;
 
 	-- Stall fetch1 if we have a miss on cache or TLB or a protection fault
 	stall_out <= not (is_hit and access_ok);