Fix re-evaluation of logic dependent on state set in DPI exports (#3091).

Verilator should now correctly re-evaluate any logic that depends on
state set in a DPI exported function, including if the DPI export is
called outside eval, or if the DPI export is called from a DPI import.

Whenever the design contains a DPI exported function that sets a
non-local variable, we create a global __Vdpi_export_trigger flag, that
is set in the body of the DPI export, and make all variables set in any
DPI exported functions dependent on this flag (this ensures correct
ordering and change detection on state set in DPI exports when needed).
The DPI export trigger flag is cleared at the end of eval, which ensured
calls to DPI exports outside of eval are detected. Additionally the
ordering is modifies to assume that any call to a 'context' DPI import
might call DPI exports by adding an edge to the ordering graph from the
logic vertex containing the call to the DPI import to the DPI export
trigger variable vertex (note the standard does not allow calls to DPI
exports from DPI imports that were not imported with 'context', so we
do not enforce ordering on those).

src/V3Order.cpp

@@ -645,6 +645,7 @@ private:
     AstSenTree* m_comboDomainp = nullptr;  // Combo activation tree
     AstSenTree* m_deleteDomainp = nullptr;  // Delete this from tree
     OrderInputsVertex* m_inputsVxp = nullptr;  // Top level vertex all inputs point from
+    OrderVarVertex* m_dpiExportTriggerVxp = nullptr;  // DPI Export trigger condition vertex
     OrderLogicVertex* m_logicVxp = nullptr;  // Current statement being tracked, nullptr=ignored
     AstTopScope* m_topScopep = nullptr;  // Current top scope being processed
     AstScope* m_scopetopp = nullptr;  // Scope under TOPSCOPE
@@ -756,6 +757,10 @@ private:
     }
 
     void nodeMarkCircular(OrderVarVertex* vertexp, OrderEdge* edgep) {
+        // To be marked circular requires being a clock assigned in a delayed assignment, or
+        // having a cutable in or out edge, none of which is true for the DPI export trigger.
+        UASSERT(vertexp != m_dpiExportTriggerVxp,
+                "DPI expor trigger should not be marked circular");
         AstVarScope* nodep = vertexp->varScp();
         OrderLogicVertex* fromLVtxp = nullptr;
         OrderLogicVertex* toLVtxp = nullptr;
@@ -953,6 +958,9 @@ private:
         // Base vertices
         m_activeSenVxp = nullptr;
         m_inputsVxp = new OrderInputsVertex(&m_graph, nullptr);
+        if (AstVarScope* const dpiExportTrigger = v3Global.rootp()->dpiExportTriggerp()) {
+            m_dpiExportTriggerVxp = newVarUserVertex(dpiExportTrigger, WV_STD);
+        }
         //
         iterateChildren(nodep);
         // Done topscope, erase extra user information
@@ -1130,6 +1138,24 @@ private:
             }
         }
     }
+    virtual void visit(AstDpiExportUpdated* nodep) override {
+        // This is under an AstAlways, sensitive to a change in the DPI export trigger. We just
+        // need to add an edge to the enclosing logic vertex (the vertex for the AstAlways).
+        OrderVarVertex* const varVxp = newVarUserVertex(nodep->varScopep(), WV_STD);
+        new OrderComboCutEdge(&m_graph, m_logicVxp, varVxp);
+        // Only used for ordering, so we can get rid of it here
+        nodep->unlinkFrBack();
+        VL_DO_DANGLING(pushDeletep(nodep), nodep);
+    }
+    virtual void visit(AstCCall* nodep) override {
+        // Calls to 'context' imported DPI function may call DPI exported functions
+        if (m_dpiExportTriggerVxp && nodep->funcp()->dpiImportWrapper()
+            && nodep->funcp()->dpiContext()) {
+            UASSERT_OBJ(m_logicVxp, nodep, "Call not under logic");
+            new OrderEdge(&m_graph, m_logicVxp, m_dpiExportTriggerVxp, WEIGHT_NORMAL);
+        }
+        iterateChildren(nodep);
+    }
     virtual void visit(AstSenTree* nodep) override {
         // Having a node derived from the sentree isn't required for
         // correctness, it merely makes the graph better connected
@@ -1187,10 +1213,10 @@ private:
         iterateNewStmt(nodep);
     }
     virtual void visit(AstCFunc*) override {
-        // Ignore for now
-        // We should detect what variables are set in the function, and make
-        // settlement code for them, then set a global flag, so we call "settle"
-        // on the next evaluation loop.
+        // Calls to DPI exports handled with AstCCall. /* verlator public */ functions are
+        // ignored for now (and hence potentially mis-ordered), but could use the same or
+        // similar mechanism as DPI exports. Every other impure function (including those
+        // that may set a non-local variable) must have been inlined in V3Task.
     }
     //--------------------
     virtual void visit(AstNode* nodep) override { iterateChildren(nodep); }