Set FST timescale correctly. (#2266)

The FST trace timescale used to be set in the constructor via
set_time_unit, but at that point we haven't normally opened the
file yet so it was just dropped. On top of that, we actually want
to use set_time_resolution... FST trace timescales now match the VCD.

include/verilated_fst_c.cpp

@@ -100,6 +100,7 @@ void VerilatedFst::open(const char* filename) VL_MT_UNSAFE {
     m_assertOne.check();
     m_fst = fstWriterCreate(filename, 1);
     fstWriterSetPackType(m_fst, FST_WR_PT_LZ4);
+    fstWriterSetTimescaleFromString(m_fst, m_timeRes.c_str());
 #ifdef VL_TRACE_THREADED
     fstWriterSetParallelMode(m_fst, 1);
 #endif

include/verilated_fst_c.h

@@ -65,6 +65,8 @@ private:
     // helpers
     std::vector<char> m_valueStrBuffer;
 
+    std::string m_timeRes;
+
 public:
     explicit VerilatedFst(void* fst = NULL);
     ~VerilatedFst();
@@ -77,11 +79,11 @@ public:
         fstWriterClose(m_fst);
         m_fst = NULL;
     }
-    void set_time_unit(const char* unitp) { fstWriterSetTimescaleFromString(m_fst, unitp); }
+    void set_time_unit(const char*) {}
     void set_time_unit(const std::string& unit) { set_time_unit(unit.c_str()); }
 
-    void set_time_resolution(const char*) {}
-    void set_time_resolution(const std::string& unit) { set_time_resolution(unit.c_str()); }
+    void set_time_resolution(const char* unitp) { m_timeRes = unitp; }
+    void set_time_resolution(const std::string& unit) { m_timeRes = unit; }
 
     // double timescaleToDouble(const char* unitp);
     // std::string doubleToTimescale(double value);

test_regress/t/t_interface_ref_trace_fst.out

@@ -1,12 +1,12 @@
 $date
-	Wed Dec  4 07:48:29 2019
+	Sun Apr 19 04:13:22 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_array_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Oct 21 21:57:08 2018
+	Sun Apr 19 04:15:23 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_complex_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Apr 12 20:15:55 2020
+	Sun Apr 19 04:15:26 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_complex_params_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Apr 12 20:17:15 2020
+	Sun Apr 19 04:15:31 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_complex_structs_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Apr 12 20:14:19 2020
+	Sun Apr 19 04:15:33 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Apr 12 20:17:58 2020
+	Sun Apr 19 04:15:36 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_packed_struct_fst.out

@@ -1,12 +1,12 @@
 $date
-	Sun Apr 12 20:19:35 2020
+	Sun Apr 19 04:15:38 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module top $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_two_dumpfst_cc.out

@@ -1,12 +1,12 @@
 $date
-	Sat Apr 18 22:18:21 2020
+	Sun Apr 19 04:15:48 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module topa $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_two_hdrfst_cc.out

@@ -1,12 +1,12 @@
 $date
-	Sat Mar  7 18:27:58 2020
+	Sun Apr 19 04:15:51 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module topa $end
 $var wire 1 ! clk $end

test_regress/t/t_trace_two_portfst_cc.out

@@ -1,12 +1,12 @@
 $date
-	Sat Apr 18 22:19:21 2020
+	Sun Apr 19 04:13:00 2020
 
 $end
 $version
 	fstWriter
 $end
 $timescale
-	1ns
+	1ps
 $end
 $scope module topa $end
 $var wire 1 ! clk $end