From: David Shah <dave@ds0.me>
Date: Wed, 2 Dec 2020 17:08:39 +0000 (+0000)
Subject: nexus: More efficient CO mapping
X-Git-Tag: working-ls180~83^2~2
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=264e924abb85424a70daf50662700e3aceada689;p=yosys.git

nexus: More efficient CO mapping

Signed-off-by: David Shah <dave@ds0.me>
---

diff --git a/techlibs/nexus/arith_map.v b/techlibs/nexus/arith_map.v
index fd9d61be3..ce14a15ea 100644
--- a/techlibs/nexus/arith_map.v
+++ b/techlibs/nexus/arith_map.v
@@ -88,9 +88,9 @@ module _80_nexus_alu (A, B, CI, BI, X, Y, CO);
 			.COUT(FCO[i+2])
 		);
 
-		assign CO[i] = (AA[i] && BB[i]) || (((i == 0) ? CI : CO[i-1]) && (AA[i] || BB[i]));
+		assign CO[i] = (AA[i] && BB[i]) || ((Y[i] ^ AA[i] ^ BB[i]) && (AA[i] || BB[i]));
 		if (i+1 < Y_WIDTH) begin
-			assign CO[i+1] = (AA[i+1] && BB[i+1]) || (CO[i] && (AA[i+1] || BB[i+1]));
+			assign CO[i + 1] = (AA[i] && BB[i]) || ((Y[i] ^ AA[i] ^ BB[i]) && (AA[i] || BB[i]));
 			assign Y[i+1] = Y1[i];
 		end
 	end endgenerate