Fix sformat string incorrectly cleared (#3515) (#3519).

docs/CONTRIBUTORS

@@ -33,6 +33,7 @@ Gianfranco Costamagna
 Glen Gibb
 Graham Rushton
 Guokai Chen
+Gustav Svensk
 Harald Heckmann
 Howard Su
 Huang Rui

include/verilated.cpp

@@ -1459,11 +1459,12 @@ void VL_SFORMAT_X(int obits, void* destp, const char* formatp, ...) VL_MT_SAFE {
 
 void VL_SFORMAT_X(int obits_ignored, std::string& output, const char* formatp, ...) VL_MT_SAFE {
     if (obits_ignored) {}
-    output = "";
+    std::string temp_output;
     va_list ap;
     va_start(ap, formatp);
-    _vl_vsformat(output, formatp, ap);
+    _vl_vsformat(temp_output, formatp, ap);
     va_end(ap);
+    output = temp_output;
 }
 
 std::string VL_SFORMATF_NX(const char* formatp, ...) VL_MT_SAFE {

test_regress/t/t_sys_sformat.v

@@ -83,6 +83,12 @@ module t;
       $swriteo(str2, 4'd12);
       if (str2 != "14") $stop;
 
+      str3 = "foo";
+      $sformat(str3, "%s", str3);  // $sformat twice so verilator does not
+      $sformat(str3, "%s", str3);  // optimize the call to $sformat(str3, "%s", "foo")
+`ifdef TEST_VERBOSE  $display("str3=%0s", str3);  `endif
+      if (str3 != "foo") $stop;
+
       $write("*-* All Finished *-*\n");
       $finish;
    end