execute2: Simplify execute2 logic to improve timing

This aims to simplify the logic in the execute2_1 process.  It is not
really necessary to preserve the contents of ex2 when stalled, except
for ex2.e.last_nia; but when stalled, bits which would initiate
downstream actions, such as ex2.e.valid, ex2.e.interrupt and ex2.se,
should be cleared.

Also, the path through stage2_stall to the bypass valid signal has
shown up as a critical path.  This dependency is there because the
mfspr instruction to a slow SPR or a PMU SPR should not forward a
result before the instruction is about to complete, because the result
might change (for example when reading the timebase).  To avoid this
dependency, we simply don't forward results for mfspr to slow/PMU
SPRs.

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

execute1.vhdl

@@ -1225,7 +1225,7 @@ begin
         v.e.rc := e_in.rc;
         v.e.write_cr_data := write_cr_data;
         v.e.write_cr_mask := write_cr_mask;
-        v.e.write_cr_enable := e_in.output_cr;
+        v.e.write_cr_enable := e_in.output_cr or e_in.rc;
         v.e.write_xerc_enable := e_in.output_xer;
         v.e.xerc := xerc_in;
         v.new_msr := ex1.msr;
@@ -2058,16 +2058,22 @@ begin
 	-- Next insn adder used in a couple of places
 	next_nia <= std_ulogic_vector(unsigned(ex1.e.last_nia) + 4);
 
-	v := ex2;
-        if stage2_stall = '0' then
-            v.e := ex1.e;
-            v.se := ex1.se;
-            v.ext_interrupt := ex1.ext_interrupt;
-            v.taken_branch_event := ex1.taken_branch_event;
-            v.br_mispredict := ex1.br_mispredict;
-            if ex1.advance_nia = '1' then
-                v.e.last_nia := next_nia;
-            end if;
+	v.log_addr_spr := ex2.log_addr_spr;
+
+        v.e := ex1.e;
+        v.se := ex1.se;
+        v.ext_interrupt := ex1.ext_interrupt and not stage2_stall;
+        v.taken_branch_event := ex1.taken_branch_event and not stage2_stall;
+        v.br_mispredict := ex1.br_mispredict and not stage2_stall;
+        if stage2_stall = '1' then
+            v.e.last_nia := ex2.e.last_nia;
+        elsif ex1.advance_nia = '1' then
+            v.e.last_nia := next_nia;
+        end if;
+        if stage2_stall = '1' then
+            v.e.valid := '0';
+            v.e.interrupt := '0';
+            v.se := side_effect_init;
         end if;
 
         if ex1.se.mult_32s = '1' and ex1.oe = '1' then
@@ -2153,14 +2159,11 @@ begin
             cr_mask(7) := '1';
         end if;
 
-	if stage2_stall = '0' then
-            v.e.write_data := ex_result;
-            v.e.write_cr_data := cr_res;
-            v.e.write_cr_mask := cr_mask;
-            if ex1.e.rc = '1' and ex1.e.write_enable = '1' and v.e.valid = '1' then
-                v.e.write_cr_enable := '1';
-            end if;
+        v.e.write_data := ex_result;
+        v.e.write_cr_data := cr_res;
+        v.e.write_cr_mask := cr_mask;
 
+	if stage2_stall = '0' then
             if ex1.se.write_msr = '1' then
                 ctrl_tmp.msr <= ex1.msr;
             end if;
@@ -2250,10 +2253,11 @@ begin
             end if;
         end if;
 
-        bypass_valid := ex1.e.valid;
-        if stage2_stall = '1' and ex1.res2_sel(1) = '1' then
-            bypass_valid := '0';
-        end if;
+        -- Don't bypass the result from mfspr to slow SPRs or PMU SPRs,
+        -- because we don't want to send the value while stalled because it
+        -- might change, and we don't want bypass_valid to depend on
+        -- stage2_stall for timing reasons.
+        bypass_valid := ex1.e.valid and not ex1.res2_sel(1);
 
         bypass2_data.tag.valid <= ex1.e.write_enable and bypass_valid;
         bypass2_data.tag.tag <= ex1.e.instr_tag.tag;