Speed up disasm_insn_t::to_string

Stringstream is very slow; it's faster to manipulate strings instead.

Also, handle the underscore-dot replacement as a preprocessing step,
rather than doing it redundantly every time.

riscv/disasm.h

@@ -6,6 +6,7 @@
 #include "decode.h"
 #include <string>
 #include <sstream>
+#include <algorithm>
 #include <vector>
 
 extern const char* xpr_name[NXPR];
@@ -23,10 +24,13 @@ class arg_t
 class disasm_insn_t
 {
  public:
-  NOINLINE disasm_insn_t(const char* name, uint32_t match, uint32_t mask,
+  NOINLINE disasm_insn_t(const char* name_, uint32_t match, uint32_t mask,
                          const std::vector<const arg_t*>& args)
-    : match(match), mask(mask), args(args), name(strdup(name)) {}
-  ~disasm_insn_t() { free(const_cast<char *>(name)); }
+    : match(match), mask(mask), args(args)
+  {
+    name = name_;
+    std::replace(name.begin(), name.end(), '_', '.');
+  }
 
   bool operator == (insn_t insn) const
   {
@@ -35,20 +39,17 @@ class disasm_insn_t
 
   const char* get_name() const
   {
-    return name;
+    return name.c_str();
   }
 
   std::string to_string(insn_t insn) const
   {
-    std::stringstream s;
-    int len;
-    for (len = 0; name[len]; len++)
-      s << (name[len] == '_' ? '.' : name[len]);
+    std::string s(name);
 
     if (args.size())
     {
       bool next_arg_optional  = false;
-      s << std::string(std::max(1, 8 - len), ' ');
+      s += std::string(std::max(1, 8 - int(name.size())), ' ');
       for (size_t i = 0; i < args.size(); i++) {
         if (args[i] == nullptr) {
           next_arg_optional = true;
@@ -59,11 +60,11 @@ class disasm_insn_t
           next_arg_optional = false;
           if (argString.empty()) continue;
         }
-        if (i != 0) s << ", ";
-        s << argString;
+        if (i != 0) s += ", ";
+        s += argString;
       }
     }
-    return s.str();
+    return s;
   }
 
   uint32_t get_match() const { return match; }
@@ -73,7 +74,7 @@ class disasm_insn_t
   uint32_t match;
   uint32_t mask;
   std::vector<const arg_t*> args;
-  const char* name;
+  std::string name;
 };
 
 class disassembler_t