From 5bba9c3640971e25544f2053b31eb152c138c3af Mon Sep 17 00:00:00 2001
From: Eddie Hung <eddie@fpgeh.com>
Date: Thu, 27 Feb 2020 16:55:55 -0800
Subject: ast: fixes #1710; do not generate RTLIL for unreachable ternary

---
 frontends/ast/genrtlil.cc | 31 ++++++++++++++++++++++---------
 tests/various/bug1710.ys  | 30 ++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 9 deletions(-)
 create mode 100644 tests/various/bug1710.ys

diff --git a/frontends/ast/genrtlil.cc b/frontends/ast/genrtlil.cc
index 3fb6b3e5c..1dfcf3e0e 100644
--- a/frontends/ast/genrtlil.cc
+++ b/frontends/ast/genrtlil.cc
@@ -1338,18 +1338,31 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
 				detectSignWidth(width_hint, sign_hint);
 
 			RTLIL::SigSpec cond = children[0]->genRTLIL();
-			RTLIL::SigSpec val1 = children[1]->genRTLIL(width_hint, sign_hint);
-			RTLIL::SigSpec val2 = children[2]->genRTLIL(width_hint, sign_hint);
+			RTLIL::SigSpec sig;
+			if (cond.is_fully_const()) {
+				if (cond.as_bool()) {
+					sig = children[1]->genRTLIL(width_hint, sign_hint);
+					widthExtend(this, sig, sig.size(), children[1]->is_signed);
+				}
+				else {
+					sig = children[2]->genRTLIL(width_hint, sign_hint);
+					widthExtend(this, sig, sig.size(), children[2]->is_signed);
+				}
+			}
+			else {
+				RTLIL::SigSpec val1 = children[1]->genRTLIL(width_hint, sign_hint);
+				RTLIL::SigSpec val2 = children[2]->genRTLIL(width_hint, sign_hint);
 
-			if (cond.size() > 1)
-				cond = uniop2rtlil(this, "$reduce_bool", 1, cond, false);
+				if (cond.size() > 1)
+					cond = uniop2rtlil(this, "$reduce_bool", 1, cond, false);
 
-			int width = max(val1.size(), val2.size());
-			is_signed = children[1]->is_signed && children[2]->is_signed;
-			widthExtend(this, val1, width, is_signed);
-			widthExtend(this, val2, width, is_signed);
+				int width = max(val1.size(), val2.size());
+				is_signed = children[1]->is_signed && children[2]->is_signed;
+				widthExtend(this, val1, width, is_signed);
+				widthExtend(this, val2, width, is_signed);
 
-			RTLIL::SigSpec sig = mux2rtlil(this, cond, val1, val2);
+				sig = mux2rtlil(this, cond, val1, val2);
+			}
 
 			if (sig.size() < width_hint)
 				sig.extend_u0(width_hint, sign_hint);
diff --git a/tests/various/bug1710.ys b/tests/various/bug1710.ys
new file mode 100644
index 000000000..c2ecf3c90
--- /dev/null
+++ b/tests/various/bug1710.ys
@@ -0,0 +1,30 @@
+logger -werror "out of bounds"
+read_verilog <<EOT
+module Example;
+
+    parameter FLAG = 1;
+    wire [3:0] inp;
+
+    reg out1;
+    initial out1 = FLAG ? &inp[2:0] : &inp[4:0];
+
+    reg out2;
+    initial
+        if (FLAG)
+            out2 = &inp[2:0];
+        else
+            out2 = &inp[4:0];
+
+    wire out3;
+    assign out3 = FLAG ? &inp[2:0] : &inp[4:0];
+
+    wire out4;
+    generate
+        if (FLAG)
+            assign out4 = &inp[2:0];
+        else
+            assign out4 = &inp[4:0];
+    endgenerate
+
+endmodule
+EOT
-- 
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