execute1: Take an extra cycle for OE=1 multiply instructions

We now expect the overflow signal from the multiplier to come along
one cycle later than the product.

This breaks up a long combinatorial path and improves timing.

This also changes some uses of v.<field> to r.<field> in the slow
op logic, which should help timing as well.

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

execute1.vhdl

@@ -56,6 +56,7 @@ architecture behaviour of execute1 is
 	lr_update : std_ulogic;
 	next_lr : std_ulogic_vector(63 downto 0);
 	mul_in_progress : std_ulogic;
+        mul_finish : std_ulogic;
         div_in_progress : std_ulogic;
         cntz_in_progress : std_ulogic;
         slow_op_insn : insn_type_t;
@@ -69,7 +70,7 @@ architecture behaviour of execute1 is
     constant reg_type_init : reg_type :=
         (e => Execute1ToWritebackInit, f => Execute1ToFetch1Init,
          busy => '0', lr_update => '0', terminate => '0',
-         mul_in_progress => '0', div_in_progress => '0', cntz_in_progress => '0',
+         mul_in_progress => '0', mul_finish => '0', div_in_progress => '0', cntz_in_progress => '0',
          slow_op_insn => OP_ILLEGAL, slow_op_rc => '0', slow_op_oe => '0', slow_op_xerc => xerc_init,
          next_lr => (others => '0'), last_nia => (others => '0'), others => (others => '0'));
 
@@ -371,6 +372,7 @@ begin
 	v.mul_in_progress := '0';
         v.div_in_progress := '0';
         v.cntz_in_progress := '0';
+        v.mul_finish := '0';
 
         -- signals to multiply and divide units
         sign1 := '0';
@@ -965,31 +967,47 @@ begin
                         when others =>
                             -- i.e. OP_MUL_L64
                             result := multiply_to_x.result(63 downto 0);
-                            overflow := multiply_to_x.overflow;
                     end case;
 		else
 		    result := divider_to_x.write_reg_data;
 		    overflow := divider_to_x.overflow;
 		end if;
-		result_en := '1';
-		v.e.write_reg := gpr_to_gspr(v.slow_op_dest);
-		v.e.rc := v.slow_op_rc;
-		v.e.xerc := v.slow_op_xerc;
-		v.e.write_xerc_enable := v.slow_op_oe;
-		-- We must test oe because the RC update code in writeback
-		-- will use the xerc value to set CR0:SO so we must not clobber
-		-- xerc if OE wasn't set.
-		if v.slow_op_oe = '1' then
-		    v.e.xerc.ov := overflow;
-		    v.e.xerc.ov32 := overflow;
-		    v.e.xerc.so := v.slow_op_xerc.so or overflow;
-		end if;
-		v.e.valid := '1';
+                if r.mul_in_progress = '1' and r.slow_op_oe = '1' then
+                    -- have to wait until next cycle for overflow indication
+                    v.mul_finish := '1';
+                    v.busy := '1';
+                else
+                    result_en := '1';
+                    v.e.write_reg := gpr_to_gspr(r.slow_op_dest);
+                    v.e.rc := r.slow_op_rc;
+                    v.e.xerc := r.slow_op_xerc;
+                    v.e.write_xerc_enable := r.slow_op_oe;
+                    -- We must test oe because the RC update code in writeback
+                    -- will use the xerc value to set CR0:SO so we must not clobber
+                    -- xerc if OE wasn't set.
+                    if r.slow_op_oe = '1' then
+                        v.e.xerc.ov := overflow;
+                        v.e.xerc.ov32 := overflow;
+                        v.e.xerc.so := r.slow_op_xerc.so or overflow;
+                    end if;
+                    v.e.valid := '1';
+                end if;
 	    else
 		v.busy := '1';
 		v.mul_in_progress := r.mul_in_progress;
 		v.div_in_progress := r.div_in_progress;
 	    end if;
+        elsif r.mul_finish = '1' then
+            result := r.e.write_data;
+            result_en := '1';
+            v.e.write_reg := gpr_to_gspr(r.slow_op_dest);
+            v.e.rc := r.slow_op_rc;
+            v.e.xerc := r.slow_op_xerc;
+            v.e.write_xerc_enable := r.slow_op_oe;
+            v.e.xerc.ov := multiply_to_x.overflow;
+            v.e.xerc.ov32 := multiply_to_x.overflow;
+            v.e.xerc.so := r.slow_op_xerc.so or multiply_to_x.overflow;
+            v.e.valid := '1';
 	end if;
 
         if illegal = '1' then

multiply.vhdl

@@ -38,12 +38,15 @@ architecture behaviour of multiply is
     end record;
 
     signal r, rin : reg_type := (multiply_pipeline => MultiplyPipelineInit);
+    signal overflow : std_ulogic;
+    signal ovf_in   : std_ulogic;
 begin
     multiply_0: process(clk)
     begin
         if rising_edge(clk) then
             m <= m_in;
             r <= rin;
+            overflow <= ovf_in;
         end if;
     end process;
 
@@ -74,9 +77,10 @@ begin
         else
             ov := (or d(127 downto 63)) and not (and d(127 downto 63));
         end if;
+        ovf_in <= ov;
 
         m_out.result <= d;
-        m_out.overflow <= ov;
+        m_out.overflow <= overflow;
         m_out.valid <= v.multiply_pipeline(PIPELINE_DEPTH-1).valid;
 
         rin <= v;

xilinx-mult.vhdl

@@ -35,6 +35,7 @@ architecture behaviour of multiply is
     signal req_32bit, r32_1 : std_ulogic;
     signal req_not, rnot_1 : std_ulogic;
     signal valid_1 : std_ulogic;
+    signal overflow, ovf_in : std_ulogic;
 
 begin
     addend <= m_in.addend;
@@ -964,9 +965,10 @@ begin
             ov := not ((p1_pat and p0_pat and not product(31)) or
                        (p1_patb and p0_patb and product(31)));
         end if;
+        ovf_in <= ov;
 
         m_out.result <= product;
-        m_out.overflow <= ov;
+        m_out.overflow <= overflow;
     end process;
 
     process(clk)
@@ -979,6 +981,7 @@ begin
             r32_1 <= m_in.is_32bit;
             req_not <= rnot_1;
             rnot_1 <= m_in.not_result;
+            overflow <= ovf_in;
         end if;
     end process;