From 8455ee709164807da770a1805158e7ff8deda8c6 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sat, 22 Aug 2020 20:04:02 -0400
Subject: [PATCH] Optimize one-statement fork/join into begin

---
 src/V3Width.cpp            | 11 ++++++++---
 test_regress/t/t_fork.out  |  5 -----
 test_regress/t/t_fork.pl   |  9 ++++-----
 test_regress/t/t_fork.v    |  8 ++++++++
 test_regress/t/t_fork2.out |  5 +++++
 test_regress/t/t_fork2.pl  | 19 +++++++++++++++++++
 test_regress/t/t_fork2.v   | 21 +++++++++++++++++++++
 7 files changed, 65 insertions(+), 13 deletions(-)
 delete mode 100644 test_regress/t/t_fork.out
 create mode 100644 test_regress/t/t_fork2.out
 create mode 100755 test_regress/t/t_fork2.pl
 create mode 100644 test_regress/t/t_fork2.v

diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index 465b22c81..3a47a1d56 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -562,9 +562,14 @@ private:
             VL_DO_DANGLING(pushDeletep(nodep->unlinkFrBack()), nodep);
             return;
         }
-        if (v3Global.opt.bboxUnsup()) {
-            AstBegin* newp
-                = new AstBegin(nodep->fileline(), nodep->name(), nodep->stmtsp()->unlinkFrBack());
+        if (v3Global.opt.bboxUnsup()
+            // With no statements, begin is identical
+            || !nodep->stmtsp()
+            // With one statement, a begin block does as good as a fork/join or join_any
+            || (!nodep->stmtsp()->nextp() && !nodep->joinType().joinNone())) {
+            AstNode* stmtsp = nullptr;
+            if (nodep->stmtsp()) stmtsp = nodep->stmtsp()->unlinkFrBack();
+            AstBegin* newp = new AstBegin{nodep->fileline(), nodep->name(), stmtsp};
             nodep->replaceWith(newp);
             VL_DO_DANGLING(nodep->deleteTree(), nodep);
         } else {
diff --git a/test_regress/t/t_fork.out b/test_regress/t/t_fork.out
deleted file mode 100644
index 03e479d9f..000000000
--- a/test_regress/t/t_fork.out
+++ /dev/null
@@ -1,5 +0,0 @@
-%Error-UNSUPPORTED: t/t_fork.v:10:14: Unsupported: fork statements
-                                    : ... In instance t
-   10 |       fork : fblk
-      |              ^~~~
-%Error: Exiting due to
diff --git a/test_regress/t/t_fork.pl b/test_regress/t/t_fork.pl
index a5846c699..1c4ba9485 100755
--- a/test_regress/t/t_fork.pl
+++ b/test_regress/t/t_fork.pl
@@ -8,12 +8,11 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 # Version 2.0.
 # SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
 
-scenarios(vlt => 1);
+scenarios(simulator => 1);
 
-lint(
-    fails => 1,
-    expect_filename => $Self->{golden_filename},
-    );
+compile();
+
+execute();
 
 ok(1);
 1;
diff --git a/test_regress/t/t_fork.v b/test_regress/t/t_fork.v
index 06f9ed036..b92cf8830 100644
--- a/test_regress/t/t_fork.v
+++ b/test_regress/t/t_fork.v
@@ -7,6 +7,14 @@
 module t (/*AUTOARG*/);
 
    initial begin
+      // With no statements this is a NOP
+      fork
+      join
+      fork
+      join_any
+      fork
+      join_none
+      // With one statement this is supported and optimized to a begin/end
       fork : fblk
          begin
             $write("*-* All Finished *-*\n");
diff --git a/test_regress/t/t_fork2.out b/test_regress/t/t_fork2.out
new file mode 100644
index 000000000..e72b77c58
--- /dev/null
+++ b/test_regress/t/t_fork2.out
@@ -0,0 +1,5 @@
+%Error-UNSUPPORTED: t/t_fork2.v:10:14: Unsupported: fork statements
+                                     : ... In instance t
+   10 |       fork : fblk
+      |              ^~~~
+%Error: Exiting due to
diff --git a/test_regress/t/t_fork2.pl b/test_regress/t/t_fork2.pl
new file mode 100755
index 000000000..a5846c699
--- /dev/null
+++ b/test_regress/t/t_fork2.pl
@@ -0,0 +1,19 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 by Wilson Snyder. 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-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(vlt => 1);
+
+lint(
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_fork2.v b/test_regress/t/t_fork2.v
new file mode 100644
index 000000000..33eb0b485
--- /dev/null
+++ b/test_regress/t/t_fork2.v
@@ -0,0 +1,21 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2003 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t (/*AUTOARG*/);
+
+   initial begin
+      fork : fblk
+         begin
+            $write("Forked");
+         end
+         begin
+            $write("*-* All Finished *-*\n");
+            $finish;
+         end
+      join : fblk
+   end
+
+endmodule