From d2de6b42cf3bd7fef44df3c05dd133dcd873f86a Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Fri, 24 Jul 2026 09:26:01 -0400
Subject: [PATCH] * Fix $fgets being mis-optimized away (#7976).

Fixes #7976.
---
 Changes                             |  1 +
 src/V3AstNodeExpr.h                 |  1 +
 test_regress/t/t_sys_fgets_loop.dat | 11 ++++++
 test_regress/t/t_sys_fgets_loop.py  | 18 +++++++++
 test_regress/t/t_sys_fgets_loop.v   | 57 +++++++++++++++++++++++++++++
 5 files changed, 88 insertions(+)
 create mode 100644 test_regress/t/t_sys_fgets_loop.dat
 create mode 100755 test_regress/t/t_sys_fgets_loop.py
 create mode 100644 test_regress/t/t_sys_fgets_loop.v

diff --git a/Changes b/Changes
index 354a0dab6..31faa4155 100644
--- a/Changes
+++ b/Changes
@@ -66,6 +66,7 @@ Verilator 5.051 devel
 * Fix lost writes when select width exceeds variable width (#7975). [Bartosz Skorowski]
 * Fix variable scope in unique on dynamic array (#7981). [Kornel Uriasz, Antmicro Ltd.]
 * Fix table optimization causing not contextually convertible to bool error (#7983). [Jakub Michalski]
+* Fix $fgets being mis-optimized away (#7976). [G-A. Kamendje]
 
 
 Verilator 5.050 2026-07-01
diff --git a/src/V3AstNodeExpr.h b/src/V3AstNodeExpr.h
index 67a4806b5..46d4c13f9 100644
--- a/src/V3AstNodeExpr.h
+++ b/src/V3AstNodeExpr.h
@@ -3259,6 +3259,7 @@ public:
     bool sizeMattersRhs() const override { return false; }
     bool isSystemFunc() const override { return true; }
     int instrCount() const override { return widthInstrs() * 64; }
+    bool isPure() override { return false; }  // SPECIAL: $display has 'visual' ordering
 };
 class AstFUngetC final : public AstNodeBiop {
 public:
diff --git a/test_regress/t/t_sys_fgets_loop.dat b/test_regress/t/t_sys_fgets_loop.dat
new file mode 100644
index 000000000..0f366c68b
--- /dev/null
+++ b/test_regress/t/t_sys_fgets_loop.dat
@@ -0,0 +1,11 @@
+This is line 0 in test file
+This is line 1 in test file
+This is line 2 in test file
+This is line 3 in test file
+This is line 4 in test file
+This is line 5 in test file
+This is line 6 in test file
+This is line 7 in test file
+This is line 8 in test file
+This is line 8 in test file
+This is line 9 in test file
diff --git a/test_regress/t/t_sys_fgets_loop.py b/test_regress/t/t_sys_fgets_loop.py
new file mode 100755
index 000000000..46d1fe4c0
--- /dev/null
+++ b/test_regress/t/t_sys_fgets_loop.py
@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=['--binary'])
+
+test.execute()
+
+test.passes()
diff --git a/test_regress/t/t_sys_fgets_loop.v b/test_regress/t/t_sys_fgets_loop.v
new file mode 100644
index 000000000..c44bb300f
--- /dev/null
+++ b/test_regress/t/t_sys_fgets_loop.v
@@ -0,0 +1,57 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+
+  task test_read(input integer f, output string l);
+    automatic string line;
+    automatic string buffer = "";
+    automatic integer code;
+
+    if (f == 0) $stop;
+    while (1) begin
+      code = $fgets(line, f);
+      // Uncomment for issue #7976 workaround
+      // $display("code %d => content: %s", code, line);
+      buffer = {buffer, line};
+      if (line == "") begin
+        break;
+      end
+    end
+    l = buffer;
+  endtask
+
+  task test_eof_tsk;
+    automatic integer fd;
+    automatic string test_file_name = "t/t_sys_fgets_loop.dat";
+    automatic string output_line;
+    // Open files
+    fd = $fopen(test_file_name, "r");
+    if (fd == 0) begin
+      $display("ERROR ould not open file '%s' for reading aborting", test_file_name);
+      $fatal(0);
+    end
+
+    while (!$feof(
+        fd
+    )) begin
+      test_read(fd, output_line);
+    end
+    $fclose(fd);
+  endtask
+
+  initial begin
+    test_eof_tsk;
+  end
+
+  initial begin
+    #100;
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+
+endmodule