Internals: Track null separately from '0

src/V3AstNodes.h

@@ -153,6 +153,13 @@ public:
         , m_num(this, 1, on) {
         dtypeSetBit();
     }
+    class Null {};
+    AstConst(FileLine* fl, Null)
+        : ASTGEN_SUPER(fl)
+        , m_num(V3Number::Null{}, this) {
+        dtypeSetBit();  // Events 1 bit, objects 64 bits, so autoExtend=1 and use bit here
+        initWithNumber();
+    }
     ASTNODE_NODE_FUNCS(Const)
     virtual string name() const override { return num().ascii(); }  // * = Value
     const V3Number& num() const { return m_num; }  // * = Value

src/V3Number.cpp

@@ -495,6 +495,7 @@ string V3Number::ascii(bool prefixed, bool cleanVerilog) const {
             out << "'";
             if (bitIs0(0)) {
                 out << '0';
+                if (isNull()) out << "[null]";
             } else if (bitIs1(0)) {
                 out << '1';
             } else if (bitIsZ(0)) {
@@ -529,6 +530,7 @@ string V3Number::ascii(bool prefixed, bool cleanVerilog) const {
         // Always deal with 4 bits at once.  Note no 4-state, it's above.
         out << displayed("%0h");
     }
+    if (isNull() && VL_UNCOVERABLE(!isEqZero())) out << "-%E-null-not-zero";
     return out.str();
 }
 

src/V3Number.h

@@ -43,6 +43,7 @@ class V3Number final {
     bool m_sized : 1;  // True if the user specified the width, else we track it.
     bool m_signed : 1;  // True if signed value
     bool m_double : 1;  // True if double real value
+    bool m_isNull : 1;  // True if "null" versus normal 0
     bool m_isString : 1;  // True if string
     bool m_fromString : 1;  // True if from string literal
     bool m_autoExtend : 1;  // True if SystemVerilog extend-to-any-width
@@ -176,6 +177,12 @@ public:
         init(nodep, 0);
         setString(value);
     }
+    class Null {};
+    V3Number(Null, AstNode* nodep) {
+        init(nodep, 0);
+        m_isNull = true;
+        m_autoExtend = true;
+    }
     explicit V3Number(const V3Number* nump, int width = 1) {
         init(nullptr, width);
         m_fileline = nump->fileline();
@@ -193,6 +200,7 @@ private:
         setNames(nodep);
         m_signed = false;
         m_double = false;
+        m_isNull = false;
         m_isString = false;
         m_autoExtend = false;
         m_fromString = false;
@@ -250,6 +258,7 @@ public:
     bool isString() const { return m_isString; }
     void isString(bool flag) { m_isString = flag; }
     bool isNegative() const { return bitIs1(width() - 1); }
+    bool isNull() const { return m_isNull; }
     bool isFourState() const;
     bool hasZ() const {
         for (int i = 0; i < words(); i++) {

src/verilog.y

@@ -4274,7 +4274,7 @@ expr<nodep>:			// IEEE: part of expression/constant_expression/primary
 	//			// Indistinguishable from function_subroutine_call:method_call
 	//
 	|	'$'					{ $$ = new AstUnbounded($<fl>1); }
-	|	yNULL					{ $$ = new AstConst($1, AstConst::StringToParse(), "'0"); }
+	|	yNULL					{ $$ = new AstConst($1, AstConst::Null{}); }
 	//			// IEEE: yTHIS
 	//			// See exprScope
 	//