From 9731ec76e3d1c77cdb782bf9e55936723795c3f2 Mon Sep 17 00:00:00 2001
From: Iru Cai <mytbk920423@gmail.com>
Date: Mon, 11 Oct 2021 20:45:19 +0800
Subject: [PATCH] Fix $urandom_range when the range is 0 ... UINT_MAX (#3161)

---
 include/verilated_funcs.h  | 3 +++
 test_regress/t/t_urandom.v | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/include/verilated_funcs.h b/include/verilated_funcs.h
index 0c11c6551..cddc44842 100644
--- a/include/verilated_funcs.h
+++ b/include/verilated_funcs.h
@@ -82,9 +82,12 @@ extern IData VL_RANDOM_SEEDED_II(int obits, IData seed) VL_MT_SAFE;
 inline IData VL_URANDOM_RANGE_I(IData hi, IData lo) {
     vluint64_t rnd = vl_rand64();
     if (VL_LIKELY(hi > lo)) {
+        // (hi - lo + 1) can be zero when hi is UINT_MAX and lo is zero
+        if (VL_UNLIKELY(hi - lo + 1 == 0)) return rnd;
         // Modulus isn't very fast but it's common that hi-low is power-of-two
         return (rnd % (hi - lo + 1)) + lo;
     } else {
+        if (VL_UNLIKELY(lo - hi + 1 == 0)) return rnd;
         return (rnd % (lo - hi + 1)) + hi;
     }
 }
diff --git a/test_regress/t/t_urandom.v b/test_regress/t/t_urandom.v
index fcb6b7b9d..bd865b990 100644
--- a/test_regress/t/t_urandom.v
+++ b/test_regress/t/t_urandom.v
@@ -37,6 +37,9 @@ module t(/*AUTOARG*/);
       v2 = $urandom_range(v1, v1);
       if (v1 != v2) $stop;
 
+      v2 = $urandom_range(0, 32'hffffffff);
+      if (v2 == v1) $stop;
+
       for (int test = 0; test < 20; ++test) begin
          v1 = 2;
          v1 = $urandom_range(0, v1);