Internals: Do not optimize variables with RW refs in Dfg

src/V3Dfg.h

@@ -68,37 +68,6 @@ class DfgVisitor;
 template <typename T_User, bool = fitsSpaceAllocatedFor<T_User, void*>()>
 class DfgUserMap;
 
-namespace V3Dfg {
-//-----------------------------------------------------------------------
-// Functions for compatibility tests
-
-// Returns true if variable can be represented in the graph
-inline bool isSupported(const AstVar* varp) {
-    if (varp->isIfaceRef()) return false;  // Cannot handle interface references
-    if (varp->delayp()) return false;  // Cannot handle delayed variables
-    if (varp->isSc()) return false;  // SystemC variables are special and rare, we can ignore
-    if (varp->dfgMultidriven()) return false;  // Discovered as multidriven on earlier DFG run
-    return DfgDataType::fromAst(varp->dtypep());
-}
-
-// Returns true if variable can be represented in the graph
-inline bool isSupported(const AstVarScope* vscp) {
-    const AstNodeModule* const modp = vscp->scopep()->modp();
-    if (VN_IS(modp, Module)) {
-        // Regular module supported
-    } else if (const AstIface* const ifacep = VN_CAST(modp, Iface)) {
-        // Interfaces supported if there are no virtual interfaces for
-        // them, otherwise they cannot be resovled statically.
-        if (ifacep->hasVirtualRef()) return false;
-    } else {
-        return false;  // Anything else (package, class, etc) not supported
-    }
-    // Check the AstVar
-    return isSupported(vscp->varp());
-}
-
-}  //namespace V3Dfg
-
 //------------------------------------------------------------------------------
 // Dataflow graph vertex type enum
 
@@ -574,6 +543,39 @@ public:
     sinkCone(const std::vector<const DfgVertex*>&) const VL_MT_DISABLED;
 };
 
+namespace V3Dfg {
+//-----------------------------------------------------------------------
+// Functions for compatibility tests
+
+// Returns true if variable can be represented in the graph
+inline bool isSupported(const AstVar* varp) {
+    if (varp->isIfaceRef()) return false;  // Cannot handle interface references
+    if (varp->delayp()) return false;  // Cannot handle delayed variables
+    if (varp->isSc()) return false;  // SystemC variables are special and rare, we can ignore
+    if (varp->dfgMultidriven()) return false;  // Discovered as multidriven on earlier DFG run
+    if (DfgVertexVar::hasRWRefs(varp)) return false;  // Referenced via READWRITE references
+    return DfgDataType::fromAst(varp->dtypep());
+}
+
+// Returns true if variable can be represented in the graph
+inline bool isSupported(const AstVarScope* vscp) {
+    const AstNodeModule* const modp = vscp->scopep()->modp();
+    if (VN_IS(modp, Module)) {
+        // Regular module supported
+    } else if (const AstIface* const ifacep = VN_CAST(modp, Iface)) {
+        // Interfaces supported if there are no virtual interfaces for
+        // them, otherwise they cannot be resovled statically.
+        if (ifacep->hasVirtualRef()) return false;
+    } else {
+        return false;  // Anything else (package, class, etc) not supported
+    }
+    if (DfgVertexVar::hasRWRefs(vscp)) return false;  // Referenced via READWRITE references
+    // Check the AstVar
+    return isSupported(vscp->varp());
+}
+
+}  //namespace V3Dfg
+
 //------------------------------------------------------------------------------
 // Map from DfgVertices to T_Value implemeneted via DfgVertex::m_userStorage
 

src/V3DfgOptimizer.cpp

@@ -246,7 +246,8 @@ class DataflowOptimize final {
     // - bit1: Written via AstVarXRef (hierarchical reference)
     // - bit2: Read by logic in same module/netlist not represented in DFG
     // - bit3: Written by logic in same module/netlist not represented in DFG
-    // - bit31-4: Reference count, how many DfgVertexVar represent this variable
+    // - bit4: Has READWRITE references
+    // - bit31-5: Reference count, how many DfgVertexVar represent this variable
     //
     // AstNode::user2/user3/user4 can be used by various DFG algorithms
     const VNUser1InUse m_user1InUse;
@@ -285,6 +286,12 @@ class DataflowOptimize final {
                     senItemp->foreach([](const AstVarRef* refp) {
                         DfgVertexVar::setHasExtRdRefs(refp->varScopep());
                     });
+                    return;
+                }
+                // Check direct references
+                if (const AstVarRef* const refp = VN_CAST(nodep, VarRef)) {
+                    if (refp->access().isRW()) DfgVertexVar::setHasRWRefs(refp->varScopep());
+                    return;
                 }
             } else {
                 if (AstVar* const varp = VN_CAST(nodep, Var)) {
@@ -296,13 +303,20 @@ class DataflowOptimize final {
                     if (hasExtWr) DfgVertexVar::setHasExtWrRefs(varp);
                     return;
                 }
+                // Check direct references
+                if (const AstVarRef* const refp = VN_CAST(nodep, VarRef)) {
+                    if (refp->access().isRW()) DfgVertexVar::setHasRWRefs(refp->varp());
+                    return;
+                }
             }
+
             // Check hierarchical references
             if (const AstVarXRef* const xrefp = VN_CAST(nodep, VarXRef)) {
                 AstVar* const tgtp = xrefp->varp();
                 if (!tgtp) return;
                 if (xrefp->access().isReadOrRW()) DfgVertexVar::setHasExtRdRefs(tgtp);
                 if (xrefp->access().isWriteOrRW()) DfgVertexVar::setHasExtWrRefs(tgtp);
+                if (xrefp->access().isRW()) DfgVertexVar::setHasRWRefs(tgtp);
                 return;
             }
             // Check cell ports

src/V3DfgSynthesize.cpp

@@ -145,6 +145,7 @@ class AstToDfgConverter final : public VNVisitor {
     // Returns {nullptr, 0}, if the given LValue expression is not supported.
     std::pair<DfgVertexSplice*, uint32_t> convertLValue(AstNodeExpr* nodep) {
         if (const AstVarRef* const vrefp = VN_CAST(nodep, VarRef)) {
+            UASSERT_OBJ(vrefp->access().isWriteOnly(), vrefp, "Non-WriteOnly reference");
             if (!isSupported(vrefp)) {
                 ++m_ctx.m_conv.nonRepLValue;
                 return {nullptr, 0};
@@ -366,7 +367,8 @@ class AstToDfgConverter final : public VNVisitor {
         UASSERT_OBJ(!nodep->user2p(), nodep, "Already has Dfg vertex");
         if (unhandled(nodep)) return;
         // This visit method is only called on RValues, where only read refs are supported
-        if (!nodep->access().isReadOnly() || !isSupported(nodep)) {
+        UASSERT_OBJ(nodep->access().isReadOnly(), nodep, "Non-ReadOnly reference");
+        if (!isSupported(nodep)) {
             m_foundUnhandled = true;
             ++m_ctx.m_conv.nonRepVarRef;
             return;

src/V3DfgVertices.h

@@ -70,8 +70,8 @@ class DfgVertexVar VL_NOT_FINAL : public DfgVertex {
 #endif
         // Increment reference count
         AstNode* const variablep = nodep();
-        variablep->user1(variablep->user1() + 0x10);
+        variablep->user1(variablep->user1() + 0x20);
-        UASSERT_OBJ((variablep->user1() >> 4) > 0, variablep, "Reference count overflow");
+        UASSERT_OBJ((variablep->user1() >> 5) > 0, variablep, "Reference count overflow");
         // Allocate sources
         newInput();
         newInput();
@@ -87,8 +87,8 @@ public:
     ~DfgVertexVar() {
         // Decrement reference count
         AstNode* const variablep = nodep();
-        variablep->user1(variablep->user1() - 0x10);
+        variablep->user1(variablep->user1() - 0x20);
-        UASSERT_OBJ((variablep->user1() >> 4) >= 0, variablep, "Reference count underflow");
+        UASSERT_OBJ((variablep->user1() >> 5) >= 0, variablep, "Reference count underflow");
     }
     ASTGEN_MEMBERS_DfgVertexVar;
 
@@ -118,7 +118,7 @@ public:
     void driverFileLine(FileLine* flp) { m_driverFileLine = flp; }
 
     // Variable referenced from other DFG in the same module/netlist
-    bool hasDfgRefs() const { return nodep()->user1() >> 5; }  // I.e.: (nodep()->user1() >> 4) > 1
+    bool hasDfgRefs() const { return nodep()->user1() >> 6; }  // I.e.: (nodep()->user1() >> 5) > 1
 
     // Variable referenced from Ast code in the same module/netlist
     static bool hasModRdRefs(const AstNode* nodep) { return nodep->user1() & 0x04; }
@@ -140,6 +140,10 @@ public:
     bool hasExtWrRefs() const { return hasExtWrRefs(nodep()); }
     bool hasExtRefs() const { return hasExtRdRefs() || hasExtWrRefs(); }
 
+    // Variable referenced via READWRITE references
+    static bool hasRWRefs(const AstNode* nodep) { return nodep->user1() & 0x10; }
+    static void setHasRWRefs(AstNode* nodep) { nodep->user1(nodep->user1() | 0x10); }
+
     // True iff the value of this variable is read outside this DfgGraph
     bool isObserved() const {
         // A DfgVarVertex is written in exactly one DfgGraph, and might be read in an arbitrary