Support `disable fork` (#4125) (#4569)

include/verilated_types.h

@@ -81,11 +81,16 @@ constexpr IData VL_CLOG2_CE_Q(QData lhs) VL_PURE {
     return lhs <= 1 ? 0 : VL_CLOG2_CE_Q((lhs + 1) >> 1ULL) + 1;
 }
 
-//===================================================================
+// Metadata of processes
-// VlProcess stores metadata of running processes
+class VlProcess;
+
+using VlProcessRef = std::shared_ptr<VlProcess>;
+
 class VlProcess final {
     // MEMBERS
     int m_state;  // Current state of the process
+    VlProcessRef m_parentp = nullptr;  // Parent process, if exists
+    std::set<VlProcess*> m_children;  // Active child processes
 
 public:
     // TYPES
@@ -98,16 +103,34 @@ public:
     };
 
     // CONSTRUCTORS
+    // Construct independent process
     VlProcess()
         : m_state{RUNNING} {}
+    // Construct child process of parent
+    VlProcess(VlProcessRef parentp)
+        : m_state{RUNNING}
+        , m_parentp{parentp} {
+        m_parentp->attach(this);
+    }
+
+    ~VlProcess() {
+        if (m_parentp) m_parentp->detach(this);
+    }
+
+    void attach(VlProcess* childp) { m_children.insert(childp); }
+    void detach(VlProcess* childp) { m_children.erase(childp); }
 
-    // METHODS
     int state() { return m_state; }
     void state(int s) { m_state = s; }
+    void disable() {
+        state(KILLED);
+        disable_fork();
+    }
+    void disable_fork() {
+        for (VlProcess* childp : m_children) childp->disable();
+    }
 };
 
-using VlProcessRef = std::shared_ptr<VlProcess>;
-
 inline std::string VL_TO_STRING(const VlProcessRef& p) { return std::string("process"); }
 
 //===================================================================

src/V3AstNodeOther.h

@@ -86,7 +86,7 @@ private:
     bool m_recursive : 1;  // Recursive or part of recursion
     bool m_underGenerate : 1;  // Under generate (for warning)
     bool m_virtual : 1;  // Virtual method in class
-    bool m_needProcess : 1;  // Implements part of a process that allocates std::process
+    bool m_needProcess : 1;  // Needs access to VlProcess of the caller
     VLifetime m_lifetime;  // Lifetime
     VIsCached m_purity;  // Pure state
 
@@ -300,7 +300,7 @@ class AstNodeProcedure VL_NOT_FINAL : public AstNode {
     // IEEE procedure: initial, final, always
     // @astgen op2 := stmtsp : List[AstNode] // Note: op1 is used in some sub-types only
     bool m_suspendable : 1;  // Is suspendable by a Delay, EventControl, etc.
-    bool m_needProcess : 1;  // Implements part of a process that allocates std::process
+    bool m_needProcess : 1;  // Uses VlProcess
 protected:
     AstNodeProcedure(VNType t, FileLine* fl, AstNode* stmtsp)
         : AstNode{t, fl} {
@@ -608,7 +608,7 @@ private:
     bool m_dpiImportPrototype : 1;  // This is the DPI import prototype (i.e.: provided by user)
     bool m_dpiImportWrapper : 1;  // Wrapper for invoking DPI import prototype from generated code
     bool m_dpiTraceInit : 1;  // DPI trace_init
-    bool m_needProcess : 1;  // Implements part of a process that allocates std::process
+    bool m_needProcess : 1;  // Needs access to VlProcess of the caller
 public:
     AstCFunc(FileLine* fl, const string& name, AstScope* scopep, const string& rtnType = "")
         : ASTGEN_SUPER_CFunc(fl) {
@@ -2096,19 +2096,23 @@ class AstBegin final : public AstNodeBlock {
     // Parents: statement
     // @astgen op1 := genforp : Optional[AstNode]
 
-    bool m_generate;  // Underneath a generate
+    bool m_generate : 1;  // Underneath a generate
-    const bool m_implied;  // Not inserted by user
+    bool m_needProcess : 1;  // Uses VlProcess
+    const bool m_implied : 1;  // Not inserted by user
 public:
     // Node that puts name into the output stream
     AstBegin(FileLine* fl, const string& name, AstNode* stmtsp, bool generate = false,
              bool implied = false)
         : ASTGEN_SUPER_Begin(fl, name, stmtsp)
         , m_generate{generate}
+        , m_needProcess{false}
         , m_implied{implied} {}
     ASTGEN_MEMBERS_AstBegin;
     void dump(std::ostream& str) const override;
     void generate(bool flag) { m_generate = flag; }
     bool generate() const { return m_generate; }
+    void setNeedProcess() { m_needProcess = true; }
+    bool needProcess() const { return m_needProcess; }
     bool implied() const { return m_implied; }
 };
 class AstFork final : public AstNodeBlock {

src/V3AstNodes.cpp

@@ -2321,6 +2321,7 @@ void AstBegin::dump(std::ostream& str) const {
     if (generate()) str << " [GEN]";
     if (genforp()) str << " [GENFOR]";
     if (implied()) str << " [IMPLIED]";
+    if (needProcess()) str << " [NPRC]";
 }
 void AstCoverDecl::dump(std::ostream& str) const {
     this->AstNodeStmt::dump(str);

src/V3EmitCFunc.cpp

@@ -410,7 +410,8 @@ void EmitCFunc::displayNode(AstNode* nodep, AstScopeName* scopenamep, const stri
     displayEmit(nodep, isScan);
 }
 
-void EmitCFunc::emitCCallArgs(const AstNodeCCall* nodep, const string& selfPointer) {
+void EmitCFunc::emitCCallArgs(const AstNodeCCall* nodep, const string& selfPointer,
+                              bool inProcess) {
     puts("(");
     bool comma = false;
     if (nodep->funcp()->isLoose() && !nodep->funcp()->isStatic()) {
@@ -422,6 +423,8 @@ void EmitCFunc::emitCCallArgs(const AstNodeCCall* nodep, const string& selfPoint
         if (comma) puts(", ");
         if (VN_IS(nodep->backp(), CAwait) || !nodep->funcp()->isCoroutine()) {
             puts("vlProcess");
+        } else if (inProcess) {
+            puts("std::make_shared<VlProcess>(vlProcess)");
         } else {
             puts("std::make_shared<VlProcess>()");
         }

src/V3EmitCFunc.h

@@ -205,7 +205,7 @@ public:
     }
     void emitOpName(AstNode* nodep, const string& format, AstNode* lhsp, AstNode* rhsp,
                     AstNode* thsp);
-    void emitCCallArgs(const AstNodeCCall* nodep, const string& selfPointer);
+    void emitCCallArgs(const AstNodeCCall* nodep, const string& selfPointer, bool inProcess);
     void emitDereference(const string& pointer);
     void emitCvtPackStr(AstNode* nodep);
     void emitCvtWideArray(AstNode* nodep, AstNode* fromp);
@@ -500,7 +500,7 @@ public:
             }
             puts(funcp->nameProtect());
         }
-        emitCCallArgs(nodep, nodep->selfPointerProtect(m_useSelfForThis));
+        emitCCallArgs(nodep, nodep->selfPointerProtect(m_useSelfForThis), m_cfuncp->needProcess());
     }
     void visit(AstCMethodCall* nodep) override {
         const AstCFunc* const funcp = nodep->funcp();
@@ -508,7 +508,7 @@ public:
         iterateConst(nodep->fromp());
         putbs("->");
         puts(funcp->nameProtect());
-        emitCCallArgs(nodep, "");
+        emitCCallArgs(nodep, "", m_cfuncp->needProcess());
     }
     void visit(AstCAwait* nodep) override {
         puts("co_await ");
@@ -613,6 +613,7 @@ public:
             puts("]);\n");
         }
     }
+    void visit(AstDisableFork* nodep) override { puts("vlProcess->disable_fork();\n"); }
     void visit(AstCReturn* nodep) override {
         puts("return (");
         iterateAndNextConstNull(nodep->lhsp());

src/V3LinkJump.cpp

@@ -330,7 +330,7 @@ private:
             AstJumpLabel* const labelp = findAddLabel(beginp, false);
             nodep->addNextHere(new AstJumpGo{nodep->fileline(), labelp});
         } else {
-            nodep->v3warn(E_UNSUPPORTED, "Unsupported: disable fork");
+            nodep->v3warn(E_UNSUPPORTED, "Unsupported: disabling fork by name");
         }
         nodep->unlinkFrBack();
         VL_DO_DANGLING(pushDeletep(nodep), nodep);

src/V3SchedTiming.cpp

@@ -279,7 +279,6 @@ void transformForks(AstNetlist* const netlistp) {
         // STATE
         bool m_inClass = false;  // Are we in a class?
         bool m_beginHasAwaits = false;  // Does the current begin have awaits?
-        bool m_beginNeedProcess = false;  // Does the current begin have process::self dependency?
         AstFork* m_forkp = nullptr;  // Current fork
         AstCFunc* m_funcp = nullptr;  // Current function
 
@@ -358,9 +357,8 @@ void transformForks(AstNetlist* const netlistp) {
             UASSERT_OBJ(m_forkp, nodep, "Begin outside of a fork");
             // Start with children, so later we only find awaits that are actually in this begin
             m_beginHasAwaits = false;
-            m_beginNeedProcess = false;
             iterateChildrenConst(nodep);
-            if (m_beginHasAwaits || m_beginNeedProcess) {
+            if (m_beginHasAwaits || nodep->needProcess()) {
                 UASSERT_OBJ(!nodep->name().empty(), nodep, "Begin needs a name");
                 // Create a function to put this begin's statements in
                 FileLine* const flp = nodep->fileline();
@@ -384,7 +382,7 @@ void transformForks(AstNetlist* const netlistp) {
                 }
                 // Put the begin's statements in the function, delete the begin
                 newfuncp->addStmtsp(nodep->stmtsp()->unlinkFrBackWithNext());
-                if (m_beginNeedProcess) {
+                if (nodep->needProcess()) {
                     newfuncp->setNeedProcess();
                     newfuncp->addStmtsp(new AstCStmt{nodep->fileline(),
                                                      "vlProcess->state(VlProcess::FINISHED);\n"});
@@ -405,10 +403,6 @@ void transformForks(AstNetlist* const netlistp) {
             m_beginHasAwaits = true;
             iterateChildrenConst(nodep);
         }
-        void visit(AstCCall* nodep) override {
-            if (nodep->funcp()->needProcess()) m_beginNeedProcess = true;
-            iterateChildrenConst(nodep);
-        }
         void visit(AstExprStmt* nodep) override { iterateChildren(nodep); }
 
         //--------------------

src/V3Timing.cpp

@@ -12,11 +12,27 @@
 // Version 2.0.
 // SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
 //
-// TimingSuspendableVisitor locates all C++ functions and processes that contain timing controls,
+// TimingSuspendableVisitor does not perform any AST transformations.
-// and marks them as suspendable. If a process calls a suspendable function, then it is also marked
+// Instead it propagates two types of flags:
-// as suspendable. If a function calls or overrides a suspendable function, it is also marked as
+// - flag "suspendable": (for detecting what will need to become a coroutine)
-// suspendable. TimingSuspendableVisitor creates a dependency graph to propagate this property. It
+//     The visitor locates all C++ functions and processes that contain timing controls,
-// does not perform any AST transformations.
+//     and marks them as suspendable. If a process calls a suspendable function,
+//     then it is also marked as suspendable. If a function calls or overrides
+//     a suspendable function, it is also marked as suspendable.
+//     TimingSuspendableVisitor creates a dependency graph to propagate this property.
+// - flag "needs process": (for detecting what needs a VlProcess argument in signature)
+//   The visitor distinguishes 4 types of nodes:
+//     - T_ALLOCS_PROC: nodes that can allocate VlProcess (forks, always, initial etc.),
+//     - T_FORCES_PROC: nodes that make it necessary for the previous type to allocate VlProcess
+//       (like process::self which then wraps it, allowing use inside Verilog).
+//     - T_NEEDS_PROC: nodes that should obtain VlProcess if it will be allocated
+//       (all of the previous type + timing controls, so they could update process state),
+//     - T_HAS_PROC: nodes that are going to be emitted with a VlProcess argument.
+//   T_FORCES_PROC and T_NEEDS_PROC are propagated upwards up to the nodes of type T_ALLOCS_PROC,
+//   this is to detect which processes have to allocate VlProcess. Then nodes of these processes
+//   get marked as T_HAS_PROC and the flag is propagated downwards through nodes
+//   type T_NEEDS_PROC. Using only nodes type T_NEEDS_PROC assures the flags are only propagated
+//   through paths leading to nodes that actually use VlProcess.
 //
 // TimingControlVisitor is the one that actually performs transformations:
 // - for each intra-assignment timing control:
@@ -70,8 +86,10 @@ VL_DEFINE_DEBUG_FUNCTIONS;
 enum NodeFlag : uint8_t {
     T_SUSPENDEE = 1 << 0,  // Suspendable (due to dependence on another suspendable)
     T_SUSPENDER = 1 << 1,  // Suspendable (has timing control)
-    T_HAS_PROC = 1 << 2,  // Has an associated std::process
+    T_ALLOCS_PROC = 1 << 2,  // Can allocate VlProcess
-    T_CALLS_PROC_SELF = 1 << 3,  // Calls std::process::self
+    T_FORCES_PROC = 1 << 3,  // Forces VlProcess allocation
+    T_NEEDS_PROC = 1 << 4,  // Needs access to VlProcess if it's allocated
+    T_HAS_PROC = 1 << 5,  // Has VlProcess argument in the signature
 };
 
 enum ForkType : uint8_t {
@@ -86,6 +104,11 @@ enum PropagationType : uint8_t {
     P_SIGNATURE = 3,  // Propagation required to maintain C++ function's signature requirements
 };
 
+// Add timing flag to a node
+static void addFlags(AstNode* const nodep, uint8_t flags) { nodep->user2(nodep->user2() | flags); }
+// Check if a node has ALL of the expected flags set
+static bool hasFlags(AstNode* const nodep, uint8_t flags) { return !(~nodep->user2() & flags); }
+
 // ######################################################################
 //  Detect nodes affected by timing and/or requiring a process
 
@@ -126,8 +149,8 @@ private:
     class SuspendDepVtx final : public DepVtx {
         VL_RTTI_IMPL(SuspendDepVtx, DepVtx)
         string dotColor() const override {
-            if (nodep()->user2() & T_SUSPENDER) return "red";
+            if (hasFlags(nodep(), T_SUSPENDER)) return "red";
-            if (nodep()->user2() & T_SUSPENDEE) return "blue";
+            if (hasFlags(nodep(), T_SUSPENDEE)) return "blue";
             return "black";
         }
 
@@ -140,8 +163,9 @@ private:
     class NeedsProcDepVtx final : public DepVtx {
         VL_RTTI_IMPL(NeedsProcDepVtx, DepVtx)
         string dotColor() const override {
-            if (nodep()->user2() & T_CALLS_PROC_SELF) return "red";
+            if (hasFlags(nodep(), T_HAS_PROC)) return "blue";
-            if (nodep()->user2() & T_HAS_PROC) return "blue";
+            if (hasFlags(nodep(), T_NEEDS_PROC)) return "green";
+            if (hasFlags(nodep(), T_FORCES_PROC)) return "red";
             return "black";
         }
 
@@ -198,10 +222,10 @@ private:
         if (!nodep->user5p()) nodep->user5p(new NeedsProcDepVtx{&m_procGraph, nodep, classp});
         return nodep->user5u().to<NeedsProcDepVtx*>();
     }
-    // Set timing flag of a node
+    // Pass timing flag between nodes
     bool passFlag(const AstNode* from, AstNode* to, NodeFlag flag) {
         if ((from->user2() & flag) && !(to->user2() & flag)) {
-            to->user2(to->user2() | flag);
+            addFlags(to, flag);
             return true;
         }
         return false;
@@ -216,6 +240,15 @@ private:
         }
     }
     template <typename Predicate>
+    void propagateFlagsIf(DepVtx* const vxp, NodeFlag flag, Predicate p) {
+        auto* const parentp = vxp->nodep();
+        for (V3GraphEdge* edgep = vxp->outBeginp(); edgep; edgep = edgep->outNextp()) {
+            auto* const depVxp = static_cast<DepVtx*>(edgep->top());
+            AstNode* const depp = depVxp->nodep();
+            if (p(edgep) && passFlag(parentp, depp, flag)) propagateFlagsIf(depVxp, flag, p);
+        }
+    }
+    template <typename Predicate>
     void propagateFlagsReversedIf(DepVtx* const vxp, NodeFlag flag, Predicate p) {
         auto* const parentp = vxp->nodep();
         for (V3GraphEdge* edgep = vxp->inBeginp(); edgep; edgep = edgep->inNextp()) {
@@ -236,17 +269,22 @@ private:
     void visit(AstNodeProcedure* nodep) override {
         VL_RESTORER(m_procp);
         m_procp = nodep;
-        if (nodep->needProcess()) nodep->user2(T_HAS_PROC | T_CALLS_PROC_SELF);
+        getNeedsProcDepVtx(nodep);
+        addFlags(nodep, T_ALLOCS_PROC);
         if (VN_IS(nodep, Always)) {
             UINFO(1, "Always does " << (nodep->needProcess() ? "" : "NOT ") << "need process\n");
         }
         iterateChildren(nodep);
     }
+    void visit(AstDisableFork* nodep) override {
+        visit(static_cast<AstNode*>(nodep));
+        addFlags(m_procp, T_FORCES_PROC | T_NEEDS_PROC);
+    }
     void visit(AstCFunc* nodep) override {
         VL_RESTORER(m_procp);
         m_procp = nodep;
         iterateChildren(nodep);
-        if (nodep->needProcess()) nodep->user2(T_HAS_PROC | T_CALLS_PROC_SELF);
+        if (nodep->needProcess()) addFlags(nodep, T_FORCES_PROC | T_NEEDS_PROC);
         DepVtx* const sVxp = getSuspendDepVtx(nodep);
         DepVtx* const pVxp = getNeedsProcDepVtx(nodep);
         if (!m_classp) return;
@@ -287,9 +325,12 @@ private:
             new V3GraphEdge{&m_suspGraph, getSuspendDepVtx(nodep->funcp()),
                             getSuspendDepVtx(m_procp), m_underFork ? P_FORK : P_CALL};
 
-        if (!m_underFork)
+        new V3GraphEdge{&m_procGraph, getNeedsProcDepVtx(nodep->funcp()),
-            new V3GraphEdge{&m_procGraph, getNeedsProcDepVtx(nodep->funcp()),
+                        getNeedsProcDepVtx(m_procp), P_CALL};
-                            getNeedsProcDepVtx(m_procp), P_CALL};
+
+        if (m_underFork && !(m_underFork & F_MIGHT_SUSPEND)) {
+            addFlags(nodep, T_NEEDS_PROC | T_ALLOCS_PROC);
+        }
 
         iterateChildren(nodep);
     }
@@ -301,9 +342,12 @@ private:
             new V3GraphEdge{&m_suspGraph, getSuspendDepVtx(nodep), getSuspendDepVtx(m_procp),
                             m_underFork ? P_FORK : P_CALL};
 
-        if (!m_underFork)
+        new V3GraphEdge{&m_procGraph, getNeedsProcDepVtx(nodep), getNeedsProcDepVtx(m_procp),
-            new V3GraphEdge{&m_procGraph, getNeedsProcDepVtx(nodep), getNeedsProcDepVtx(m_procp),
+                        P_CALL};
-                            P_CALL};
+
+        if (m_underFork && !(m_underFork & F_MIGHT_SUSPEND)) {
+            addFlags(nodep, T_NEEDS_PROC | T_ALLOCS_PROC);
+        }
 
         m_procp = nodep;
         m_underFork = 0;
@@ -316,7 +360,7 @@ private:
                                    // so that transformForks() in V3SchedTiming gets called and
                                    // removes all forks and begins
         if (nodep->isTimingControl() && m_procp) {
-            m_procp->user2(T_SUSPENDEE | T_SUSPENDER);
+            addFlags(m_procp, T_SUSPENDEE | T_SUSPENDER);
             m_underFork |= F_MIGHT_SUSPEND;
         }
         m_underFork |= F_MIGHT_NEED_PROC;
@@ -325,7 +369,7 @@ private:
     void visit(AstNode* nodep) override {
         if (nodep->isTimingControl()) {
             v3Global.setUsesTiming();
-            if (m_procp) m_procp->user2(T_SUSPENDEE | T_SUSPENDER);
+            if (m_procp) addFlags(m_procp, T_SUSPENDEE | T_SUSPENDER | T_NEEDS_PROC);
         }
         iterateChildren(nodep);
     }
@@ -342,22 +386,35 @@ public:
         // Propagate suspendability
         for (V3GraphVertex* vxp = m_suspGraph.verticesBeginp(); vxp; vxp = vxp->verticesNextp()) {
             DepVtx* const depVxp = static_cast<DepVtx*>(vxp);
-            if (depVxp->nodep()->user2() & T_SUSPENDEE) propagateFlags(depVxp, T_SUSPENDEE);
+            if (hasFlags(depVxp->nodep(), T_SUSPENDEE)) propagateFlags(depVxp, T_SUSPENDEE);
         }
         if (dumpGraphLevel() >= 6) m_suspGraph.dumpDotFilePrefixed("timing_deps");
-        // Propagate process
+
+        // Propagate T_HAS_PROCESS
         for (V3GraphVertex* vxp = m_procGraph.verticesBeginp(); vxp; vxp = vxp->verticesNextp()) {
             DepVtx* const depVxp = static_cast<DepVtx*>(vxp);
-            if (depVxp->nodep()->user2() & T_HAS_PROC) propagateFlags(depVxp, T_HAS_PROC);
+            // Find processes that'll allocate VlProcess
-        }
+            if (hasFlags(depVxp->nodep(), T_FORCES_PROC)) {
-        // Propagate process downwards (from caller to callee) for suspendable calls
+                propagateFlagsIf(depVxp, T_FORCES_PROC, [&](const V3GraphEdge* e) -> bool {
-        for (V3GraphVertex* vxp = m_suspGraph.verticesBeginp(); vxp; vxp = vxp->verticesNextp()) {
+                    return !hasFlags(static_cast<DepVtx*>(e->fromp())->nodep(), T_ALLOCS_PROC);
-            DepVtx* const depVxp = static_cast<DepVtx*>(vxp);
-            if (depVxp->nodep()->user2() & T_HAS_PROC)
-                propagateFlagsReversedIf(depVxp, T_HAS_PROC, [&](const V3GraphEdge* e) -> bool {
-                    return (e->weight() != P_FORK)
-                           && (static_cast<DepVtx*>(e->top())->nodep()->user2() & T_SUSPENDEE);
                 });
+            }
+            // Mark nodes on paths between processes and statements that use VlProcess
+            if (hasFlags(depVxp->nodep(), T_NEEDS_PROC)) {
+                propagateFlagsIf(depVxp, T_NEEDS_PROC, [&](const V3GraphEdge* e) -> bool {
+                    return !hasFlags(static_cast<DepVtx*>(e->top())->nodep(), T_ALLOCS_PROC);
+                });
+            }
+        }
+        for (V3GraphVertex* vxp = m_procGraph.verticesBeginp(); vxp; vxp = vxp->verticesNextp()) {
+            DepVtx* const depVxp = static_cast<DepVtx*>(vxp);
+            // Mark nodes that will be emitted with a VlProcess argument
+            if (hasFlags(depVxp->nodep(), T_ALLOCS_PROC | T_FORCES_PROC)) {
+                addFlags(depVxp->nodep(), T_HAS_PROC);
+                propagateFlagsReversedIf(depVxp, T_HAS_PROC, [&](const V3GraphEdge* e) -> bool {
+                    return hasFlags(static_cast<DepVtx*>(e->fromp())->nodep(), T_NEEDS_PROC);
+                });
+            }
         }
         if (dumpGraphLevel() >= 6) m_procGraph.dumpDotFilePrefixed("proc_deps");
     }
@@ -586,7 +643,7 @@ private:
     void addProcessInfo(AstCMethodHard* const methodp) const {
         FileLine* const flp = methodp->fileline();
         AstCExpr* const ap = new AstCExpr{
-            flp, m_procp && (m_procp->user2() & T_HAS_PROC) ? "vlProcess" : "nullptr", 0};
+            flp, m_procp && (hasFlags(m_procp, T_HAS_PROC)) ? "vlProcess" : "nullptr", 0};
         ap->dtypeSetVoid();
         methodp->addPinsp(ap);
     }
@@ -697,8 +754,8 @@ private:
         VL_RESTORER(m_procp);
         m_procp = nodep;
         iterateChildren(nodep);
-        if (nodep->user2() & T_SUSPENDEE) nodep->setSuspendable();
+        if (hasFlags(nodep, T_SUSPENDEE)) nodep->setSuspendable();
-        if (nodep->user2() & T_HAS_PROC) nodep->setNeedProcess();
+        if (hasFlags(nodep, T_HAS_PROC)) nodep->setNeedProcess();
     }
     void visit(AstInitial* nodep) override {
         visit(static_cast<AstNodeProcedure*>(nodep));
@@ -719,11 +776,11 @@ private:
 
         // Workaround for killing `always` processes (doing that is pretty much UB)
         // TODO: Disallow killing `always` at runtime (throw an error)
-        if (nodep->user2() & T_HAS_PROC) nodep->user2(nodep->user2() | T_SUSPENDEE);
+        if (hasFlags(nodep, T_HAS_PROC)) addFlags(nodep, T_SUSPENDEE);
 
         iterateChildren(nodep);
-        if (nodep->user2() & T_HAS_PROC) nodep->setNeedProcess();
+        if (hasFlags(nodep, T_HAS_PROC)) nodep->setNeedProcess();
-        if (!(nodep->user2() & T_SUSPENDEE)) return;
+        if (!hasFlags(nodep, T_SUSPENDEE)) return;
         nodep->setSuspendable();
         FileLine* const flp = nodep->fileline();
         AstSenTree* const sensesp = m_activep->sensesp();
@@ -744,8 +801,8 @@ private:
         VL_RESTORER(m_procp);
         m_procp = nodep;
         iterateChildren(nodep);
-        if (nodep->user2() & T_HAS_PROC) nodep->setNeedProcess();
+        if (hasFlags(nodep, T_HAS_PROC)) nodep->setNeedProcess();
-        if (!(nodep->user2() & T_SUSPENDEE)) return;
+        if (!(hasFlags(nodep, T_SUSPENDEE))) return;
 
         nodep->rtnType("VlCoroutine");
         // If in a class, create a shared pointer to 'this'
@@ -768,7 +825,7 @@ private:
         }
     }
     void visit(AstNodeCCall* nodep) override {
-        if ((nodep->funcp()->user2() & T_SUSPENDEE) && !nodep->user1SetOnce()) {  // If suspendable
+        if (hasFlags(nodep->funcp(), T_SUSPENDEE) && !nodep->user1SetOnce()) {  // If suspendable
             VNRelinker relinker;
             nodep->unlinkFrBack(&relinker);
             AstCAwait* const awaitp = new AstCAwait{nodep->fileline(), nodep};
@@ -1048,6 +1105,7 @@ private:
     void visit(AstBegin* nodep) override {
         VL_RESTORER(m_procp);
         m_procp = nodep;
+        if (hasFlags(nodep, T_HAS_PROC)) nodep->setNeedProcess();
         iterateChildren(nodep);
     }
     void visit(AstFork* nodep) override {
@@ -1061,6 +1119,7 @@ private:
         while (stmtp) {
             if (!VN_IS(stmtp, Begin)) {
                 auto* const beginp = new AstBegin{stmtp->fileline(), "", nullptr};
+                if (hasFlags(stmtp, T_HAS_PROC)) addFlags(beginp, T_HAS_PROC);
                 stmtp->replaceWith(beginp);
                 beginp->addStmtsp(stmtp);
                 stmtp = beginp;

src/V3Width.cpp

@@ -696,8 +696,14 @@ private:
         }
     }
     void visit(AstDisableFork* nodep) override {
-        nodep->v3warn(E_UNSUPPORTED, "Unsupported: disable fork statements");
+        if (nodep->fileline()->timingOn()) {
-        VL_DO_DANGLING(nodep->unlinkFrBack()->deleteTree(), nodep);
+            if (v3Global.opt.timing().isSetFalse()) {
+                nodep->v3warn(E_NOTIMING, "Support for disable fork statement requires --timing");
+            } else if (!v3Global.opt.timing().isSetTrue()) {
+                nodep->v3warn(E_NEEDTIMINGOPT, "Use --timing or --no-timing to specify how "
+                                                   << "disable fork should be handled");
+            }
+        }
     }
     void visit(AstWaitFork* nodep) override {
         nodep->v3warn(E_UNSUPPORTED, "Unsupported: wait fork statements");

test_regress/t/t_disable.out

@@ -0,0 +1,5 @@
+%Error-UNSUPPORTED: t/t_disable.v:11:10: Unsupported: disabling fork by name
+   11 |          disable foo;
+      |          ^~~~~~~
+                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
+%Error: Exiting due to

test_regress/t/t_disable.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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(linter => 1);
+
+lint(
+    verilator_flags2 => ['--lint-only --timing'],
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_disable.v

@@ -0,0 +1,19 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t(/*AUTOARG*/);
+
+   initial begin
+      fork : foo
+         disable foo;
+         #1 $stop;
+      join_none
+      #2;
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+
+endmodule

test_regress/t/t_disable_fork1.pl

@@ -0,0 +1,23 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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(simulator => 1);
+
+compile(
+    verilator_flags2 => ["--exe --main --timing"],
+    make_main => 0,
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_disable_fork1.v

@@ -0,0 +1,49 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Antmicro Ltd.
+// SPDX-License-Identifier: CC0-1.0
+
+`define N 3
+
+class Cls;
+   task runforks(integer n);
+      for (integer i = 0; i < n; i++) fork
+         #1 $stop;
+      join_none
+   endtask
+endclass
+
+module t;
+   Cls cls = new;
+
+   initial begin
+      // run forks
+      for (integer i = 0; i < `N; i++) fork
+         #1 $stop;
+      join_none
+
+      // run forks inside a method
+      cls.runforks(`N);
+
+      // run forks in forks
+      for (integer i = 0; i < `N; i++) fork
+         for (integer j = 0; j < `N; j++) fork
+            #1 $stop;
+         join_none
+      join_none
+
+      for (integer i = 0; i < `N; i++) fork
+         cls.runforks(`N);
+      join_none
+
+      // kill them all
+      disable fork;
+
+      // check if we can still fork
+      fork
+         #2 $write("*-* All Finished *-*\n");
+         #3 $finish;
+      join_none
+   end
+endmodule

test_regress/t/t_disable_fork2.pl

@@ -0,0 +1,23 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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(simulator => 1);
+
+compile(
+    verilator_flags2 => ["--exe --main --timing"],
+    make_main => 0,
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_disable_fork2.v

@@ -0,0 +1,58 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Antmicro Ltd.
+// SPDX-License-Identifier: CC0-1.0
+
+// USING THIS FOR DEBUGGING PROCESS PROPAGATION:
+//
+// The example contains most cases that were problematic during the
+// works on support of 'disable fork' statement, including:
+//
+// - indirect use of disable fork (through a task),
+// - indirect use of forks that are to be disabled,
+// - forks in forks,
+// - a function taking VlProcess argument shared between a process that
+//   allocates VlProcess, and one that doesnt,
+// - a function that has a delay and obtains VlProcess argument,
+// - a function that has a delay and doesn't obtain it.
+//
+// Blocks below contain info on whether they should (YES) or shouldn't (NO)
+// be emitted as functions with a VlProcess argument.
+//
+// To check if that corresponds to reality, see blue nodes in proc_deps.dot
+
+class Cls;
+   task print; /*NO*/
+      $write("*-* All ");
+   endtask
+   task disable_fork_func; /*YES*/
+      disable fork;
+   endtask
+   task common_func; /*YES*/
+      fork /*YES*/ #1; join_none
+   endtask
+   task fork_func; /*YES*/
+      fork /*YES*/ #1 $stop; join_none
+   endtask
+   task delay_func; /*NO*/
+      fork /*NO*/ #1 $write("Finished *-*\n"); join_none
+   endtask
+endclass
+
+module t;
+   Cls cls = new;
+
+   initial begin /*YES*/
+      fork /*YES*/ cls.common_func(); join_none
+      cls.fork_func();
+      cls.disable_fork_func();
+      cls.print();
+   end
+
+   initial begin /*NO*/
+      cls.delay_func();
+      cls.common_func();
+      fork /*YES*/ disable fork; join_none
+   end
+endmodule

test_regress/t/t_disable_fork3.pl

@@ -0,0 +1,23 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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(simulator => 1);
+
+compile(
+    verilator_flags2 => ["--exe --main --timing"],
+    make_main => 0,
+    );
+
+execute(
+    check_finished => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_disable_fork3.v

@@ -0,0 +1,18 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Antmicro Ltd.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+   initial begin
+      fork begin
+         fork begin
+            #3 $stop;
+         end join_none
+         #1;
+      end join_none
+      #2 disable fork;
+   end
+   initial #4 $write("*-* All Finished *-*\n");
+endmodule

test_regress/t/t_disable_fork_notiming.out

@@ -0,0 +1,6 @@
+%Error-NOTIMING: t/t_disable_fork_notiming.v:8:12: Support for disable fork statement requires --timing
+                                                 : ... note: In instance 't'
+    8 |    initial disable fork;
+      |            ^~~~~~~
+                 ... For error description see https://verilator.org/warn/NOTIMING?v=latest
+%Error: Exiting due to

test_regress/t/t_disable_fork_notiming.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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(simulator => 1);
+
+compile(
+    expect_filename => $Self->{golden_filename},
+    v_flags2 => ["--no-timing"],
+    fails => 1,
+    );
+
+ok(1);
+1;

test_regress/t/t_disable_fork_notiming.v

@@ -0,0 +1,9 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Antmicro Ltd.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+   initial disable fork;
+endmodule

test_regress/t/t_fork_disable.out

@@ -1,10 +0,0 @@
-%Error-UNSUPPORTED: t/t_fork_disable.v:16:7: Unsupported: disable fork statements
-                                           : ... note: In instance 't'
-   16 |       disable fork;
-      |       ^~~~~~~
-                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
-%Error-UNSUPPORTED: t/t_fork_disable.v:17:7: Unsupported: wait fork statements
-                                           : ... note: In instance 't'
-   17 |       wait fork;
-      |       ^~~~
-%Error: Exiting due to

test_regress/t/t_uvm_pkg_todo.vh

@@ -11190,9 +11190,7 @@ task uvm_phase::execute_phase();
                end
              end
            join_any
-//TODO issue #4125 - Support disable fork
+           disable fork;
-//TODO  %Error-UNSUPPORTED: t/t_uvm_pkg_todo.vh:11187:12: Unsupported: disable fork statements
-//TODO           disable fork;
           end
         join
     end
@@ -15172,9 +15170,7 @@ class uvm_heartbeat extends uvm_object;
       end
       @(m_stop_event);
     join_any
-//TODO issue #4125 - Support disable fork
+    disable fork;
-//TODO  %Error-UNSUPPORTED: t/t_uvm_pkg_todo.vh:15167:12: Unsupported: disable fork statements
-//TODO    disable fork;
   endtask
 endclass
 class uvm_heartbeat_callback extends uvm_objection_callback;
@@ -19218,9 +19214,7 @@ task uvm_sequencer_base::m_wait_for_available_sequence();
           join_any
         end
       join_any
-//TODO issue #4125 - Support disable fork
+      disable fork;
-//TODO  %Error-UNSUPPORTED: t/t_uvm_pkg_todo.vh:19213:12: Unsupported: disable fork statements
-//TODO      disable fork;
     end
   join
 endtask

test_regress/t/t_wait_fork.out

@@ -0,0 +1,6 @@
+%Error-UNSUPPORTED: t/t_wait_fork.v:17:7: Unsupported: wait fork statements
+                                        : ... note: In instance 't'
+   17 |       wait fork;
+      |       ^~~~
+                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
+%Error: Exiting due to