execute1: Simplify no-op behaviour of mfspr

When mfspr is performed to one of the reserved no-op SPRs, or to an
undefined SPR in privileged state, the behaviour is a no-op, that is,
the destination register is not written.  Previously this was done by
writing back the same value that the register had before the
instruction, but in fact it can be done simply by negating the write
enable signal so that the result GPR is not written.  This gives a
small reduction in logic complexity.

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

execute1.vhdl

@@ -106,7 +106,6 @@ architecture behaviour of execute1 is
         scv_trap : std_ulogic;
         write_tbl : std_ulogic;
         write_tbu : std_ulogic;
-        noop_spr_read : std_ulogic;
         send_hmsg : std_ulogic_vector(NCPUS-1 downto 0);
         clr_hmsg : std_ulogic;
     end record;
@@ -1434,7 +1433,9 @@ begin
                         report "MFSPR to slow SPR " & integer'image(decode_spr_num(e_in.insn));
                     end if;
                     slow_op := '1';
-                    v.se.noop_spr_read := e_in.spr_select.noop;
+                    if e_in.spr_select.noop = '1' then
+                        v.e.write_enable := '0';
+                    end if;
                     if e_in.spr_select.ispmu = '0' then
                         case e_in.spr_select.sel is
                             when SPRSEL_LOGR =>
@@ -1455,8 +1456,7 @@ begin
                             " invalid";
                     end if;
                     slow_op := '1';
-                    v.se.noop_spr_read := '1';
-                    v.res2_sel := "10";
+                    v.e.write_enable := '0';
                     if ex1.msr(MSR_PR) = '1' then
                         illegal := '1';
                     end if;
@@ -2114,9 +2114,7 @@ begin
         else
             rcresult := countbits_result;
         end if;
-        if ex1.se.noop_spr_read = '1' then
-            sprres := ex1.spr_write_data;
-        elsif ex1.res2_sel(0) = '0' then
+        if ex1.res2_sel(0) = '0' then
             sprres := spr_result;
         else
             sprres := pmu_to_x.spr_val;