Support $sampled (#3569)

src/V3Assert.cpp

@@ -45,6 +45,7 @@ private:
     VDouble0 m_statAsNotImm;  // Statistic tracking
     VDouble0 m_statAsImm;  // Statistic tracking
     VDouble0 m_statAsFull;  // Statistic tracking
+    bool m_inSampled = false;  // True inside a sampled expression
 
     // METHODS
     string assertDisplayMessage(AstNode* nodep, const string& prefix, const string& message) {
@@ -66,6 +67,11 @@ private:
             nodep->fmtp()->scopeNamep(new AstScopeName{nodep->fileline(), true});
         }
     }
+    AstSampled* newSampledExpr(AstNode* nodep) {
+        const auto sampledp = new AstSampled{nodep->fileline(), nodep};
+        sampledp->dtypeFrom(nodep);
+        return sampledp;
+    }
     AstVarRef* newMonitorNumVarRefp(AstNode* nodep, VAccess access) {
         if (!m_monitorNumVarp) {
             m_monitorNumVarp = new AstVar{nodep->fileline(), VVarType::MODULETEMP, "__VmonitorNum",
@@ -332,7 +338,8 @@ private:
             ticks = VN_AS(nodep->ticksp(), Const)->toUInt();
         }
         UASSERT_OBJ(ticks >= 1, nodep, "0 tick should have been checked in V3Width");
-        AstNode* inp = nodep->exprp()->unlinkFrBack();
+        AstNode* const exprp = nodep->exprp()->unlinkFrBack();
+        AstNode* inp = newSampledExpr(exprp);
         AstVar* invarp = nullptr;
         AstSenTree* const sentreep = nodep->sentreep();
         sentreep->unlinkFrBack();
@@ -353,10 +360,41 @@ private:
         }
         nodep->replaceWith(inp);
     }
+
+    //========== Move $sampled down to read-only variables
     virtual void visit(AstSampled* nodep) override {
+        if (nodep->user1()) return;
+        VL_RESTORER(m_inSampled);
+        {
+            m_inSampled = true;
+            iterateChildren(nodep);
+        }
         nodep->replaceWith(nodep->exprp()->unlinkFrBack());
         VL_DO_DANGLING(pushDeletep(nodep), nodep);
     }
+    virtual void visit(AstVarRef* nodep) override {
+        iterateChildren(nodep);
+        if (m_inSampled) {
+            if (!nodep->access().isReadOnly()) {
+                nodep->v3warn(E_UNSUPPORTED,
+                              "Unsupported: Write to variable in sampled expression");
+            } else {
+                VNRelinker relinkHandle;
+                nodep->unlinkFrBack(&relinkHandle);
+                AstSampled* const newp = newSampledExpr(nodep);
+                relinkHandle.relink(newp);
+                newp->user1(1);
+            }
+        }
+    }
+    // Don't sample sensitivities
+    virtual void visit(AstSenItem* nodep) override {
+        VL_RESTORER(m_inSampled);
+        {
+            m_inSampled = false;
+            iterateChildren(nodep);
+        }
+    }
 
     //========== Statements
     virtual void visit(AstDisplay* nodep) override {