Support $ungetc.

Changes

@@ -4,7 +4,7 @@ The contributors that suggested a given feature are shown in []. Thanks!
 
 * Verilator 4.023 devel
 
-****  Support $rewind.
+****  Support $rewind and $ungetc.
 
 ****  Add -Wpedantic for compliance testing.
 

bin/verilator

@@ -3562,8 +3562,7 @@ MCDs, which includes the mode parameter to $fopen being mandatory.
 
 =item $fscanf, $sscanf
 
-Only integer formats are supported; %e, %f, %m, %r, %v, and %z are not
-supported.
+The formats %r, %v, and %z are not supported.
 
 =item $fullskew, $hold, $nochange, $period, $recovery, $recrem, $removal,
 $setup, $setuphold, $skew, $timeskew, $width

src/V3AstNodes.h

@@ -4335,6 +4335,28 @@ public:
     AstNode* filep() const { return lhsp(); }
 };
 
+class AstFUngetC : public AstNodeBiop {
+public:
+    AstFUngetC(FileLine* fl, AstNode* lhsp, AstNode* rhsp)
+        : AstNodeBiop(fl, lhsp, rhsp) {}
+    ASTNODE_NODE_FUNCS(FUngetC)
+    virtual void numberOperate(V3Number& out, const V3Number& lhs, const V3Number& rhs) { V3ERROR_NA; }
+    virtual AstNode* cloneType(AstNode* lhsp, AstNode* rhsp) {
+        return new AstFUngetC(this->fileline(), lhsp, rhsp); }
+    virtual string emitVerilog() { return "%f$ungetc(%r, %l)"; }
+    // Non-existent filehandle returns EOF
+    virtual string emitC() { return "(%li ? (ungetc(%ri, VL_CVT_I_FP(%li)) >= 0 ? 0 : -1) : -1)"; }
+    virtual bool cleanOut() const { return false; }
+    virtual bool cleanLhs() const { return true; }
+    virtual bool cleanRhs() const { return true; }
+    virtual bool sizeMattersLhs() const { return false; }
+    virtual bool sizeMattersRhs() const { return false; }
+    virtual int instrCount() const { return widthInstrs() * 64; }
+    virtual bool isPure() const { return false; }  // SPECIAL: $display has 'visual' ordering
+    AstNode* filep() const { return lhsp(); }
+    AstNode* charp() const { return rhsp(); }
+};
+
 class AstNodeSystemUniop : public AstNodeUniop {
 public:
     AstNodeSystemUniop(FileLine* fl, AstNode* lhsp) : AstNodeUniop(fl, lhsp) {

src/V3LinkLValue.cpp

@@ -149,6 +149,14 @@ private:
         }
         m_setRefLvalue = last_setRefLvalue;
     }
+    virtual void visit(AstFUngetC* nodep) {
+        bool last_setRefLvalue = m_setRefLvalue;
+        {
+            m_setRefLvalue = true;
+            iterateAndNextNull(nodep->filep());
+        }
+        m_setRefLvalue = last_setRefLvalue;
+    }
     virtual void visit(AstSScanF* nodep) {
         bool last_setRefLvalue = m_setRefLvalue;
         {

src/V3Width.cpp

@@ -2380,6 +2380,13 @@ private:
             userIterateAndNext(nodep->strgp(), WidthVP(SELF, BOTH).p());
         }
     }
+    virtual void visit(AstFUngetC* nodep) {
+        if (m_vup->prelim()) {
+            iterateCheckFileDesc(nodep, nodep->filep(), BOTH);
+            iterateCheckSigned32(nodep, "$fungetc character", nodep->charp(), BOTH);
+            nodep->dtypeSetLogicUnsized(32, 8, AstNumeric::SIGNED);  // Spec says integer return
+        }
+    }
     virtual void visit(AstFRead* nodep) {
         if (m_vup->prelim()) {
             nodep->dtypeSetSigned32();  // Spec says integer return

src/verilog.l

@@ -220,6 +220,7 @@ vnum    {vnum1}|{vnum2}|{vnum3}|{vnum4}|{vnum5}
   "$test$plusargs"      { FL; return yD_TESTPLUSARGS; }
   "$time"               { FL; return yD_TIME; }
   "$timeskew"           { FL; return yaTIMINGSPEC; }
+  "$ungetc"             { FL; return yD_UNGETC; }
   "$value$plusargs"     { FL; return yD_VALUEPLUSARGS; }
   "$width"              { FL; return yaTIMINGSPEC; }
   "$write"              { FL; return yD_WRITE; }

src/verilog.y

@@ -557,6 +557,7 @@ class AstSenTree;
 %token<fl>		yD_TANH		"$tanh"
 %token<fl>		yD_TESTPLUSARGS	"$test$plusargs"
 %token<fl>		yD_TIME		"$time"
+%token<fl>		yD_UNGETC	"$ungetc"
 %token<fl>		yD_UNIT		"$unit"
 %token<fl>		yD_UNPACKED_DIMENSIONS "$unpacked_dimensions"
 %token<fl>		yD_UNSIGNED	"$unsigned"
@@ -2970,6 +2971,7 @@ system_f_call_or_t<nodep>:	// IEEE: part of system_tf_call (can be task or func)
 	|	yD_TANH '(' expr ')'			{ $$ = new AstTanhD($1,$3); }
 	|	yD_TESTPLUSARGS '(' str ')'		{ $$ = new AstTestPlusArgs($1,*$3); }
 	|	yD_TIME	parenE				{ $$ = new AstTime($1); }
+	|	yD_UNGETC '(' expr ',' expr ')'		{ $$ = new AstFUngetC($1, $5, $3); }  // Arg swap to file first
 	|	yD_UNPACKED_DIMENSIONS '(' exprOrDataType ')'	{ $$ = new AstAttrOf($1,AstAttrType::DIM_UNPK_DIMENSIONS,$3); }
 	|	yD_UNSIGNED '(' expr ')'		{ $$ = new AstUnsigned($1,$3); }
 	|	yD_VALUEPLUSARGS '(' expr ',' expr ')'	{ $$ = new AstValuePlusArgs($1,$3,$5); }

test_regress/t/t_sys_file_basic.v

@@ -90,6 +90,10 @@ module t;
 	 if ($fgetc(file) != "i") $stop;
 	 if ($fgetc(file) != "\n") $stop;
 
+	 // $ungetc
+	 if ($ungetc("x", file) != 0) $stop;
+	 if ($fgetc(file) != "x") $stop;
+
 	 // $fgets
 	 chars = $fgets(letterl, file);
 	 if (`verbose) $write("c=%0d l=%s\n", chars, letterl);