[dv] Fix performance counter printing in simple system

This will avoid printing out a load of spurious zeros if the Ibex
config doesn't enable the corresponding counter.

dv/verilator/pcount/cpp/ibex_pcounts.cc

@@ -10,6 +10,7 @@
 #include <svdpi.h>
 
 extern "C" {
+extern unsigned int mhpmcounter_num();
 extern unsigned long long mhpmcounter_get(int index);
 }
 
@@ -32,14 +33,31 @@ const std::vector<std::string> ibex_counter_names = {
     "Multiply Wait",
     "Divide Wait"};
 
+static bool has_hpm_counter(int index)
+{
+  // The "cycles" and "instructions retired" counters are special and always exist.
+  if (index == 0 || index == 2) return true;
+
+  // The "NONE" counter is fake. That never exists.
+  if (index == 1) return false;
+
+  // Otherwise, a counter exists if the index is strictly less than
+  // the MHPMCounterNum parameter that got passed to the
+  // ibex_cs_registers module.
+  return index < mhpmcounter_num();
+}
+
 std::string ibex_pcount_string(bool csv) {
   char separator = csv ? ',' : ':';
   std::string::size_type longest_name_length;
 
   if (!csv) {
     longest_name_length = 0;
-    for (const std::string &counter_name : ibex_counter_names) {
-      longest_name_length = std::max(longest_name_length, counter_name.length());
+    for (int i = 0; i < ibex_counter_names.size(); ++i) {
+      if (has_hpm_counter(i)) {
+        longest_name_length = std::max(longest_name_length,
+                                       ibex_counter_names[i].length());
+      }
     }
 
     // Add 1 to always get at least once space after the separator
@@ -49,6 +67,8 @@ std::string ibex_pcount_string(bool csv) {
   std::stringstream pcount_ss;
 
   for (int i = 0; i < ibex_counter_names.size(); ++i) {
+    if (!has_hpm_counter(i)) continue;
+
     pcount_ss << ibex_counter_names[i] << separator;
 
     if (!csv) {

examples/simple_system/rtl/ibex_simple_system.sv

@@ -318,6 +318,12 @@ module ibex_simple_system (
       .timer_intr_o   (timer_irq)
     );
 
+  export "DPI-C" function mhpmcounter_num;
+
+  function automatic int unsigned mhpmcounter_num();
+    return u_top.u_ibex_top.u_ibex_core.cs_registers_i.MHPMCounterNum;
+  endfunction
+
   export "DPI-C" function mhpmcounter_get;
 
   function automatic longint unsigned mhpmcounter_get(int index);