From 13b112dcb8f67778c6a140cb5ce8f1ab21aa6fb9 Mon Sep 17 00:00:00 2001
From: Alejandro Soto <alejandro@34project.org>
Date: Tue, 8 Nov 2022 15:59:12 -0600
Subject: Improve ALU performance

---
 rtl/core/alu/add.sv | 37 +++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

(limited to 'rtl/core/alu/add.sv')

diff --git a/rtl/core/alu/add.sv b/rtl/core/alu/add.sv
index 12bd237..a15a6b6 100644
--- a/rtl/core/alu/add.sv
+++ b/rtl/core/alu/add.sv
@@ -1,20 +1,45 @@
 module core_alu_add
-#(parameter W=16)
+#
+(
+	parameter W=16,
+	parameter SUB=0
+)
 (
 	input  logic[W - 1:0] a,
 	                      b,
-	                      c_in,
+	input  logic          c_in,
 
 	output logic[W - 1:0] q,
 	output logic          c,
 	                      v
 );
 
-	logic sgn_a, sgn_b, sgn_q;
+	logic sgn_a, sgn_b, sgn_q, maybe_v;
+	logic[W:0] out;
+
+	/* Quartus infiere dos sumadores si se zero-extendea el cin
+	 * para complacer a Verilator, lo cual es malo para Fmax.
+	 */
+`ifdef VERILATOR
+	logic[W:0] ext_carry;
+	assign ext_carry = {{W{1'b0}}, c_in};
+`else
+	logic ext_carry;
+	assign ext_carry = c_in;
+`endif
+
+	assign v = maybe_v & (sgn_a ^ sgn_q);
+	assign {c, q} = out;
 	assign {sgn_a, sgn_b, sgn_q} = {a[W - 1], b[W - 1], q[W - 1]};
 
-	//TODO: No sirve el carry
-	assign {c, q} = {1'b0, a} + {1'b0, b} + {1'b0, c_in};
-	assign v = (sgn_a ~^ sgn_b) & (sgn_a ^ sgn_q);
+	generate
+		if(SUB) begin
+			assign out = {1'b1, a} - {1'b0, b} - ext_carry;
+			assign maybe_v = sgn_a ^ sgn_b;
+		end else begin
+			assign out = {1'b0, a} + {1'b0, b} + ext_carry;
+			assign maybe_v = sgn_a ~^ sgn_b;
+		end
+	endgenerate
 
 endmodule
-- 
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