From a860efa8ac37dfe20cd74324dd02136082aa2c1c Mon Sep 17 00:00:00 2001
From: Clifford Wolf <clifford@clifford.at>
Date: Thu, 15 Aug 2013 21:00:06 +0200
Subject: [PATCH] Implemented same div-by-zero behavior as found in other
 synthesis tools

---
 kernel/satgen.h     | 33 ++++++++++++++++++++++++++++-----
 techlibs/stdcells.v | 33 +++++++++++++++++++++++++++++++--
 2 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/kernel/satgen.h b/kernel/satgen.h
index 5b4abfc21..27063776e 100644
--- a/kernel/satgen.h
+++ b/kernel/satgen.h
@@ -300,20 +300,43 @@ struct SatGen
 				chain_buf.erase(chain_buf.begin() + a_u.size(), chain_buf.end());
 			}
 
+			std::vector<int> y_tmp = ignore_div_by_zero ? y : ez->vec_var(y.size());
 			if (cell->type == "$div") {
 				if (cell->parameters["\\A_SIGNED"].as_bool() && cell->parameters["\\B_SIGNED"].as_bool())
-					ez->assume(ez->vec_eq(y, ez->vec_ite(ez->XOR(a.back(), b.back()), ez->vec_neg(y_u), y_u)));
+					ez->assume(ez->vec_eq(y_tmp, ez->vec_ite(ez->XOR(a.back(), b.back()), ez->vec_neg(y_u), y_u)));
 				else
-					ez->assume(ez->vec_eq(y, y_u));
+					ez->assume(ez->vec_eq(y_tmp, y_u));
 			} else {
 				if (cell->parameters["\\A_SIGNED"].as_bool() && cell->parameters["\\B_SIGNED"].as_bool())
-					ez->assume(ez->vec_eq(y, ez->vec_ite(a.back(), ez->vec_neg(chain_buf), chain_buf)));
+					ez->assume(ez->vec_eq(y_tmp, ez->vec_ite(a.back(), ez->vec_neg(chain_buf), chain_buf)));
 				else
-					ez->assume(ez->vec_eq(y, chain_buf));
+					ez->assume(ez->vec_eq(y_tmp, chain_buf));
 			}
 
-			if (ignore_div_by_zero)
+			if (ignore_div_by_zero) {
 				ez->assume(ez->expression(ezSAT::OpOr, b));
+			} else {
+				std::vector<int> div_zero_result;
+				if (cell->type == "$div") {
+					if (cell->parameters["\\A_SIGNED"].as_bool() && cell->parameters["\\B_SIGNED"].as_bool()) {
+						std::vector<int> all_ones(y.size(), ez->TRUE);
+						std::vector<int> only_first_one(y.size(), ez->FALSE);
+						only_first_one.at(0) = ez->TRUE;
+						div_zero_result = ez->vec_ite(a.back(), only_first_one, all_ones);
+					} else {
+						div_zero_result.insert(div_zero_result.end(), cell->connections.at("\\A").width, ez->TRUE);
+						div_zero_result.insert(div_zero_result.end(), y.size() - div_zero_result.size(), ez->FALSE);
+					}
+				} else {
+					int copy_a_bits = std::min(cell->connections.at("\\A").width, cell->connections.at("\\B").width);
+					div_zero_result.insert(div_zero_result.end(), a.begin(), a.begin() + copy_a_bits);
+					if (cell->parameters["\\A_SIGNED"].as_bool() && cell->parameters["\\B_SIGNED"].as_bool())
+						div_zero_result.insert(div_zero_result.end(), y.size() - div_zero_result.size(), div_zero_result.back());
+					else
+						div_zero_result.insert(div_zero_result.end(), y.size() - div_zero_result.size(), ez->FALSE);
+				}
+				ez->assume(ez->vec_eq(y, ez->vec_ite(ez->expression(ezSAT::OpOr, b), y_tmp, div_zero_result)));
+			}
 
 			return true;
 		}
diff --git a/techlibs/stdcells.v b/techlibs/stdcells.v
index c411ba35b..d861d796e 100644
--- a/techlibs/stdcells.v
+++ b/techlibs/stdcells.v
@@ -1077,6 +1077,9 @@ input [A_WIDTH-1:0] A;
 input [B_WIDTH-1:0] B;
 output [Y_WIDTH-1:0] Y;
 
+wire [Y_WIDTH-1:0] Y_buf;
+wire [Y_WIDTH-1:0] Y_div_zero;
+
 \$div_mod #(
 	.A_SIGNED(A_SIGNED),
 	.B_SIGNED(B_SIGNED),
@@ -1086,9 +1089,20 @@ output [Y_WIDTH-1:0] Y;
 ) div_mod (
 	.A(A),
 	.B(B),
-	.Y(Y)
+	.Y(Y_buf)
 );
 
+// explicitly force the division-by-zero behavior found in other synthesis tools 
+generate begin
+	if (A_SIGNED && B_SIGNED) begin:make_div_zero
+		assign Y_div_zero = A[A_WIDTH-1] ? {Y_WIDTH{1'b0}} | 1'b1 : {Y_WIDTH{1'b1}};
+	end else begin:make_div_zero
+		assign Y_div_zero = {A_WIDTH{1'b1}};
+	end
+end endgenerate
+
+assign Y = B ? Y_buf : Y_div_zero;
+
 endmodule
 
 // --------------------------------------------------------
@@ -1105,6 +1119,9 @@ input [A_WIDTH-1:0] A;
 input [B_WIDTH-1:0] B;
 output [Y_WIDTH-1:0] Y;
 
+wire [Y_WIDTH-1:0] Y_buf;
+wire [Y_WIDTH-1:0] Y_div_zero;
+
 \$div_mod #(
 	.A_SIGNED(A_SIGNED),
 	.B_SIGNED(B_SIGNED),
@@ -1114,9 +1131,21 @@ output [Y_WIDTH-1:0] Y;
 ) div_mod (
 	.A(A),
 	.B(B),
-	.R(Y)
+	.R(Y_buf)
 );
 
+// explicitly force the division-by-zero behavior found in other synthesis tools 
+localparam div_zero_copy_a_bits = A_WIDTH < B_WIDTH ? A_WIDTH : B_WIDTH;
+generate begin
+	if (A_SIGNED && B_SIGNED) begin:make_div_zero
+		assign Y_div_zero = $signed(A[div_zero_copy_a_bits-1:0]);
+	end else begin:make_div_zero
+		assign Y_div_zero = $unsigned(A[div_zero_copy_a_bits-1:0]);
+	end
+end endgenerate
+
+assign Y = B ? Y_buf : Y_div_zero;
+
 endmodule
 
 /****
-- 
2.30.2