Add -fno-merge-cond-motion option

This disables code motion during V3MergeCond, for debugging.

docs/guide/exe_verilator.rst

@@ -459,6 +459,8 @@ Summary:
 
 .. option:: -fno-merge-cond
 
+.. option:: -fno-merge-cond-motion
+
 .. option:: -fno-merge-const-pool
 
 .. option:: -fno-reloop

src/V3MergeCond.cpp

@@ -477,10 +477,12 @@ private:
             AstNode* currp = m_workQueuep->front();
             m_workQueuep->pop();
 
-            // Analyse sub-tree list for code motion
+            // Analyse sub-tree list for code motion and conditional merging
             CodeMotionAnalysisVisitor::analyze(currp, stmtProperties);
             // Perform the code motion within the whole sub-tree list
-            currp = CodeMotionOptimizeVisitor::optimize(currp, stmtProperties);
+            if (v3Global.opt.fMergeCondMotion()) {
+                currp = CodeMotionOptimizeVisitor::optimize(currp, stmtProperties);
+            }
 
             // Merge conditionals in the whole sub-tree list (this might create new work items)
             iterateAndNextNull(currp);

src/V3Options.cpp

@@ -1097,6 +1097,7 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
     DECL_OPTION("-flife-post", FOnOff, &m_fLifePost);
     DECL_OPTION("-flocalize", FOnOff, &m_fLocalize);
     DECL_OPTION("-fmerge-cond", FOnOff, &m_fMergeCond);
+    DECL_OPTION("-fmerge-cond-motion", FOnOff, &m_fMergeCondMotion);
     DECL_OPTION("-fmerge-const-pool", FOnOff, &m_fMergeConstPool);
     DECL_OPTION("-freloop", FOnOff, &m_fReloop);
     DECL_OPTION("-freorder", FOnOff, &m_fReorder);

src/V3Options.h

@@ -354,7 +354,8 @@ private:
     bool m_fLifePost;    // main switch: -fno-life-post: delayed assignment elimination
     bool m_fLocalize;    // main switch: -fno-localize: convert temps to local variables
     bool m_fMergeCond;   // main switch: -fno-merge-cond: merge conditionals
-    bool m_fMergeConstPool = true;  // main switch: --fmerge-const-pool
+    bool m_fMergeCondMotion = true; // main switch: -fno-merge-cond-motion: perform code motion
+    bool m_fMergeConstPool = true;  // main switch: -fno-merge-const-pool
     bool m_fReloop;      // main switch: -fno-reloop: reform loops
     bool m_fReorder;     // main switch: -fno-reorder: reorder assignments in blocks
     bool m_fSplit;       // main switch: -fno-split: always assignment splitting
@@ -587,6 +588,7 @@ public:
     bool fLifePost() const { return m_fLifePost; }
     bool fLocalize() const { return m_fLocalize; }
     bool fMergeCond() const { return m_fMergeCond; }
+    bool fMergeCondMotion() const { return m_fMergeCondMotion; }
     bool fMergeConstPool() const { return m_fMergeConstPool; }
     bool fReloop() const { return m_fReloop; }
     bool fReorder() const { return m_fReorder; }

test_regress/t/t_merge_cond_no_motion.pl

@@ -0,0 +1,34 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2022 by Geza Lore. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(vlt_all => 1);
+
+top_filename("t/t_merge_cond.v");
+
+compile(
+    verilator_flags2 => ["-unroll-count 64", "--stats", "-fno-merge-cond-motion"],
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+if ($Self->{vlt}) {
+    # Note, with vltmt this might be split differently, so only checking vlt
+    file_grep($Self->{stats}, qr/Optimizations, MergeCond merges\s+(\d+)/i,
+              10);
+    file_grep($Self->{stats}, qr/Optimizations, MergeCond merged items\s+(\d+)/i,
+              580);
+    file_grep($Self->{stats}, qr/Optimizations, MergeCond longest merge\s+(\d+)/i,
+              64);
+}
+
+ok(1);
+1;