path: root/src/context/context.h

/*********************                                                        */
/*! \file context.h
 ** \verbatim
 ** Top contributors (to current version):
 **   Morgan Deters, Clark Barrett, Tim King
 ** This file is part of the CVC4 project.
 ** Copyright (c) 2009-2017 by the authors listed in the file AUTHORS
 ** in the top-level source directory) and their institutional affiliations.
 ** All rights reserved.  See the file COPYING in the top-level source
 ** directory for licensing information.\endverbatim
 **
 ** \brief Context class and context manager.
 **
 ** Context class and context manager.
 **/

#include "cvc4_private.h"

#ifndef __CVC4__CONTEXT__CONTEXT_H
#define __CVC4__CONTEXT__CONTEXT_H

#include <cstdlib>
#include <cstring>
#include <iostream>
#include <memory>
#include <new>
#include <typeinfo>
#include <vector>

#include "base/cvc4_assert.h"
#include "base/output.h"
#include "context/context_mm.h"


namespace CVC4 {
namespace context {

class Context;
class Scope;
class ContextObj;
class ContextNotifyObj;

/** Pretty-printing of Contexts (for debugging) */
std::ostream& operator<<(std::ostream&, const Context&);

/** Pretty-printing of Scopes (for debugging) */
std::ostream& operator<<(std::ostream&, const Scope&);

/**
 * A Context encapsulates all of the dynamic state of the system.  Its main
 * methods are push() and pop().  A call to push() saves the current state,
 * and a call to pop() restores the state saved by the most recent call to
 * push().
 *
 * Objects which want to participate in this global save and restore
 * mechanism must inherit from ContextObj (see below).
 *
 * For more flexible context-dependent behavior, objects may implement the
 * ContextNotifyObj interface and simply get a notification when a pop has
 * occurred.
 *
 * Context also uses a helper class called Scope which stores information
 * specific to the portion of the Context since the last call to push() (see
 * below).
 *
 * Memory allocation in Contexts is done with the help of the
 * ContextMemoryManager.  A copy is stored in each Scope object for quick
 * access.
 */
class Context {

  /**
   * Pointer to the ContextMemoryManager for this Context.
   */
  ContextMemoryManager* d_pCMM;

  /**
   * List of all scopes for this context.
   */
  std::vector<Scope*> d_scopeList;

  /**
   * Doubly-linked list of objects to notify before every pop.  See
   * ContextNotifyObj for structure of linked list.
   */
  ContextNotifyObj* d_pCNOpre;

  /**
   * Doubly-linked list of objects to notify after every pop.  See
   * ContextNotifyObj for structure of linked list.
   */
  ContextNotifyObj* d_pCNOpost;

  friend std::ostream& operator<<(std::ostream&, const Context&);

  // disable copy, assignment
  Context(const Context&) CVC4_UNDEFINED;
  Context& operator=(const Context&) CVC4_UNDEFINED;

public:

  /**
   * A mechanism by which a "scoped" bit of contextual speculation can
   * be applied.  One might create a Context::ScopedPush in a function
   * (as a local variable on the stack), then manipulate some
   * context-dependent data structures in some fashion, speculatively.
   * When the ScopedPush goes out of scope and is destructed, the
   * context-dependent data structures will return to their original
   * state.
   *
   * When such speculation occurs in a lexically-scoped manner, like
   * described above, it is FAR preferable to use ScopedPush than to
   * call ->push() and ->pop() on the Context directly.  If you do the
   * latter, it's extremely easy to forget to pop() on exceptional
   * exit of the function, or if a short-circuited "early" return is
   * later added to the function, etc.  Further, ScopedPush includes
   * an assertion that the Context at the end looks like the Context
   * at the beginning (the topmost Scope pointer should be the same).
   * This assertion is only an approximate check for correct behavior,
   * but should catch egregious mismatches of ->push() and ->pop()
   * while the ScopedPush is being applied---egregious mismatches that
   * could exist, for example, if a Theory does some speculative
   * reasoning but accidently gives control back to some other mechanism
   * which does some speculation which isn't properly scoped inside the
   * first.
   */
  class ScopedPush {
    Context* const d_context;
    const Scope* const d_scope;
  public:
    ScopedPush(Context* ctxt) :
      d_context(ctxt),
      d_scope(d_context->getTopScope()) {
      d_context->push();
    }
    ~ScopedPush() noexcept(false) {
      d_context->pop();
      AlwaysAssert(d_context->getTopScope() == d_scope,
                   "Context::ScopedPush observed an uneven Context (at pop, "
                   "top scope doesn't match what it was at the time the "
                   "ScopedPush was applied)");
    }
  };/* Context::ScopedPush */

  /**
   * Constructor: create ContextMemoryManager and initial Scope
   */
  Context();

  /**
   * Destructor: pop all scopes, delete ContextMemoryManager
   */
  ~Context();

  /**
   * Return the current (top) scope
   */
  Scope* getTopScope() const { return d_scopeList.back(); }

  /**
   * Return the initial (bottom) scope
   */
  Scope* getBottomScope() const { return d_scopeList[0]; }

  /**
   * Return the current Scope level.
   */
  int getLevel() const { return d_scopeList.size() - 1; }

  /**
   * Return the ContextMemoryManager associated with the context.
   */
  ContextMemoryManager* getCMM() { return d_pCMM; }

  /**
   * Save the current state, create a new Scope
   */
  void push();

  /**
   * Restore the previous state, delete the top Scope
   */
  void pop();

  /**
   * Pop all the way back to given level
   */
  void popto(int toLevel);

  /**
   * Add pCNO to the list of objects notified before every pop
   */
  void addNotifyObjPre(ContextNotifyObj* pCNO);

  /**
   * Add pCNO to the list of objects notified after every pop
   */
  void addNotifyObjPost(ContextNotifyObj* pCNO);

};/* class Context */


/**
 * A UserContext is different from a Context only because it's used for
 * different purposes---so separating the two types gives type errors where
 * appropriate.
 */
class UserContext : public Context {
private:
  // disable copy, assignment
  UserContext(const UserContext&) CVC4_UNDEFINED;
  UserContext& operator=(const UserContext&) CVC4_UNDEFINED;
public:
  UserContext() {}
};/* class UserContext */


/**
 * Conceptually, a Scope encapsulates that portion of the context that
 * changes after a call to push() and must be undone on a subsequent call to
 * pop().  In particular, each call to push() creates a new Scope object .
 * This new Scope object becomes the top scope and it points (via the
 * d_pScopePrev member) to the previous top Scope.  Each call to pop()
 * deletes the current top scope and restores the previous one.  The main
 * purpose of a Scope is to maintain a linked list of ContexObj objects which
 * change while the Scope is the top scope and which must be restored when
 * the Scope is deleted.
 *
 * A Scope is also associated with a ContextMemoryManager.  All memory
 * allocated by the Scope is allocated in a single region using the
 * ContextMemoryManager and released all at once when the Scope is popped.
 */
class Scope {

  /**
   * Context that created this Scope
   */
  Context* d_pContext;

  /**
   * Memory manager for this Scope.  Same as in Context, but stored here too
   * for faster access by ContextObj objects.
   */
  ContextMemoryManager* d_pCMM;

  /**
   * Scope level (total number of outstanding push() calls when this Scope was
   * created).
   */
  int d_level;

  /**
   * Linked list of objects which changed in this scope,
   * and thus need to be restored when the scope is deleted.
   */
  ContextObj* d_pContextObjList;

  /**
   * A list of ContextObj to be garbage collected before the destruction of this
   * scope. deleteSelf() will be called on each element during ~Scope().
   *
   * This is either nullptr or list owned by this scope.
   */
  std::unique_ptr<std::vector<ContextObj*>> d_garbage;

  friend std::ostream& operator<<(std::ostream&, const Scope&);

 public:
  /**
   * Constructor: Create a new Scope; set the level and the previous Scope
   * if any.
   */
  Scope(Context* pContext, ContextMemoryManager* pCMM, int level)
      : d_pContext(pContext),
        d_pCMM(pCMM),
        d_level(level),
        d_pContextObjList(nullptr),
        d_garbage()
  {
  }

  /**
   * Destructor: Clears out all of the garbage and restore all of the objects
   * in ContextObjList.
   */
  ~Scope();

  /**
   * Get the Context for this Scope
   */
  Context* getContext() const { return d_pContext; }

  /**
   * Get the ContextMemoryManager for this Scope
   */
  ContextMemoryManager* getCMM() const { return d_pCMM; }

  /**
   * Get the level of the current Scope
   */
  int getLevel() const { return d_level; }

  /**
   * Return true iff this Scope is the current top Scope
   */
  bool isCurrent() const { return this == d_pContext->getTopScope(); }

  /**
   * When a ContextObj object is modified for the first time in this
   * Scope, it should call this method to add itself to the list of
   * objects that will need to be restored.  Defined inline below.
   */
  void addToChain(ContextObj* pContextObj);

  /**
   * Overload operator new for use with ContextMemoryManager to allow
   * creation of new Scope objects in the current memory region.
   */
  static void* operator new(size_t size, ContextMemoryManager* pCMM)
  {
    Trace("context_mm") << "Scope::new " << size << " in " << pCMM << std::endl;
    return pCMM->newData(size);
  }

  /**
   * Enqueues a ContextObj to be garbage collected via a call to deleteSelf()
   * during the destruction of this scope.
   */
  void enqueueToGarbageCollect(ContextObj* obj);

  /**
   * Overload operator delete for Scope objects allocated using
   * ContextMemoryManager.  No need to do anything because memory is
   * freed automatically when the ContextMemoryManager pop() method is
   * called.  Include both placement and standard delete for
   * completeness.
   */
  static void operator delete(void* pMem, ContextMemoryManager* pCMM) {}
  static void operator delete(void* pMem) {}

  //FIXME:  //! Check for memory leaks
  //  void check();

};/* class Scope */

/**
 * This is an abstract base class from which all objects that are
 * context-dependent should inherit.  For any data structure that you
 * want to have be automatically backtracked, do the following:
 *
 * 1. Create a class for the data structure that inherits from ContextObj
 *
 * 2. Implement save()
 *    The role of save() is to create a new ContexObj object that contains
 *    enough information to restore the object to its current state, even if
 *    it gets changed later.  Note that save should call the (default)
 *    ContextObj Copy Constructor to copy the information in the base class.
 *    It is recommended that any memory allocated by save() should be done
 *    through the ContextMemoryManager.  This way, it does not need to be
 *    explicitly freed when restore is called.  However, it is only required
 *    that the ContextObj itself be allocated using the
 *    ContextMemoryManager.  If other memory is allocated not using the
 *    ContextMemoryManager, it should be freed when restore() is called.  In
 *    fact, any clean-up work on a saved object must be done by restore().
 *    This is because the destructor is never called explicitly on saved
 *    objects.
 *
 * 3. Implement restore()
 *    The role of restore() is, given the ContextObj returned by a previous
 *    call to save(), to restore the current object to the state it was in
 *    when save() was called.  Note that the implementation of restore does
 *    *not* need to worry about restoring the base class data.  This is done
 *    automatically.  Ideally, restore() should not have to free any memory
 *    as any memory allocated by save() should have been done using the
 *    ContextMemoryManager (see item 2 above).
 *
 * 4. In the subclass implementation, any time the state is about to be
 *    changed, first call makeCurrent().
 *
 * 5. In the subclass implementation, the destructor should call destroy(),
 *    which repeatedly calls restore() until the object is restored to context
 *    level 0.  Note, however, that if there is additional cleanup required at
 *    level 0, destroy() does not do this.  It has to be implemented in the
 *    destructor of the subclass.  The reason the destroy() functionality
 *    cannot be in the ContextObj destructor is that it needs to call the
 *    subclass-specific restore() method in order to properly clean up saved
 *    copies.
 *
 * GOTCHAS WHEN ALLOCATING CONTEXTUAL OBJECTS WITH NON-STANDARD ALLOCATORS
 *
 * Be careful if you intend to allocate ContextObj in (for example)
 * ContextMemoryManager memory.  ContextMemoryManager doesn't call the
 * destructors of contained objects, which means the objects aren't
 * properly unregistered from the Context (as in point #5, above).
 * This can be remedied by allocating the ContextObj in the "top
 * scope" rather than the "bottom scope" (which is what the
 * "allocatedInCMM" flag to the special constructor in ContextObj
 * does).
 *
 * But why allocate in CMM if it's so complicated?  There's a big
 * advantage: you don't have to track the lifetime of the ContextObj.
 * The object simply ceases to exist when the Context is popped.  Thus
 * you can create an object in context memory, and if you stow
 * pointers to it only in context memory as well, all is cleaned up
 * for you when the scope pops.  Here's a list of gotchas:
 *
 * 1. For data structures intended solely to be allocated in context
 *    memory, privately declare (but don't define) an operator
 *    new(size_t) and destructor (as currently in the Link class, in
 *    src/theory/uf/ecdata.h).
 *
 * 2. For data structures that may or may not be allocated in context
 *    memory, and are designed to be that way (esp. if they contain
 *    ContextObj instances), they should be heavily documented --
 *    especially the destructor, since it _may_or_may_not_be_called_.
 *
 * 3. There's also an issue for generic code -- some class Foo<T>
 *    might be allocated in context memory, and that might normally be
 *    fine, but if T is a ContextObj this requires certain care.
 *
 * 4. Note that certain care is required for ContextObj inside of data
 *    structures.  If the (enclosing) data structure can be allocated
 *    in CMM, that means the (enclosed) ContextObj can be, even if it
 *    was not designed to be allocated in that way.  In many
 *    instances, it may be required that data structures enclosing
 *    ContextObj be parameterized with a similar "bool allocatedInCMM"
 *    argument as the special constructor in this class (and pass it
 *    on to all ContextObj instances).
 */
class ContextObj {
  /**
   * Pointer to Scope in which this object was last modified.
   */
  Scope* d_pScope;

  /**
   * Pointer to most recent version of same ContextObj in a previous Scope
   */
  ContextObj* d_pContextObjRestore;

  /**
   * Next link in ContextObjList list maintained by Scope class.
   */
  ContextObj* d_pContextObjNext;

  /**
   * Previous link in ContextObjList list maintained by Scope class.  We use
   * double-indirection here to make element deletion easy.
   */
  ContextObj** d_ppContextObjPrev;

  /**
   * Helper method for makeCurrent (see below).  Separated out to allow common
   * case to be inlined without making a function call.  It calls save() and
   * does the necessary bookkeeping to ensure that object can be restored to
   * its current state when restore is called.
   */
  void update();

  // The rest of the private methods are for the benefit of the Scope.  We make
  // Scope our friend so it is the only one that can use them.

  friend class Scope;

  friend std::ostream& operator<<(std::ostream&, const Scope&);

  /**
   * Return reference to next link in ContextObjList.  Used by
   * Scope::addToChain method.
   */
  ContextObj*& next() { return d_pContextObjNext; }

  /**
   * Return reference to prev link in ContextObjList.  Used by
   * Scope::addToChain method.
   */
  ContextObj**& prev() { return d_ppContextObjPrev; }

  /**
   * This method is called by Scope during a pop: it does the necessary work to
   * restore the object from its saved copy and then returns the next object in
   * the list that needs to be restored.
   */
  ContextObj* restoreAndContinue();

 protected:
  /**
   * This is a method that must be implemented by all classes inheriting from
   * ContextObj.  See the comment before the class declaration.
   */
  virtual ContextObj* save(ContextMemoryManager* pCMM) = 0;

  /**
   * This is a method that must be implemented by all classes inheriting from
   * ContextObj.  See the comment before the class declaration.
   */
  virtual void restore(ContextObj* pContextObjRestore) = 0;

  /**
   * This method checks if the object has been modified in this Scope
   * yet.  If not, it calls update().
   */
  inline void makeCurrent();

  /**
   * Just calls update(), but given a different name for the derived
   * class-facing interface.  This is a "forced" makeCurrent(), useful
   * for ContextObjs allocated in CMM that need a special "bottom"
   * case when they disappear out of existence (kind of a destructor).
   * See CDOhash_map (in cdhashmap.h) for an example.
   */
  inline void makeSaveRestorePoint();

  /**
   * Should be called from sub-class destructor: calls restore until restored
   * to initial version (version at context level 0).  Also removes object from
   * all Scope lists.  Note that this doesn't actually free the memory
   * allocated by the ContextMemoryManager for this object.  This isn't done
   * until the corresponding Scope is popped.
   */
  void destroy();

  /////
  //
  //  These next four accessors return properties of the Scope to
  //  derived classes without giving them the Scope object directly.
  //
  /////

  /**
   * Get the Context with which this ContextObj was created.  This is
   * part of the protected interface, intended for derived classes to
   * use if necessary.
   */
  Context* getContext() const { return d_pScope->getContext(); }

  /**
   * Get the ContextMemoryManager with which this ContextObj was
   * created.  This is part of the protected interface, intended for
   * derived classes to use if necessary.  If a ContextObj-derived
   * class needs to allocate memory somewhere other than the save()
   * member function (where it is explicitly given a
   * ContextMemoryManager), it can use this accessor to get the memory
   * manager.
   */
  ContextMemoryManager* getCMM() const { return d_pScope->getCMM(); }

  /**
   * Get the level associated to this ContextObj.  Useful if a
   * ContextObj-derived class needs to compare the level of its last
   * update with another ContextObj.
   */
  int getLevel() const { return d_pScope->getLevel(); }

  /**
   * Returns true if the object is "current"-- that is, updated in the
   * topmost contextual scope.  Useful if a ContextObj-derived class
   * needs to determine if it has been modified in the current scope.
   * Note that it is always safe to call makeCurrent() without first
   * checking if the object is current, so this function need not be
   * used under normal circumstances.
   */
  bool isCurrent() const { return d_pScope->isCurrent(); }

 public:
  /**
   * Disable delete: objects allocated with new(ContextMemorymanager) should
   * never be deleted.  Objects allocated with new(bool) should be deleted by
   * calling deleteSelf().
   */
  static void operator delete(void* pMem) {
    AlwaysAssert(false, "It is not allowed to delete a ContextObj this way!");
  }

  /**
   * operator new using ContextMemoryManager (common case used by
   * subclasses during save()).  No delete is required for memory
   * allocated this way, since it is automatically released when the
   * context is popped.  Also note that allocations using this
   * operator never have their destructor called, so any clean-up has
   * to be done using the restore method.
   */
  static void* operator new(size_t size, ContextMemoryManager* pCMM) {
    Trace("context_mm") << "Context::new " << size << " in " << pCMM << std::endl;
    return pCMM->newData(size);
  }

  /**
   * Corresponding placement delete.  Note that this is provided just
   * to satisfy the requirement that a placement delete should be
   * provided for every placement new.  It would only be called if a
   * ContextObj constructor throws an exception after a successful
   * call to the above new operator.
   */
  static void operator delete(void* pMem, ContextMemoryManager* pCMM) {}

  /**
   * Create a new ContextObj.  The initial scope is set to the bottom
   * scope of the Context.  Note that in the common case, the copy
   * constructor is called to create new ContextObj objects.  The
   * default copy constructor does the right thing, so we do not
   * explicitly define it.
   */
  ContextObj(Context* context);

  /**
   * Create a new ContextObj.  This constructor takes an argument that
   * specifies whether this ContextObj is itself allocated in context
   * memory.  If it is, it's invalid below the current scope level, so
   * we don't put it in scope 0.
   *
   * WARNING: Read the notes above on "Gotchas when allocating
   * contextual objects with non-standard allocators."
   */
  ContextObj(bool allocatedInCMM, Context* context);

  /**
   * Destructor does nothing: subclass must explicitly call destroy() instead.
   */
  virtual ~ContextObj() {}

  /**
   * If you want to allocate a ContextObj object on the heap, use this
   * special new operator.  To free this memory, instead of
   * "delete p", use "p->deleteSelf()".
   */
  static void* operator new(size_t size, bool) {
    return ::operator new(size);
  }

  /**
   * Corresponding placement delete.  Note that this is provided for
   * the compiler in case the ContextObj constructor throws an
   * exception.  The client can't call it.
   */
  static void operator delete(void* pMem, bool) {
    ::operator delete(pMem);
  }

  /**
   * Use this instead of delete to delete memory allocated using the special
   * new function provided above that takes a bool argument.  Do not call this
   * function on memory allocated using the new that takes a
   * ContextMemoryManager as an argument.
   */
  void deleteSelf() {
    Debug("context") << "deleteSelf(" << this << ") " << typeid(*this).name() << std::endl;
    this->~ContextObj();
    ::operator delete(this);
  }

  /**
   * Use this to enqueue calling deleteSelf() at the time of the destruction of
   * the enclosing Scope.
   */
  void enqueueToGarbageCollect();

};/* class ContextObj */

/**
 * For more flexible context-dependent behavior than that provided by
 * ContextObj, objects may implement the ContextNotifyObj interface
 * and simply get a notification when a pop has occurred.  See
 * Context class for how to register a ContextNotifyObj with the
 * Context (you can choose to have notification come before or after
 * the ContextObj objects have been restored).
 */
class ContextNotifyObj {

  /**
   * Context is our friend so that when the Context is deleted, any
   * remaining ContextNotifyObj can be removed from the Context list.
   */
  friend class Context;

  /**
   * Pointer to next ContextNotifyObject in this List
   */
  ContextNotifyObj* d_pCNOnext;

  /**
   * Pointer to previous ContextNotifyObject in this list
   */
  ContextNotifyObj** d_ppCNOprev;

  /**
   * Return reference to next link in ContextNotifyObj list.  Used by
   * Context::addNotifyObj methods.
   */
  ContextNotifyObj*& next() { return d_pCNOnext; }

  /**
   * Return reference to prev link in ContextNotifyObj list.  Used by
   * Context::addNotifyObj methods.
   */
  ContextNotifyObj**& prev() { return d_ppCNOprev; }

 protected:
  /**
   * This is the method called to notify the object of a pop.  It must be
   * implemented by the subclass. It is protected since context is out
   * friend.
   */
  virtual void contextNotifyPop() = 0;

public:

  /**
   * Constructor for ContextNotifyObj.  Parameters are the context to
   * which this notify object will be added, and a flag which, if
   * true, tells the context to notify this object *before* the
   * ContextObj objects are restored.  Otherwise, the context notifies
   * the object after the ContextObj objects are restored.  The
   * default is for notification after.
   */
  ContextNotifyObj(Context* pContext, bool preNotify = false);

  /**
   * Destructor: removes object from list
   */
  virtual ~ContextNotifyObj();

};/* class ContextNotifyObj */

inline void ContextObj::makeCurrent()
{
  if(!(d_pScope->isCurrent())) {
    update();
  }
}

inline void ContextObj::makeSaveRestorePoint() { update(); }

inline void Scope::addToChain(ContextObj* pContextObj)
{
  if(d_pContextObjList != NULL) {
    d_pContextObjList->prev() = &pContextObj->next();
  }

  pContextObj->next() = d_pContextObjList;
  pContextObj->prev() = &d_pContextObjList;
  d_pContextObjList = pContextObj;
}

}/* CVC4::context namespace */
}/* CVC4 namespace */

#endif /* __CVC4__CONTEXT__CONTEXT_H */