merge_irept: do not store fresh irept::dt unless necessary

This optimisation ensures that operator== can use pointer equality when
submitting an already-shared irept to merge_irept. On SV-COMP/ReachSafety-ECA
this reduces the number of recursive operator== invocations from 5,814,764,546
(generated by 4,241,423,614 invocations) to 4,673,896,121 (generated from
4,295,188,693 invocations). That is, despite ~50M additional invocations
(because the performance improvement permits more symbolic execution runs)
around 1.1B recursive calls are avoided.

Author: tautschnig

src/util/merge_irep.cpp

@@ -145,18 +145,26 @@ void merge_irept::operator()(irept &irep)
 
 const irept &merge_irept::merged(const irept &irep)
 {
+  if(irep.get_sub().empty() && irep.get_named_sub().empty())
+    return *irep_store.insert(irep).first;
+
   irep_storet::const_iterator entry=irep_store.find(irep);
   if(entry!=irep_store.end())
     return *entry;
 
+  bool same_irep = true;
   irept new_irep(irep.id());
 
   const irept::subt &src_sub=irep.get_sub();
   irept::subt &dest_sub=new_irep.get_sub();
   dest_sub.reserve(src_sub.size());
 
   forall_irep(it, src_sub)
-    dest_sub.push_back(merged(*it)); // recursive call
+  {
+    const irept &op = merged(*it); // recursive call
+    same_irep &= &op.read() == &(it->read());
+    dest_sub.push_back(op);
+  }
 
   const irept::named_subt &src_named_sub=irep.get_named_sub();
   irept::named_subt &dest_named_sub=new_irep.get_named_sub();
@@ -166,16 +174,20 @@ const irept &merge_irept::merged(const irept &irep)
 #endif
   forall_named_irep(it, src_named_sub)
   {
+    const irept &op = merged(it->second); // recursive call
+    same_irep &= &op.read() == &(it->second.read());
 #ifdef NAMED_SUB_IS_FORWARD_LIST
-    dest_named_sub.emplace_after(
-      before, it->first, merged(it->second)); // recursive call
+    dest_named_sub.emplace_after(before, it->first, op);
     ++before;
 #else
-    dest_named_sub[it->first]=merged(it->second); // recursive call
+    dest_named_sub.emplace(it->first, op);
 #endif
   }
 
-  return *irep_store.insert(std::move(new_irep)).first;
+  if(same_irep)
+    return *irep_store.insert(irep).first;
+  else
+    return *irep_store.insert(std::move(new_irep)).first;
 }
 
 void merge_full_irept::operator()(irept &irep)