Make incorrect soft ordering constraint into a hard constraint. (#5520)

An ordering constraint between NBA commit blocks ('Post' logic) and the
written variable were previously added as soft constraints (cutable
edges). However these are required for correctness, so if it ever is
cut we will have incorrect simulation results.

Change these into hard constraints instead. This necessitates adding a
flag on AstVar to ignore special variables constructed during V3Delayed
that might otherwise appear as degenerate logic loops. E.g.:

if (VdlySet) {
   VdlySet = 0;  // <- This write to VdlySet can and must be ignored
   LHS = VdlyVal;
}

No functional change, but you might get an error if this constraint was
ever violated. (Theoretically it should never be, as these variables
were inserted in a way that does not require violating these constraints
...)

src/V3AstNodeOther.h

@@ -1847,6 +1847,7 @@ class AstVar final : public AstNode {
     bool m_isForcedByCode : 1;  // May be forced/released from AstAssignForce/AstRelease
     bool m_isWrittenByDpi : 1;  // This variable can be written by a DPI Export
     bool m_isWrittenBySuspendable : 1;  // This variable can be written by a suspendable process
+    bool m_ignorePostWrite : 1;  // Ignore writes in 'Post' blocks during ordering
 
     void init() {
         m_ansi = false;
@@ -1891,6 +1892,7 @@ class AstVar final : public AstNode {
         m_isForcedByCode = false;
         m_isWrittenByDpi = false;
         m_isWrittenBySuspendable = false;
+        m_ignorePostWrite = false;
         m_attrClocker = VVarAttrClocker::CLOCKER_UNKNOWN;
     }
 
@@ -2046,6 +2048,8 @@ public:
     void setWrittenByDpi() { m_isWrittenByDpi = true; }
     bool isWrittenBySuspendable() const { return m_isWrittenBySuspendable; }
     void setWrittenBySuspendable() { m_isWrittenBySuspendable = true; }
+    bool ignorePostWrite() const { return m_ignorePostWrite; }
+    void setIgnorePostWrite() { m_ignorePostWrite = true; }
 
     // METHODS
     void name(const string& name) override { m_name = name; }

src/V3Delayed.cpp

@@ -510,6 +510,7 @@ class DelayedVisitor final : public VNVisitor {
 
         // Create new flag
         AstVarScope* const flagVscp = createTemp(flp, scopep, "__VdlySet" + baseName, 1);
+        flagVscp->varp()->setIgnorePostWrite();
         // Set the flag at the original NBA
         nodep->addHereThisAsNext(  //
             new AstAssign{flp, new AstVarRef{flp, flagVscp, VAccess::WRITE},
@@ -543,6 +544,7 @@ class DelayedVisitor final : public VNVisitor {
         const std::string name = "__VdlyCommitQueue" + vscp->varp()->shortName();
         AstVarScope* const queueVscp = createTemp(flp, scopep, name, cqDTypep);
         queueVscp->varp()->noReset(true);
+        queueVscp->varp()->setIgnorePostWrite();
         vscpInfo.valueQueueKit().vscp = queueVscp;
         // Create the AstActive for the Post logic
         AstActive* const activep
@@ -556,7 +558,7 @@ class DelayedVisitor final : public VNVisitor {
         AstCMethodHard* const callp
             = new AstCMethodHard{flp, new AstVarRef{flp, queueVscp, VAccess::READWRITE}, "commit"};
         callp->dtypeSetVoid();
-        callp->addPinsp(new AstVarRef{flp, vscp, VAccess::READWRITE});
+        callp->addPinsp(new AstVarRef{flp, vscp, VAccess::WRITE});
         postp->addStmtsp(callp->makeStmt());
     }
 

src/V3OrderGraphBuilder.cpp

@@ -215,24 +215,20 @@ class OrderGraphBuilder final : public VNVisitor {
             // Update VarUsage
             varscp->user2(varscp->user2() | VU_GEN);
             // Add edges for produced variables
-            if (!m_inClocked || m_inPost) {
+            if (m_inPost) {
-                // Combinational logic
+                if (!varscp->varp()->ignorePostWrite()) {
-                OrderVarVertex* const varVxp = getVarVertex(varscp, VarVertexType::STD);
+                    // Add edge from producing LogicVertex -> produced VarStdVertex
-                // Add edge from producing LogicVertex -> produced VarStdVertex
+                    OrderVarVertex* const varVxp = getVarVertex(varscp, VarVertexType::STD);
-                if (m_inPost) {
-                    m_graphp->addSoftEdge(m_logicVxp, varVxp, WEIGHT_COMBO);
-                } else {
                     m_graphp->addHardEdge(m_logicVxp, varVxp, WEIGHT_NORMAL);
                 }
-
+                OrderVarVertex* const postVxp = getVarVertex(varscp, VarVertexType::POST);
+                // Add edge from produced VarPostVertex -> to producing LogicVertex
+                m_graphp->addHardEdge(postVxp, m_logicVxp, WEIGHT_POST);
+            } else if (!m_inClocked) {  // Combinational logic
+                // Add edge from producing LogicVertex -> produced VarStdVertex
+                OrderVarVertex* const varVxp = getVarVertex(varscp, VarVertexType::STD);
+                m_graphp->addHardEdge(m_logicVxp, varVxp, WEIGHT_NORMAL);
                 // Add edge from produced VarPostVertex -> to producing LogicVertex
-
-                // For m_inPost:
-                //    Add edge consumed_var_POST->logic_vertex
-                //    This prevents a consumer of the "early" value to be scheduled
-                //   after we've changed to the next-cycle value
-                // ALWAYS do it:
-                //    There maybe a wire a=b; between the two blocks
                 OrderVarVertex* const postVxp = getVarVertex(varscp, VarVertexType::POST);
                 m_graphp->addHardEdge(postVxp, m_logicVxp, WEIGHT_POST);
             } else if (m_inPre) {  // AstAssignPre