From 7be5f5dc43c9511ae9a8109ae4a92de29f541fc5 Mon Sep 17 00:00:00 2001
From: Nick Gasson <nick@nickg.me.uk>
Date: Sat, 29 Oct 2022 10:37:26 +0100
Subject: [PATCH] Do not rely on behaviour of __builtin_ctz with zero argument

---
 src/rt/model.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/rt/model.c b/src/rt/model.c
index ee79d611..c3ef6e26 100644
--- a/src/rt/model.c
+++ b/src/rt/model.c
@@ -1091,8 +1091,10 @@ static void build_index(rt_signal_t *signal)
    rt_nexus_t *n = &(signal->nexus);
    for (int i = 0, offset = 0; i < signal->n_nexus;
         i++, offset += n->width, n = n->chain) {
-      const int tzc = __builtin_ctz(offset);
-      shift = MIN(shift, tzc);
+      if (offset > 0) {
+         const int tzc = __builtin_ctz(offset);
+         shift = MIN(shift, tzc);
+      }
 
       // Compute greatest common divisor
       for (int b = offset; b > 0;) {
-- 
2.39.2