diff --git a/src/Makefile_obj.in b/src/Makefile_obj.in
index eb9b83e247..c626c454ae 100644
--- a/src/Makefile_obj.in
+++ b/src/Makefile_obj.in
@@ -199,10 +199,14 @@ RAW_OBJS_PCH_ASTMT = \
 	V3File.o \
 	V3Global.o \
 	V3Hasher.o \
+	V3LinkLValue.o \
 	V3Number.o \
 	V3Options.o \
+	V3Randomize.o \
 	V3Stats.o \
 	V3StatsReport.o \
+	V3Task.o \
+	V3Width.o \
 
 RAW_OBJS_PCH_ASTNOMT = \
 	V3Active.o \
@@ -255,7 +259,6 @@ RAW_OBJS_PCH_ASTNOMT = \
 	V3LinkDot.o \
 	V3LinkInc.o \
 	V3LinkJump.o \
-	V3LinkLValue.o \
 	V3LinkLevel.o \
 	V3LinkParse.o \
 	V3LinkResolve.o \
@@ -267,7 +270,6 @@ RAW_OBJS_PCH_ASTNOMT = \
 	V3Partition.o \
 	V3Premit.o \
 	V3ProtectLib.o \
-	V3Randomize.o \
 	V3Reloop.o \
 	V3Sched.o \
 	V3SchedAcyclic.o \
@@ -283,7 +285,6 @@ RAW_OBJS_PCH_ASTNOMT = \
 	V3Subst.o \
 	V3TSP.o \
 	V3Table.o \
-	V3Task.o \
 	V3Timing.o \
 	V3Trace.o \
 	V3TraceDecl.o \
@@ -292,7 +293,6 @@ RAW_OBJS_PCH_ASTNOMT = \
 	V3Unknown.o \
 	V3Unroll.o \
 	V3VariableOrder.o \
-	V3Width.o \
 	V3WidthCommit.o \
 	V3WidthSel.o \
 
@@ -337,6 +337,13 @@ ifneq ($(filter -Wthread-safety,$(CPPFLAGS) $(CXXFLAGS)),)
 THREAD_SAFETY_NEGATIVE_OBJS := \
 	V3Ast.o \
 	V3AstNodes.o \
+	V3Const__gen.o \
+	V3Expand.o \
+	V3Graph.o \
+	V3LinkLValue.o \
+	V3Randomize.o \
+	V3Task.o \
+	V3Width.o \
 
 $(THREAD_SAFETY_NEGATIVE_OBJS): override CXXFLAGS_PRIV = -Wthread-safety-negative
 endif
diff --git a/src/V3Const.cpp b/src/V3Const.cpp
index 088e2df932..f113b7304f 100644
--- a/src/V3Const.cpp
+++ b/src/V3Const.cpp
@@ -20,8 +20,6 @@
 //          If operands are constant, replace this node with constant.
 //*************************************************************************
 
-#define VL_MT_DISABLED_CODE_UNIT 1
-
 #include "config_build.h"
 #include "verilatedos.h"
 
@@ -29,6 +27,7 @@
 
 #include "V3Ast.h"
 #include "V3Global.h"
+#include "V3Mutex.h"
 #include "V3Simulate.h"
 #include "V3Stats.h"
 #include "V3String.h"
@@ -52,17 +51,17 @@ VL_DEFINE_DEBUG_FUNCTIONS;
 //######################################################################
 // Utilities
 
-static bool isConst(const AstNode* nodep, uint64_t v) {
+static bool isConst(const AstNode* nodep, uint64_t v) VL_MT_SAFE {
     const AstConst* const constp = VN_CAST(nodep, Const);
     return constp && constp->toUQuad() == v;
 }
 
 template <class T>
-static typename std::enable_if<std::is_integral<T>::value, bool>::type isPow2(T val) {
+static typename std::enable_if<std::is_integral<T>::value, bool>::type isPow2(T val) VL_MT_SAFE {
     return (val & (val - 1)) == 0;
 }
 
-static int countTrailingZeroes(uint64_t val) {
+static int countTrailingZeroes(uint64_t val) VL_MT_SAFE {
     UASSERT(val, "countTrailingZeroes argument must be non-zero");
 #if defined(__GNUC__) && !defined(VL_NO_BUILTINS)
     return __builtin_ctzll(val);
@@ -77,10 +76,15 @@ static int countTrailingZeroes(uint64_t val) {
 #endif
 }
 
+// Guards use of AstNode::user4. Must be locked when VNUser4InUse object exists.
+static V3Mutex s_user4InUseLock;
+
 // This visitor can be used in the post-expanded Ast from V3Expand, where the Ast satisfies:
 // - Constants are 64 bit at most (because words are accessed via AstWordSel)
 // - Variables are scoped.
 class ConstBitOpTreeVisitor final : public VNVisitorConst {
+    V3LockGuard m_oneInstanceLockGuard{s_user4InUseLock};
+
     // NODE STATE
     // AstVarRef::user4u      -> Base index of m_varInfos that points VarInfo
     // AstVarScope::user4u    -> Same as AstVarRef::user4
@@ -650,6 +654,7 @@ class ConstBitOpTreeVisitor final : public VNVisitorConst {
 
     // CONSTRUCTORS
     ConstBitOpTreeVisitor(AstNodeExpr* nodep, unsigned externalOps)
+        VL_REQUIRES_UNLOCKED(s_user4InUseLock)
         : m_ops{externalOps}
         , m_rootp{nodep} {
         // Fill nullptr at [0] because AstVarScope::user4 is 0 by default
@@ -681,7 +686,8 @@ class ConstBitOpTreeVisitor final : public VNVisitorConst {
     // Reduction ops are transformed in the same way.
     // &{v[0], v[1]} => 2'b11 == (2'b11 & v)
     static AstNodeExpr* simplify(AstNodeExpr* nodep, int resultWidth, unsigned externalOps,
-                                 VDouble0& reduction, VNDeleter& deleterr) {
+                                 VDouble0& reduction, VNDeleter& deleterr)
+        VL_REQUIRES_UNLOCKED(s_user4InUseLock) {
         UASSERT_OBJ(1 <= resultWidth && resultWidth <= 64, nodep, "resultWidth out of range");
 
         // Walk tree, gathering all terms referenced in expression
@@ -1168,7 +1174,7 @@ class ConstVisitor final : public VNVisitor {
         return false;
     }
 
-    bool matchBitOpTree(AstNodeExpr* nodep) {
+    bool matchBitOpTree(AstNodeExpr* nodep) VL_REQUIRES_UNLOCKED(s_user4InUseLock) {
         if (nodep->widthMin() != 1) return false;
         if (!v3Global.opt.fConstBitOpTree()) return false;
 
@@ -2040,7 +2046,7 @@ class ConstVisitor final : public VNVisitor {
                 && varNotReferenced(nodep->op4p(), varp, level + 1));
     }
 
-    bool replaceNodeAssign(AstNodeAssign* nodep) {
+    bool replaceNodeAssign(AstNodeAssign* nodep) VL_REQUIRES_UNLOCKED(s_user4InUseLock) {
         if (VN_IS(nodep->lhsp(), VarRef) && VN_IS(nodep->rhsp(), VarRef)
             && VN_AS(nodep->lhsp(), VarRef)->sameNoLvalue(VN_AS(nodep->rhsp(), VarRef))
             && !VN_IS(nodep, AssignDly)) {
@@ -2057,6 +2063,7 @@ class ConstVisitor final : public VNVisitor {
             if (m_warn && !VN_IS(nodep, AssignDly)) {  // Is same var on LHS and RHS?
                 // Note only do this (need user4) when m_warn, which is
                 // done as unique visitor
+                const V3LockGuard user4Lock{s_user4InUseLock};
                 const VNUser4InUse m_inuser4;
                 nodep->lhsp()->foreach([](const AstVarRef* nodep) {
                     if (nodep->varp()) nodep->varp()->user4(1);
@@ -2988,7 +2995,7 @@ class ConstVisitor final : public VNVisitor {
 
     //-----
     // Zero elimination
-    void visit(AstNodeAssign* nodep) override {
+    void visit(AstNodeAssign* nodep) override VL_REQUIRES_UNLOCKED(s_user4InUseLock) {
         iterateChildren(nodep);
         if (nodep->timingControlp()) m_hasJumpDelay = true;
         if (m_doNConst && replaceNodeAssign(nodep)) return;
@@ -2999,7 +3006,7 @@ class ConstVisitor final : public VNVisitor {
     void visit(AstAssignVarScope* nodep) override {
         // Don't perform any optimizations, the node won't be linked yet
     }
-    void visit(AstAssignW* nodep) override {
+    void visit(AstAssignW* nodep) override VL_REQUIRES_UNLOCKED(s_user4InUseLock) {
         iterateChildren(nodep);
         if (m_doNConst && replaceNodeAssign(nodep)) return;
         AstNodeVarRef* const varrefp = VN_CAST(
@@ -3646,7 +3653,7 @@ class ConstVisitor final : public VNVisitor {
     TREEOPV("AstRedOr {$lhsp.castExtend}",      "AstRedOr {$lhsp->castExtend()->lhsp()}");
     TREEOPV("AstRedXor{$lhsp.castExtend}",      "AstRedXor{$lhsp->castExtend()->lhsp()}");
     TREEOP ("AstRedXor{$lhsp.castXor, VN_IS(VN_AS($lhsp,,Xor)->lhsp(),,Const)}", "AstXor{AstRedXor{$lhsp->castXor()->lhsp()}, AstRedXor{$lhsp->castXor()->rhsp()}}");  // ^(const ^ a) => (^const)^(^a)
-    TREEOPC("AstAnd {$lhsp.castConst, $rhsp.castRedXor, matchBitOpTree(nodep)}", "DONE");
+    TREEOPC("AstAnd {$lhsp.castConst, $rhsp.castRedXor, matchBitOpTree(nodep)}", "DONE", "VL_REQUIRES_UNLOCKED(s_user4InUseLock)");
     TREEOPV("AstOneHot{$lhsp.width1}",          "replaceWLhs(nodep)");
     TREEOPV("AstOneHot0{$lhsp.width1}",         "replaceNum(nodep,1)");
     // Binary AND/OR is faster than logical and/or (usually)
@@ -3671,9 +3678,9 @@ class ConstVisitor final : public VNVisitor {
     TREEOP ("AstAnd {operandShiftSame(nodep)}",         "replaceShiftSame(nodep)");
     TREEOP ("AstOr  {operandShiftSame(nodep)}",         "replaceShiftSame(nodep)");
     TREEOP ("AstXor {operandShiftSame(nodep)}",         "replaceShiftSame(nodep)");
-    TREEOPC("AstAnd {matchBitOpTree(nodep)}", "DONE");
-    TREEOPC("AstOr  {matchBitOpTree(nodep)}", "DONE");
-    TREEOPC("AstXor {matchBitOpTree(nodep)}", "DONE");
+    TREEOPC("AstAnd {matchBitOpTree(nodep)}", "DONE", "VL_REQUIRES_UNLOCKED(s_user4InUseLock)");
+    TREEOPC("AstOr  {matchBitOpTree(nodep)}", "DONE", "VL_REQUIRES_UNLOCKED(s_user4InUseLock)");
+    TREEOPC("AstXor {matchBitOpTree(nodep)}", "DONE", "VL_REQUIRES_UNLOCKED(s_user4InUseLock)");
     // Note can't simplify a extend{extends}, extends{extend}, as the sign
     // bits end up in the wrong places
     TREEOPV("AstExtend {$lhsp.castExtend}",  "replaceExtend(nodep, VN_AS(nodep->lhsp(), Extend)->lhsp())");
@@ -3854,7 +3861,7 @@ AstNode* V3Const::constifyParamsNoWarnEdit(AstNode* nodep) {
 //! something a generate block can depend on, we can wait until later to do the
 //! width check.
 //! @return  Pointer to the edited node.
-AstNode* V3Const::constifyGenerateParamsEdit(AstNode* nodep) {
+AstNode* V3Const::constifyGenerateParamsEdit(AstNode* nodep) VL_MT_DISABLED {
     // if (debug() > 0) nodep->dumpTree("-  forceConPRE:: ");
     // Resize even if the node already has a width, because buried in the tree
     // we may have a node we just created with signing, etc, that isn't sized
@@ -3876,7 +3883,7 @@ AstNode* V3Const::constifyGenerateParamsEdit(AstNode* nodep) {
     return nodep;
 }
 
-void V3Const::constifyAllLint(AstNetlist* nodep) {
+void V3Const::constifyAllLint(AstNetlist* nodep) VL_MT_DISABLED {
     // Only call from Verilator.cpp, as it uses user#'s
     UINFO(2, __FUNCTION__ << ": " << endl);
     {
@@ -3886,7 +3893,7 @@ void V3Const::constifyAllLint(AstNetlist* nodep) {
     v3Global.dumpCheckGlobalTree("const", 0, dumpTreeLevel() >= 3);
 }
 
-void V3Const::constifyCpp(AstNetlist* nodep) {
+void V3Const::constifyCpp(AstNetlist* nodep) VL_MT_DISABLED {
     UINFO(2, __FUNCTION__ << ": " << endl);
     {
         ConstVisitor visitor{ConstVisitor::PROC_CPP, /* globalPass: */ true};
@@ -3895,19 +3902,19 @@ void V3Const::constifyCpp(AstNetlist* nodep) {
     v3Global.dumpCheckGlobalTree("const_cpp", 0, dumpTreeLevel() >= 3);
 }
 
-AstNode* V3Const::constifyEdit(AstNode* nodep) {
+AstNode* V3Const::constifyEdit(AstNode* nodep) VL_MT_DISABLED {
     ConstVisitor visitor{ConstVisitor::PROC_V_NOWARN, /* globalPass: */ false};
     nodep = visitor.mainAcceptEdit(nodep);
     return nodep;
 }
 
-AstNode* V3Const::constifyEditCpp(AstNode* nodep) {
+AstNode* V3Const::constifyEditCpp(AstNode* nodep) VL_MT_ENABLED {
     ConstVisitor visitor{ConstVisitor::PROC_CPP, /* globalPass: */ false};
     nodep = visitor.mainAcceptEdit(nodep);
     return nodep;
 }
 
-void V3Const::constifyAllLive(AstNetlist* nodep) {
+void V3Const::constifyAllLive(AstNetlist* nodep) VL_MT_DISABLED {
     // Only call from Verilator.cpp, as it uses user#'s
     // This only pushes constants up, doesn't make any other edits
     // IE doesn't prune dead statements, as we need to do some usability checks after this
@@ -3919,7 +3926,7 @@ void V3Const::constifyAllLive(AstNetlist* nodep) {
     v3Global.dumpCheckGlobalTree("const", 0, dumpTreeLevel() >= 3);
 }
 
-void V3Const::constifyAll(AstNetlist* nodep) {
+void V3Const::constifyAll(AstNetlist* nodep) VL_MT_DISABLED {
     // Only call from Verilator.cpp, as it uses user#'s
     UINFO(2, __FUNCTION__ << ": " << endl);
     {
@@ -3929,7 +3936,7 @@ void V3Const::constifyAll(AstNetlist* nodep) {
     v3Global.dumpCheckGlobalTree("const", 0, dumpTreeLevel() >= 3);
 }
 
-AstNode* V3Const::constifyExpensiveEdit(AstNode* nodep) {
+AstNode* V3Const::constifyExpensiveEdit(AstNode* nodep) VL_MT_DISABLED {
     ConstVisitor visitor{ConstVisitor::PROC_V_EXPENSIVE, /* globalPass: */ false};
     nodep = visitor.mainAcceptEdit(nodep);
     return nodep;
diff --git a/src/V3Const.h b/src/V3Const.h
index e992faca83..6d128bc0ee 100644
--- a/src/V3Const.h
+++ b/src/V3Const.h
@@ -27,12 +27,12 @@
 
 class V3Const final {
 public:
-    static AstNode* constifyParamsEdit(AstNode* nodep) VL_MT_DISABLED;
-    static AstNodeExpr* constifyParamsEdit(AstNodeExpr* exprp) VL_MT_DISABLED {
+    static AstNode* constifyParamsEdit(AstNode* nodep);
+    static AstNodeExpr* constifyParamsEdit(AstNodeExpr* exprp) {
         return VN_AS(constifyParamsEdit(static_cast<AstNode*>(exprp)), NodeExpr);
     }
-    static AstNode* constifyParamsNoWarnEdit(AstNode* nodep) VL_MT_DISABLED;
-    static AstNodeExpr* constifyParamsNoWarnEdit(AstNodeExpr* exprp) VL_MT_DISABLED {
+    static AstNode* constifyParamsNoWarnEdit(AstNode* nodep);
+    static AstNodeExpr* constifyParamsNoWarnEdit(AstNodeExpr* exprp) {
         return VN_AS(constifyParamsNoWarnEdit(static_cast<AstNode*>(exprp)), NodeExpr);
     }
     static AstNode* constifyGenerateParamsEdit(AstNode* nodep) VL_MT_DISABLED;
@@ -46,14 +46,14 @@ class V3Const final {
     static void constifyCpp(AstNetlist* nodep) VL_MT_DISABLED;
     // Only the current node and lower
     // Return new node that may have replaced nodep
-    static AstNode* constifyEditCpp(AstNode* nodep) VL_MT_DISABLED;
-    static AstNodeExpr* constifyEditCpp(AstNodeExpr* exprp) VL_MT_DISABLED {
+    static AstNode* constifyEditCpp(AstNode* nodep);
+    static AstNodeExpr* constifyEditCpp(AstNodeExpr* exprp) {
         return VN_AS(constifyEditCpp(static_cast<AstNode*>(exprp)), NodeExpr);
     }
     // Only the current node and lower
     // Return new node that may have replaced nodep
-    static AstNode* constifyEdit(AstNode* nodep) VL_MT_DISABLED;
-    static AstNodeExpr* constifyEdit(AstNodeExpr* exprp) VL_MT_DISABLED {
+    static AstNode* constifyEdit(AstNode* nodep);
+    static AstNodeExpr* constifyEdit(AstNodeExpr* exprp) {
         return VN_AS(constifyEdit(static_cast<AstNode*>(exprp)), NodeExpr);
     }
     // Only the current node and lower, with special SenTree optimization
diff --git a/src/V3Expand.cpp b/src/V3Expand.cpp
index d4f048f403..7a91c2a262 100644
--- a/src/V3Expand.cpp
+++ b/src/V3Expand.cpp
@@ -25,7 +25,7 @@
 //
 //*************************************************************************
 
-#include "V3PchAstNoMT.h"  // VL_MT_DISABLED_CODE_UNIT
+#include "V3PchAstMT.h"
 
 #include "V3Expand.h"
 
diff --git a/src/V3Expand.h b/src/V3Expand.h
index e3f8fc6306..d119b9391e 100644
--- a/src/V3Expand.h
+++ b/src/V3Expand.h
@@ -28,7 +28,7 @@ class AstNetlist;
 
 class V3Expand final {
 public:
-    static void expandAll(AstNetlist* nodep) VL_MT_DISABLED;
+    static void expandAll(AstNetlist* nodep);
 };
 
 #endif  // Guard
diff --git a/src/V3Global.h b/src/V3Global.h
index e48c1276c8..1cd4041757 100644
--- a/src/V3Global.h
+++ b/src/V3Global.h
@@ -222,7 +222,8 @@ class V3Global final {
     void shutdown();  // Release allocated resources
 
     // ACCESSORS (general)
-    AstNetlist* rootp() const VL_MT_DISABLED { return m_rootp; }
+    //AstNetlist* rootp() const VL_MT_DISABLED { return m_rootp; }
+    AstNetlist* rootp() const { return m_rootp; }
     VNetlist* netlistp() const VL_MT_SAFE { return m_netlist.get(); }
     VWidthMinUsage widthMinUsage() const { return m_widthMinUsage; }
     bool assertDTypesResolved() const { return m_assertDTypesResolved; }
diff --git a/src/V3Graph.cpp b/src/V3Graph.cpp
index f77f1d056f..dd9c9a40f1 100644
--- a/src/V3Graph.cpp
+++ b/src/V3Graph.cpp
@@ -14,8 +14,6 @@
 //
 //*************************************************************************
 
-#define VL_MT_DISABLED_CODE_UNIT 1
-
 #include "config_build.h"
 #include "verilatedos.h"
 
diff --git a/src/V3Graph.h b/src/V3Graph.h
index c2973cae7d..bca3f6a9fa 100644
--- a/src/V3Graph.h
+++ b/src/V3Graph.h
@@ -96,12 +96,12 @@ class V3Graph VL_NOT_FINAL {
     // ACCESSORS
 
 public:
-    V3Graph() VL_MT_DISABLED;
-    virtual ~V3Graph() VL_MT_DISABLED;
+    V3Graph();
+    virtual ~V3Graph();
     virtual string dotRankDir() const { return "TB"; }  // rankdir for dot plotting
 
     // METHODS
-    void clear() VL_MT_DISABLED;  // Empty it of all vertices/edges, as if making a new object
+    void clear();  // Empty it of all vertices/edges, as if making a new object
     bool empty() const { return m_vertices.empty(); }
     V3GraphVertex* verticesBeginp() const { return m_vertices.begin(); }
 
@@ -152,7 +152,7 @@ class V3Graph VL_NOT_FINAL {
 
     /// Remove any redundant edges, weights become SUM of any other weight
     /// Side-effect: changes user()
-    void removeRedundantEdgesSum(V3EdgeFuncP edgeFuncp) VL_MT_DISABLED;
+    void removeRedundantEdgesSum(V3EdgeFuncP edgeFuncp);
 
     /// Remove any transitive edges.  E.g. if have edges A->B, B->C, and A->C
     /// then A->C is a "transitive" edge; it's implied by the first two
@@ -175,9 +175,8 @@ class V3Graph VL_NOT_FINAL {
 
     /// Debugging
     void dump(std::ostream& os = std::cout) const VL_MT_DISABLED;
-    void dumpDotFile(const string& filename, bool colorAsSubgraph) const VL_MT_DISABLED;
-    void dumpDotFilePrefixed(const string& nameComment,
-                             bool colorAsSubgraph = false) const VL_MT_DISABLED;
+    void dumpDotFile(const string& filename, bool colorAsSubgraph) const;
+    void dumpDotFilePrefixed(const string& nameComment, bool colorAsSubgraph = false) const;
     void dumpDotFilePrefixedAlways(const string& nameComment,
                                    bool colorAsSubgraph = false) const VL_MT_DISABLED;
     static void selfTest() VL_MT_DISABLED;
@@ -208,17 +207,17 @@ class V3GraphVertex VL_NOT_FINAL {
         uint32_t m_user;  // Marker for some algorithms
     };
     // METHODS
-    void verticesPushBack(V3Graph* graphp) VL_MT_DISABLED;
+    void verticesPushBack(V3Graph* graphp);
     // ACCESSORS
     void fanout(double fanout) { m_fanout = fanout; }
     void inUnlink() { m_ins.reset(); }  // Low level; normally unlinkDelete is what you want
     void outUnlink() { m_outs.reset(); }  // Low level; normally unlinkDelete is what you want
 protected:
     // CONSTRUCTORS
-    V3GraphVertex(V3Graph* graphp, const V3GraphVertex& old) VL_MT_DISABLED;
+    V3GraphVertex(V3Graph* graphp, const V3GraphVertex& old);
 
 public:
-    explicit V3GraphVertex(V3Graph* graphp) VL_MT_DISABLED;
+    explicit V3GraphVertex(V3Graph* graphp);
     //! Clone copy constructor. Doesn't copy edges or user/userp.
     virtual V3GraphVertex* clone(V3Graph* graphp) const VL_MT_DISABLED {
         return new V3GraphVertex{graphp, *this};
@@ -338,21 +337,21 @@ class V3GraphEdge VL_NOT_FINAL {
     };
     // METHODS
     void init(V3Graph* graphp, V3GraphVertex* fromp, V3GraphVertex* top, int weight,
-              bool cutable = false) VL_MT_DISABLED;
+              bool cutable = false);
     void cut() { m_weight = 0; }  // 0 weight is same as disconnected
-    void outPushBack() VL_MT_DISABLED;
-    void inPushBack() VL_MT_DISABLED;
+    void outPushBack();
+    void inPushBack();
     // CONSTRUCTORS
 protected:
     V3GraphEdge(V3Graph* graphp, V3GraphVertex* fromp, V3GraphVertex* top,
-                const V3GraphEdge& old) VL_MT_DISABLED {
+                const V3GraphEdge& old) {
         init(graphp, fromp, top, old.m_weight, old.m_cutable);
     }
 
 public:
     //! Add DAG from one node to the specified node
     V3GraphEdge(V3Graph* graphp, V3GraphVertex* fromp, V3GraphVertex* top, int weight,
-                bool cutable = false) VL_MT_DISABLED {
+                bool cutable = false) {
         init(graphp, fromp, top, weight, cutable);
     }
     //! Clone copy constructor.  Doesn't copy existing vertices or user/userp.
diff --git a/src/V3LinkLValue.cpp b/src/V3LinkLValue.cpp
index b0ce942d9f..f768843c5c 100644
--- a/src/V3LinkLValue.cpp
+++ b/src/V3LinkLValue.cpp
@@ -18,7 +18,7 @@
 //          Set lvalue() attributes on appropriate VARREFs.
 //*************************************************************************
 
-#include "V3PchAstNoMT.h"  // VL_MT_DISABLED_CODE_UNIT
+#include "V3PchAstMT.h"
 
 #include "V3LinkLValue.h"
 
diff --git a/src/V3LinkLValue.h b/src/V3LinkLValue.h
index 26cbaa4e3f..16656936d0 100644
--- a/src/V3LinkLValue.h
+++ b/src/V3LinkLValue.h
@@ -20,8 +20,6 @@
 #include "config_build.h"
 #include "verilatedos.h"
 
-#include "V3ThreadSafety.h"
-
 class AstNetlist;
 class AstNode;
 
@@ -29,8 +27,8 @@ class AstNode;
 
 class V3LinkLValue final {
 public:
-    static void linkLValue(AstNetlist* nodep) VL_MT_DISABLED;
-    static void linkLValueSet(AstNode* nodep) VL_MT_DISABLED;
+    static void linkLValue(AstNetlist* nodep);
+    static void linkLValueSet(AstNode* nodep);
 };
 
 #endif  // Guard
diff --git a/src/V3Randomize.cpp b/src/V3Randomize.cpp
index 340cbf8a7b..6e01777379 100644
--- a/src/V3Randomize.cpp
+++ b/src/V3Randomize.cpp
@@ -24,7 +24,7 @@
 //
 //*************************************************************************
 
-#include "V3PchAstNoMT.h"  // VL_MT_DISABLED_CODE_UNIT
+#include "V3PchAstMT.h"
 
 #include "V3Randomize.h"
 
diff --git a/src/V3Randomize.h b/src/V3Randomize.h
index 5de300667b..2dff583138 100644
--- a/src/V3Randomize.h
+++ b/src/V3Randomize.h
@@ -28,10 +28,10 @@ class AstNetlist;
 
 class V3Randomize final {
 public:
-    static void randomizeNetlist(AstNetlist* nodep) VL_MT_DISABLED;
+    static void randomizeNetlist(AstNetlist* nodep);
 
-    static AstFunc* newRandomizeFunc(AstClass* nodep) VL_MT_DISABLED;
-    static AstFunc* newSRandomFunc(AstClass* nodep) VL_MT_DISABLED;
+    static AstFunc* newRandomizeFunc(AstClass* nodep);
+    static AstFunc* newSRandomFunc(AstClass* nodep);
 };
 
 #endif  // Guard
diff --git a/src/V3Task.cpp b/src/V3Task.cpp
index 58dd51eaf4..8d28046d03 100644
--- a/src/V3Task.cpp
+++ b/src/V3Task.cpp
@@ -23,7 +23,7 @@
 //
 //*************************************************************************
 
-#include "V3PchAstNoMT.h"  // VL_MT_DISABLED_CODE_UNIT
+#include "V3PchAstMT.h"
 
 #include "V3Task.h"
 
diff --git a/src/V3Task.h b/src/V3Task.h
index feea7e17ea..7ba223c9d5 100644
--- a/src/V3Task.h
+++ b/src/V3Task.h
@@ -55,16 +55,15 @@ class V3Task final {
     static void taskAll(AstNetlist* nodep) VL_MT_DISABLED;
     /// Return vector of [port, pin-connects-to]  (SLOW)
     static V3TaskConnects taskConnects(AstNodeFTaskRef* nodep, AstNode* taskStmtsp,
-                                       V3TaskConnectState* statep = nullptr) VL_MT_DISABLED;
+                                       V3TaskConnectState* statep = nullptr);
     static void taskConnectWrap(AstNodeFTaskRef* nodep, const V3TaskConnects& tconnects,
                                 V3TaskConnectState* statep,
-                                const std::set<const AstVar*>& argWrap) VL_MT_DISABLED;
+                                const std::set<const AstVar*>& argWrap);
     static AstNodeFTask* taskConnectWrapNew(AstNodeFTask* taskp, const string& newname,
                                             const V3TaskConnects& tconnects,
-                                            const std::set<const AstVar*>& argWrap) VL_MT_DISABLED;
+                                            const std::set<const AstVar*>& argWrap);
     static string assignInternalToDpi(AstVar* portp, bool isPtr, const string& frSuffix,
-                                      const string& toSuffix,
-                                      const string& frPrefix = "") VL_MT_DISABLED;
+                                      const string& toSuffix, const string& frPrefix = "");
     static string assignDpiToInternal(const string& lhsName, AstVar* rhsp) VL_MT_DISABLED;
     static const char* dpiTemporaryVarSuffix() VL_MT_SAFE { return s_dpiTemporaryVarSuffix; }
 };
diff --git a/src/V3ThreadSafety.h b/src/V3ThreadSafety.h
index 1753069303..e478453ea8 100644
--- a/src/V3ThreadSafety.h
+++ b/src/V3ThreadSafety.h
@@ -73,4 +73,8 @@ using V3MtDisabledLockGuard = V3LockGuardImp<V3MtDisabledLock>;
     VL_CLANG_ATTR(annotate("MT_DISABLED")) \
     VL_REQUIRES(V3MtDisabledLock::instance())
 
+#define VL_MT_ENABLED \
+    VL_CLANG_ATTR(annotate("MT_ENABLED")) \
+    VL_REQUIRES_UNLOCKED(V3MtDisabledLock::instance())
+
 #endif  // guard
diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index 9597ecf1d5..3bcc9c3812 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -63,7 +63,7 @@
 // iterateSubtreeReturnEdits.
 //*************************************************************************
 
-#include "V3PchAstNoMT.h"  // VL_MT_DISABLED_CODE_UNIT
+#include "V3PchAstMT.h"
 
 #include "V3Width.h"
 
diff --git a/src/V3Width.h b/src/V3Width.h
index 0ac3a6b25e..e5cb9b3953 100644
--- a/src/V3Width.h
+++ b/src/V3Width.h
@@ -31,13 +31,13 @@ class AstNodeDType;
 class V3Width final {
 public:
     static void width(AstNetlist* nodep) VL_MT_DISABLED;
-    static AstNode* widthParamsEdit(AstNode* nodep) VL_MT_DISABLED;
+    static AstNode* widthParamsEdit(AstNode* nodep);
     static AstNode* widthGenerateParamsEdit(AstNode* nodep) VL_MT_DISABLED;
 
     // For use only in WidthVisitor
     // Replace AstSelBit, etc with AstSel/AstArraySel
     // Returns replacement node if nodep was deleted, or null if none.
-    static AstNode* widthSelNoIterEdit(AstNode* nodep) VL_MT_DISABLED;
+    static AstNode* widthSelNoIterEdit(AstNode* nodep);
 };
 
 #endif  // Guard
diff --git a/src/Verilator.cpp b/src/Verilator.cpp
index 3c3b1a3fef..d04c636ce0 100644
--- a/src/Verilator.cpp
+++ b/src/Verilator.cpp
@@ -197,9 +197,13 @@ static void process() {
         // Coverage insertion
         //    Before we do dead code elimination and inlining, or we'll lose it.
         if (v3Global.opt.coverage()) V3Coverage::coverage(v3Global.rootp());
+    }
 
-        // Add randomize() class methods if they are used by the design
-        if (v3Global.useRandomizeMethods()) V3Randomize::randomizeNetlist(v3Global.rootp());
+    // Add randomize() class methods if they are used by the design
+    if (v3Global.useRandomizeMethods()) V3Randomize::randomizeNetlist(v3Global.rootp());
+
+    {
+        const V3MtDisabledLockGuard mtDisabler{v3MtDisabledLock()};
 
         // Push constants, but only true constants preserving liveness
         // so V3Undriven sees variables to be eliminated, ie "if (0 && foo) ..."
@@ -485,11 +489,15 @@ static void process() {
             // Move wide constants to BLOCK temps / ConstPool.
             V3Premit::premitAll(v3Global.rootp());
         }
+    }
 
-        // Expand macros and wide operators into C++ primitives
-        if (!v3Global.opt.lintOnly() && !v3Global.opt.xmlOnly() && v3Global.opt.fExpand()) {
-            V3Expand::expandAll(v3Global.rootp());
-        }
+    // Expand macros and wide operators into C++ primitives
+    if (!v3Global.opt.lintOnly() && !v3Global.opt.xmlOnly() && v3Global.opt.fExpand()) {
+        V3Expand::expandAll(v3Global.rootp());
+    }
+
+    {
+        const V3MtDisabledLockGuard mtDisabler{v3MtDisabledLock()};
 
         // Propagate constants across WORDSEL arrayed temporaries
         if (!v3Global.opt.xmlOnly() && v3Global.opt.fSubst()) {