From 8f6058a7d6654de5d28bca42e57ba118dbe44468 Mon Sep 17 00:00:00 2001
From: George Rennie <georgerennie@gmail.com>
Date: Thu, 7 Nov 2024 11:48:48 +0100
Subject: [PATCH 1/2] bufnorm: preserve constant bits driving wires

---
 passes/techmap/bufnorm.cc | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/passes/techmap/bufnorm.cc b/passes/techmap/bufnorm.cc
index 3c2cd9230..163e42c0f 100644
--- a/passes/techmap/bufnorm.cc
+++ b/passes/techmap/bufnorm.cc
@@ -438,8 +438,13 @@ struct BufnormPass : public Pass {
 				bool chain_this_wire = chain_this_wire_f(wire);
 
 				SigSpec keysig = sigmap(wire), insig = wire, outsig = wire;
-				for (int i = 0; i < GetSize(insig); i++)
-					insig[i] = mapped_bits.at(keysig[i], State::Sx);
+				for (int i = 0; i < GetSize(insig); i++) {
+					if (keysig[i].is_wire())
+						insig[i] = mapped_bits.at(keysig[i], State::Sx);
+					else
+						insig[i] = keysig[i];
+				}
+
 				if (chain_this_wire) {
 					for (int i = 0; i < GetSize(outsig); i++)
 						mapped_bits[keysig[i]] = outsig[i];

From a31c968340346294c4414db4217a525b8f892543 Mon Sep 17 00:00:00 2001
From: George Rennie <georgerennie@gmail.com>
Date: Thu, 7 Nov 2024 12:55:50 +0100
Subject: [PATCH 2/2] tests/bufnorm: add test for bufnorm of constant

---
 tests/techmap/bufnorm.ys | 11 +++++++++++
 1 file changed, 11 insertions(+)
 create mode 100644 tests/techmap/bufnorm.ys

diff --git a/tests/techmap/bufnorm.ys b/tests/techmap/bufnorm.ys
new file mode 100644
index 000000000..b81b80aed
--- /dev/null
+++ b/tests/techmap/bufnorm.ys
@@ -0,0 +1,11 @@
+# Check wires driven by constants are kept
+read_verilog <<EOT
+module top(output wire [7:0] y);
+assign y = 27;
+endmodule
+EOT
+
+equiv_opt -assert bufnorm
+design -load postopt
+select -assert-count 1 t:$buf
+select -assert-count 1 w:y %ci t:$buf %i