For UNUSED, ignore black boxed sys calls

src/V3AstNodes.h

@@ -1679,6 +1679,22 @@ struct AstSFormat : public AstNode {
     void 	lhsp(AstNode* nodep) { setOp3p(nodep); }
 };
 
+struct AstSysIgnore : public AstNode {
+    // Parents: stmtlist
+    // Children: varrefs or exprs
+    AstSysIgnore(FileLine* fileline, AstNode* exprsp)
+	: AstNode (fileline) { setNOp1p(exprsp); }
+    ASTNODE_NODE_FUNCS(SysIgnore, SYSIGNORE)
+    virtual string verilogKwd() const { return "$ignored"; }
+    virtual bool isGateOptimizable() const { return false; }  // Though deleted before opt
+    virtual bool isPredictOptimizable() const { return false; }  // Though deleted before opt
+    virtual bool isSplittable() const { return false; }  // Though deleted before opt
+    virtual bool isOutputter() const { return true; }  // Though deleted before opt
+    virtual int instrCount()	const { return instrCountPli(); }
+    AstNode*	exprsp()	const { return op1p()->castNode(); }	// op1 = Expressions to output
+    void 	exprsp(AstNode* nodep)	{ addOp1p(nodep); }	// op1 = Expressions to output
+};
+
 struct AstFClose : public AstNodeStmt {
     // Parents: stmtlist
     // Children: file which must be a varref

src/V3Const.cpp

@@ -1508,6 +1508,14 @@ private:
     // If output of a presel didn't get consted, chances are V3Param didn't visit properly
     virtual void visit(AstNodePreSel* nodep, AstNUser*) {}
 
+    // Ignored, can eliminate early
+    virtual void visit(AstSysIgnore* nodep, AstNUser*) {
+	nodep->iterateChildren(*this);
+	if (m_doNConst) {
+	    nodep->unlinkFrBack()->deleteTree(); nodep=NULL;
+	}
+    }
+
     //-----
     // Jump elimination
 

src/V3EmitV.cpp

@@ -276,6 +276,12 @@ class EmitVBaseVisitor : public EmitCBaseVisitor {
 	if (nodep->msbp()) { putbs(","); nodep->msbp()->iterateChildren(*this); }
 	puts(");\n");
     }
+    virtual void visit(AstSysIgnore* nodep, AstNUser*) {
+	putfs(nodep,nodep->verilogKwd());
+	putbs(" (");
+	nodep->exprsp()->iterateChildren(*this);
+	puts(");\n");
+    }
     virtual void visit(AstNodeFor* nodep, AstNUser*) {
 	putfs(nodep,"for (");
 	m_suppressSemi = true;

src/V3LinkLValue.cpp

@@ -154,6 +154,12 @@ private:
 	}
 	m_setRefLvalue = last_setRefLvalue;
     }
+    virtual void visit(AstSysIgnore* nodep, AstNUser*) {
+	// Can't know if lvalue or not; presume so as stricter
+	bool last_setRefLvalue = m_setRefLvalue;
+	nodep->iterateChildren(*this);
+	m_setRefLvalue = last_setRefLvalue;
+    }
     virtual void visit(AstReadMem* nodep, AstNUser*) {
 	bool last_setRefLvalue = m_setRefLvalue;
 	{

src/V3Undriven.cpp

@@ -197,6 +197,7 @@ private:
 
     // STATE
     vector<UndrivenVarEntry*>	m_entryps;	// Nodes to delete when we are finished
+    bool		m_markBoth;	// Mark as driven+used
 
     // METHODS
     static int debug() {
@@ -242,8 +243,8 @@ private:
 	if (varrefp && constp && !constp->num().isFourState()) {
 	    UndrivenVarEntry* entryp = getEntryp (varrefp->varp());
 	    int lsb = constp->toUInt();
-	    if (varrefp->lvalue()) entryp->drivenBit(lsb, nodep->width());
-	    else entryp->usedBit(lsb, nodep->width());
+	    if (m_markBoth || varrefp->lvalue()) entryp->drivenBit(lsb, nodep->width());
+	    if (m_markBoth || !varrefp->lvalue()) entryp->usedBit(lsb, nodep->width());
 	} else {
 	    // else other varrefs handled as unknown mess in AstVarRef
 	    nodep->iterateChildren(*this);
@@ -252,8 +253,16 @@ private:
     virtual void visit(AstVarRef* nodep, AstNUser*) {
 	// Any variable
 	UndrivenVarEntry* entryp = getEntryp (nodep->varp());
-	if (nodep->lvalue()) entryp->drivenWhole();
-	else entryp->usedWhole();
+	if (m_markBoth || nodep->lvalue()) entryp->drivenWhole();
+	if (m_markBoth || !nodep->lvalue()) entryp->usedWhole();
+    }
+
+    // Don't know what black boxed calls do, assume in+out
+    virtual void visit(AstSysIgnore* nodep, AstNUser*) {
+	bool prevMark = m_markBoth;
+	m_markBoth = true;
+	nodep->iterateChildren(*this);
+	m_markBoth = prevMark;
     }
 
     // Until we support tables, primitives will have undriven and unused I/Os
@@ -274,6 +283,7 @@ private:
 public:
     // CONSTUCTORS
     UndrivenVisitor(AstNetlist* nodep) {
+	m_markBoth = false;
 	AstNode::user1ClearTree();	// user1p() used on entire tree
 	nodep->accept(*this);
     }

src/V3Width.cpp

@@ -809,6 +809,9 @@ private:
 	    nodep->width(32,32);
 	}
     }
+    virtual void visit(AstSysIgnore* nodep, AstNUser* vup) {
+	nodep->exprsp()->iterateAndNext(*this,WidthVP(ANYSIZE,0,BOTH).p());
+    }
     virtual void visit(AstReadMem* nodep, AstNUser*) {
 	nodep->filenamep()->iterateAndNext(*this,WidthVP(ANYSIZE,0,BOTH).p());
 	nodep->memp()->iterateAndNext(*this,WidthVP(ANYSIZE,0,BOTH).p());

src/verilog.y

@@ -2063,8 +2063,8 @@ function_subroutine_callNoMethod<nodep>:	// IEEE: function_subroutine_call (as f
 
 system_t_call<nodep>:		// IEEE: system_tf_call (as task)
 	//
-		yaD_IGNORE  parenE			{ $$ = NULL; }
-	|	yaD_IGNORE  '(' exprList ')'		{ $$ = NULL; }
+		yaD_IGNORE  parenE			{ $$ = new AstSysIgnore($<fl>1,NULL); }
+	|	yaD_IGNORE  '(' exprList ')'		{ $$ = new AstSysIgnore($<fl>1,$3); }
 	//
 	|	yaD_DPI parenE				{ $$ = new AstTaskRef($<fl>1,*$1,NULL); }
 	|	yaD_DPI '(' exprList ')'		{ $$ = new AstTaskRef($2,*$1,$3); }

test_regress/t/t_lint_unused_bad.pl

@@ -8,7 +8,7 @@ if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); di
 # Version 2.0.
 
 compile (
-    v_flags2 => ["--lint-only -Wall -Wno-DECLFILENAME"],
+    v_flags2 => ["--lint-only --bbox-sys -Wall -Wno-DECLFILENAME"],
     fails=>1,
     verilator_make_gcc => 0,
     make_top_shell => 0,

test_regress/t/t_lint_unused_bad.v

@@ -18,6 +18,10 @@ module t (/*AUTOARG*/
    // Check we don't warn about unused UDP signals
    udp_mux2 udpsub (out, in, in, in);
 
+   // Check ignoreds mark as used
+   reg 	  sysused;
+   initial $bboxed(sysused);
+
 endmodule
 
 module sub;