From: Paul Mackerras <paulus@ozlabs.org>
Date: Thu, 16 Jan 2020 02:18:56 +0000 (+1100)
Subject: execute1: Use truth table embedded in instruction for CR logical ops
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=441160d8655762d115b2c6eab2c58bb146b96860;p=microwatt.git

execute1: Use truth table embedded in instruction for CR logical ops

It turns out that CR logical instructions have the truth table of
the operation embedded in the instruction word.  This means that we
can collect the two input operand bits into a 2-bit value and use
that as the index to select the appropriate bit from the instruction
word.

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

diff --git a/execute1.vhdl b/execute1.vhdl
index ae13c72..e41a743 100644
--- a/execute1.vhdl
+++ b/execute1.vhdl
@@ -200,6 +200,7 @@ begin
 	variable bo, bi : std_ulogic_vector(4 downto 0);
 	variable bf, bfa : std_ulogic_vector(2 downto 0);
 	variable cr_op : std_ulogic_vector(9 downto 0);
+        variable cr_operands : std_ulogic_vector(1 downto 0);
 	variable bt, ba, bb : std_ulogic_vector(4 downto 0);
 	variable btnum, banum, bbnum : integer range 0 to 31;
 	variable crresult : std_ulogic;
@@ -532,27 +533,10 @@ begin
 		    btnum := 31 - to_integer(unsigned(bt));
 		    banum := 31 - to_integer(unsigned(ba));
 		    bbnum := 31 - to_integer(unsigned(bb));
-		    case cr_op(8 downto 5) is
-	            when "1001" => -- CREQV
-			crresult := not(e_in.cr(banum) xor e_in.cr(bbnum));
-	            when "0111" => -- CRNAND
-			crresult := not(e_in.cr(banum) and e_in.cr(bbnum));
-	            when "0100" => -- CRANDC
-			crresult := (e_in.cr(banum) and not e_in.cr(bbnum));
-	            when "1000" => -- CRAND
-			crresult := (e_in.cr(banum) and e_in.cr(bbnum));
-	            when "0001" => -- CRNOR
-			crresult := not(e_in.cr(banum) or e_in.cr(bbnum));
-	            when "1101" => -- CRORC
-			crresult := (e_in.cr(banum) or not e_in.cr(bbnum));
-	            when "0110" => -- CRXOR
-			crresult := (e_in.cr(banum) xor e_in.cr(bbnum));
-	            when "1110" => -- CROR
-			crresult := (e_in.cr(banum) or e_in.cr(bbnum));
-		    when others =>
-			crresult := '0';
-		        report "BAD CR?";
-	            end case;
+                    -- Bits 5-8 of cr_op give the truth table of the requested
+                    -- logical operation
+                    cr_operands := e_in.cr(banum) & e_in.cr(bbnum);
+                    crresult := cr_op(5 + to_integer(unsigned(cr_operands)));
 		    v.e.write_cr_mask := num_to_fxm((31-btnum) / 4);
 		    for i in 0 to 31 loop
 			if i = btnum then