introduce to_dstring

As std::to_string, to_dstring produces a string from a number.  A key
feature is that dstrings for values <=64 are pre-computed; generating
strings for such numbers is 50x faster than going via std::to_string.

int main()
{
  for(unsigned i=0; i<20000000; i++)
  {
    dstringt d;
    int j=20;
    d=dstringt(std::to_string(j));
  }
}

This is 2.5s as above, and 0.05s with to_dstring.

Author: Daniel Kroening

src/util/dstring.cpp

@@ -17,3 +17,15 @@ std::ostream &dstringt::operator<<(std::ostream &out) const
 {
   return out << as_string();
 }
+
+dstringt get_dstring_number(std::size_t value)
+{
+  static const dstringt *const dstring_numbers = [] {
+    dstringt *array = new dstringt[DSTRING_NUMBERS_MAX + 1];
+    for(std::size_t i = 0; i <= DSTRING_NUMBERS_MAX; i++)
+      array[i] = dstringt(std::to_string(i));
+    return array;
+  }();
+
+  return dstring_numbers[value];
+}

src/util/dstring.h

@@ -209,4 +209,21 @@ struct diagnostics_helpert<dstringt>
   }
 };
 
+#define DSTRING_NUMBERS_MAX 64
+
+dstringt get_dstring_number(std::size_t);
+
+/// equivalent to dstringt(std::to_string(value)), i.e., produces a string
+/// from a number
+template <typename T>
+static inline
+  typename std::enable_if<std::is_integral<T>::value, dstringt>::type
+  to_dstring(T value)
+{
+  if(value >= 0 && value <= DSTRING_NUMBERS_MAX)
+    return get_dstring_number(value);
+  else
+    return std::to_string(value);
+}
+
 #endif // CPROVER_UTIL_DSTRING_H

src/util/irep.cpp

@@ -263,7 +263,7 @@ long long irept::get_long_long(const irep_namet &name) const
 
 void irept::set(const irep_namet &name, const long long value)
 {
-  add(name).id(std::to_string(value));
+  add(name).id(to_dstring(value));
 }
 
 void irept::remove(const irep_namet &name)