Fix $stacktrace to decode through internal-c++filt (#6985).

include/verilated.cpp

@@ -69,6 +69,7 @@
 # include <direct.h>  // mkdir
 #endif
 #ifdef __GLIBC__
+# include <cxxabi.h>
 # include <execinfo.h>
 # define _VL_HAVE_STACKTRACE
 #endif
@@ -1918,6 +1919,40 @@ IData VL_FREAD_I(int width, int array_lsb, int array_size, void* memp, IData fpi
     return read_count;
 }
 
+#ifdef _VL_HAVE_STACKTRACE
+static std::string _vl_stacktrace_demangle(const std::string& input) {
+    std::string result;
+    result.reserve(input.size());
+
+    std::string word;
+    for (const char c : input) {
+        if (std::isalpha(c) || c == '_') {
+            word += c;
+        } else if (!word.empty() && std::isdigit(c)) {
+            word += c;
+        } else {
+            if (!word.empty()) {
+                // abi::__cxa_demangle mallocs demangled_name
+                int status = 0;
+                char* const demangled_name
+                    = abi::__cxa_demangle(word.c_str(), NULL, NULL, &status);
+                if (status == 0) {
+                    result += std::string{demangled_name};
+                    std::free(demangled_name);  // Free the allocated memory
+                } else {
+                    result += word;
+                }
+                word.clear();
+            }
+            result += c;
+        }
+    }
+    // input requires final newline, so last word can't be symbol
+    result += word;
+    return result;
+}
+#endif
+
 std::string VL_STACKTRACE_N() VL_MT_SAFE {
     static VerilatedMutex s_stackTraceMutex;
     const VerilatedLockGuard lock{s_stackTraceMutex};
@@ -1935,8 +1970,12 @@ std::string VL_STACKTRACE_N() VL_MT_SAFE {
     // cppcheck-suppress knownConditionTrueFalse
     if (!strings) return "Unable to backtrace\n";
 
+#ifdef _VL_HAVE_STACKTRACE
     std::string result = "Backtrace:\n";
-    for (int j = 0; j < nptrs; ++j) result += std::string{strings[j]} + "\n"s;
+    for (int j = 0; j < nptrs; ++j)
+        result += _vl_stacktrace_demangle(std::string{strings[j]} + "\n"s);
+#endif
+
     free(strings);
     return result;
 }