Partially revert "Use small intrusive pointer in irep"

This partially reverts commit b741d4b.

While safer to use, these changes prevented sharing on irept's where non-const
references had been taken to any irept stored in their sub/named_sub
collections. The newly introduced cow.h and small_shared_ptr.h are kept in
place, but all modifications to irept are reverted.

Author: tautschnig

src/util/fresh_symbol.cpp

@@ -11,6 +11,7 @@ Author: Chris Smowton, [email protected]
 
 #include "fresh_symbol.h"
 
+#include "invariant.h"
 #include "namespace.h"
 #include "rename.h"
 #include "symbol.h"

src/util/irep.cpp

@@ -26,6 +26,12 @@ Author: Daniel Kroening, [email protected]
 #include <iostream>
 #endif
 
+irept nil_rep_storage;
+
+#ifdef SHARING
+irept::dt irept::empty_d;
+#endif
+
 #ifdef SUB_IS_LIST
 static inline bool named_subt_order(
   const std::pair<irep_namet, irept> &a,
@@ -49,20 +55,157 @@ static inline irept::named_subt::iterator named_subt_lower_bound(
 
 const irept &get_nil_irep()
 {
-  static irept nil_rep_storage;
   if(nil_rep_storage.id().empty()) // initialized?
     nil_rep_storage.id(ID_nil);
   return nil_rep_storage;
 }
 
+#ifdef SHARING
+void irept::detach()
+{
+  #ifdef IREP_DEBUG
+  std::cout << "DETACH1: " << data << '\n';
+  #endif
+
+  if(data==&empty_d)
+  {
+    data=new dt;
+
+    #ifdef IREP_DEBUG
+    std::cout << "ALLOCATED " << data << '\n';
+    #endif
+  }
+  else if(data->ref_count>1)
+  {
+    dt *old_data(data);
+    data=new dt(*old_data);
+
+    #ifdef IREP_DEBUG
+    std::cout << "ALLOCATED " << data << '\n';
+    #endif
+
+    data->ref_count=1;
+    remove_ref(old_data);
+  }
+
+  POSTCONDITION(data->ref_count==1);
+
+  #ifdef IREP_DEBUG
+  std::cout << "DETACH2: " << data << '\n';
+  #endif
+}
+#endif
+
+#ifdef SHARING
+void irept::remove_ref(dt *old_data)
+{
+  if(old_data==&empty_d)
+    return;
+
+  #if 0
+  nonrecursive_destructor(old_data);
+  #else
+
+  PRECONDITION(old_data->ref_count!=0);
+
+  #ifdef IREP_DEBUG
+  std::cout << "R: " << old_data << " " << old_data->ref_count << '\n';
+  #endif
+
+  old_data->ref_count--;
+  if(old_data->ref_count==0)
+  {
+    #ifdef IREP_DEBUG
+    std::cout << "D: " << pretty() << '\n';
+    std::cout << "DELETING " << old_data->data
+              << " " << old_data << '\n';
+    old_data->clear();
+    std::cout << "DEALLOCATING " << old_data << "\n";
+    #endif
+
+    // may cause recursive call
+    delete old_data;
+
+    #ifdef IREP_DEBUG
+    std::cout << "DONE\n";
+    #endif
+  }
+  #endif
+}
+#endif
+
+/// Does the same as remove_ref, but using an explicit stack instead of
+/// recursion.
+#ifdef SHARING
+void irept::nonrecursive_destructor(dt *old_data)
+{
+  std::vector<dt *> stack(1, old_data);
+
+  while(!stack.empty())
+  {
+    dt *d=stack.back();
+    stack.erase(--stack.end());
+    if(d==&empty_d)
+      continue;
+
+    INVARIANT(d->ref_count!=0, "All contents of the stack must be in use");
+    d->ref_count--;
+
+    if(d->ref_count==0)
+    {
+      stack.reserve(stack.size()+
+                    d->named_sub.size()+
+                    d->comments.size()+
+                    d->sub.size());
+
+      for(named_subt::iterator
+          it=d->named_sub.begin();
+          it!=d->named_sub.end();
+          it++)
+      {
+        stack.push_back(it->second.data);
+        it->second.data=&empty_d;
+      }
+
+      for(named_subt::iterator
+          it=d->comments.begin();
+          it!=d->comments.end();
+          it++)
+      {
+        stack.push_back(it->second.data);
+        it->second.data=&empty_d;
+      }
+
+      for(subt::iterator
+          it=d->sub.begin();
+          it!=d->sub.end();
+          it++)
+      {
+        stack.push_back(it->data);
+        it->data=&empty_d;
+      }
+
+      // now delete, won't do recursion
+      delete d;
+    }
+  }
+}
+#endif
+
 void irept::move_to_named_sub(const irep_namet &name, irept &irep)
 {
+  #ifdef SHARING
+  detach();
+  #endif
   add(name).swap(irep);
   irep.clear();
 }
 
 void irept::move_to_sub(irept &irep)
 {
+  #ifdef SHARING
+  detach();
+  #endif
   get_sub().push_back(get_nil_irep());
   get_sub().back().swap(irep);
 }