From dfe28f7ed0e67f2794ddc5e167bdcfaacc40d31f Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sat, 11 Jan 2025 08:48:40 -0500
Subject: [PATCH] Fix misoptimizing away `$urandom` (#5703).

---
 Changes                          |  1 +
 src/V3AstNodeExpr.h              |  2 +-
 test_regress/t/t_sys_rand_seed.v | 27 ++++++++++++++++++++++++++-
 3 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/Changes b/Changes
index 1ab276672..a60e8b4f0 100644
--- a/Changes
+++ b/Changes
@@ -23,6 +23,7 @@ Verilator 5.033 devel
 * Fix pattern assignment to real inside struct (#5713).
 * Fix %p format output for real inside struct (#5713).
 * Fix segfault when only enum value referenced in package (#5714). [Dan Katz]
+* Fix misoptimizing away `$urandom` (#5703). [Parker Schless]
 * Fix matching language extension options including dots.
 
 
diff --git a/src/V3AstNodeExpr.h b/src/V3AstNodeExpr.h
index 13382f64b..db32a6de9 100644
--- a/src/V3AstNodeExpr.h
+++ b/src/V3AstNodeExpr.h
@@ -1820,7 +1820,6 @@ public:
                 return "VL_RANDOM_SEEDED_%nq%lq(%li)";
             }
         }
-
         if (isWide()) {
             return "VL_RANDOM_%nq(%nw, %P)";
         } else {
@@ -1830,6 +1829,7 @@ public:
     bool cleanOut() const override { return false; }
     bool isGateOptimizable() const override { return false; }
     bool isPredictOptimizable() const override { return false; }
+    bool isPure() override { return !m_reset && !seedp(); }
     int instrCount() const override { return INSTR_COUNT_PLI; }
     bool sameNode(const AstNode* /*samep*/) const override { return true; }
     bool combinable(const AstRand* samep) const {
diff --git a/test_regress/t/t_sys_rand_seed.v b/test_regress/t/t_sys_rand_seed.v
index 549f5e8c8..3a9acec20 100644
--- a/test_regress/t/t_sys_rand_seed.v
+++ b/test_regress/t/t_sys_rand_seed.v
@@ -12,6 +12,9 @@ module t;
    int valuea;
    int valueb;
    int valuec;
+   int igna;
+   int ignb;
+   int ignc;
 
    initial begin
       // $random unlike $urandom updates the value if given
@@ -32,7 +35,29 @@ module t;
 
       valuea = $urandom(10);
       valueb = $urandom(10);
-      if (valuea !== valueb) $stop;
+      valuec = $urandom(10);
+      if (valuea !== valueb && valueb != valuec) $stop;
+
+      valuea = $urandom(10);
+      valueb = $urandom(11);
+      valuec = $urandom(12);
+      if (valuea == valueb && valueb == valuec) $stop;  // May false fail 1 in 1^64
+
+      $urandom(10);
+      valuea = $urandom;
+      $urandom(10);
+      valueb = $urandom;
+      $urandom(10);
+      valuec = $urandom;
+      if (valuea != valueb && valueb != valuec) $stop;  // May false fail 1 in 1^64
+
+      igna = $urandom(10);
+      valuea = $urandom;
+      ignb = $urandom(10);
+      valueb = $urandom;
+      ignc = $urandom(10);
+      valuec = $urandom;
+      if (valuea != valueb && valueb != valuec) $stop;  // May false fail 1 in 1^64
 
       valuea = $urandom(10);
       valueb = $urandom();