diff --git a/verilog/rtl/user_module.x b/verilog/rtl/user_module.x
new file mode 100644
index 0000000..975e504
--- /dev/null
+++ b/verilog/rtl/user_module.x
@@ -0,0 +1,23 @@
+// Copyright 2022 Google LLC.
+// SPDX-License-Identifier: Apache-2.0
+
+// ported from: https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+fn popcount(v: u8) -> u8 {
+  let v = v - ((v >> 1) & u8:0x55);
+  let v = (v & u8:0x33) + ((v >> 2) & u8:0x33);
+  (v + (v >> 4) & u8:0x0F)
+}
+
+#[test]
+fn popcount_test() {
+  let _= assert_eq(popcount(u8:0b0000_0000), u8:0);
+  let _= assert_eq(popcount(u8:0b0000_0001), u8:1);
+  let _= assert_eq(popcount(u8:0b1111_0000), u8:4);
+  let _= assert_eq(popcount(u8:0b0101_0101), u8:4);
+  let _= assert_eq(popcount(u8:0b1111_1111), u8:8);
+  _
+}
+
+pub fn user_module(io_in: u8) -> u8 {
+  popcount(io_in)
+}