N2527: Minimal Support for Garbage Collection and Reachability-Based Leak Detection (revised)

Its purpose is to support both garbage collected implementations and reachability-based leak detectors. This is done by giving undefined behavior to programs that "hide a pointer" by, for example, xor-ing it with another value, and then later turn it back into an ordinary pointer and dereference it. Such programs may currently produce incorrect results with conservative garbage collectors, since an object referenced only by such a "hidden pointer" may be prematurely collected. For the same reason, reachability-based leak detectors may erroneously report that such programs leak memory.

Note that for programs using the quick_exit() facility ( N2440, voted into the working paper at the Kona meeting), reachability-based leak detectors are arguably the only viable form of leak detection.

For a more general discussion, and the reasons to support transparent garbage collection, please see N2310.

Core Language Wording

3.7.3.3 Safely derived Pointers

[Note: Objects allocated by allocation functions must be properly referenced through chains of pointers until they are dereferenced or deallocated. The following makes this requirement precise. This allows, but does not require, implementations to reclaim unreferenced objects. -- end note.]

A traceable pointer location is one of the following:

• An object of pointer type.
• An object of an integral type at least as large as intptr_t
• A sequence of characters in an array of characters, whose size and alignment matches that of a pointer type.

A pointer value contained in a traceable pointer location is a reconstituted pointer if it was computed by:

• Copying from other than a traceable pointer location,
• Performing arithmetic or bit-wise operations on integer arguments,
• Copying a reconstituted pointer,
• Arithmetic operations on a reconstituted pointer,
• Taking the address of an object obtained by dereferencing a reconstituted pointer, or
• Reading it from a file.

A pointer value that is not a reconstituted pointer is called a safely derived pointer.

[Note: Whether or not a pointer value is safely derived is a property of how it was computed, and usually not of its actual representation. In particular, two pointers may be equal, and one of them may be safely derived, while the other is reconstituted. --end note]

A pointer to storage obtained from an allocation function shall be dereferenced or passed to a deallocation function only if it was either safely derived, or the referenced object was previously declared reachable (see [library:declare_reachable]).

Observations and issues:

1. I'm sure this is nowhere near standardese.
2. This is really an inductive definition on the chain of operations used to compute a pointer value.
3. It is unclear where we should allow pointers to be stored. We tentatively went with a fairly lax interpretation of what the current standard allows. We do need to allow copies in char arrays, and pointers in intptr_t variables. Whether we should allow them in other sufficiently large integer variables is up to debate.
4. Although this is very similar in spirit to N2310, The N2310 rules based on reachability had a fundamental problem, which is corrected here. Consider

   T *p = new ...;
   intptr_t x = reinterpret_cast<intptr_t>(p) ^ 0x555;
   a:
   T *q = reinterpret_cast<T *>(x ^ 0x555);
   T y = *q;
   

   The newly allocated object N referenced by p is always reachable by the N2310 definition. But at the label a, p is dead, and is quite likely to no longer be visible to the garbage collector, since the register containing p may well have been reused, possibly to hold x. This means that if a garbage collection occurs at point a, N may not appear to be reachable, and thus may be collected anyway.
5. We could probably get closer to the N2310 definitions if we allowed reconstituted pointers to be dereferenced if a safely derived pointer to the same object is stored in a non-stack location. That seems worth considering. It would technically eliminate the need for declare_reachable(), since it could be implemented by the user. It would outlaw some kinds of dead global dead variable and dead field elimination in garbage-collected implementations. These are probably rarely practical for C++ in any case.
6. Similar issues apply to declare_reachable calls. The only safe way to ensure that a pointer is always visible to the collector is to require that the argument to declare_reachable was safely derived. Thus a pointer to the object is guaranteed to be visible before the call, and the collector treats the object as being reachable after the call.

Library Wording

Objects constructed by the standard library that may hold a user-supplied pointer value, or an integer of type intptr_t, shall store them in a traceable pointer location (see 3.7.3.3). [Note: Other libraries are strongly encouraged to do the same, since not doing so may result in accidental use of reconstituted pointers. Libraries that store pointers outside the user's address space should make it appear that they are stored and retrieved from a traceable pointer location. --end note]

Add, possibly between 20.6.7 and 20.6.8:

void declare_reachable( void* p ) throw(std::bad_alloc)

Effects:

The argument is subsequently declared reachable (see 3.7.3.3). Reconstituted pointers to the same object may be dereferenced while the object is declared reachable.

Throws:

May throw std::bad_alloc if the system cannot allocate additional memory that may be required to track objects declared reachable.

Requires:

The argument p shall be a safely derived non-null pointer.

template < typename T >
T* undeclare_reachable( T* p ) throw()

Returns:

A safely derived copy of p. The result will compare equal to p.

Effects:

Once the number of calls to undeclare_reachable(p) equals the number of calls to declare_reachable(p), the argument is no longer declared reachable (see [above section]). When this happens, reconstituted pointers to the object referenced by p may not be subsequently dereferenced. [Note: Since the returned pointer is safely derived, it may be used to access the referenced object, even if previously no safely derived pointer existed. -- end note]

Requires:

The object referenced by p shall have been previously declared reachable, and shall be live from the time of the call until the last undeclare_reachable(p) call on the object.

[Note: It is expected that calls to declare_reachable(p) will consume a small amount of memory until the matching call to undeclare_reachable(p) is encountered. In addition, the referenced object cannot be deallocated during this period, and garbage collecting implementations will not be able to collect the object while it is declared reachable. Long running programs should arrange that calls are matched. -- end note.]

void declare_no_pointers( char* p, size_t n ) throw()

Effects:

The n bytes starting at p no longer contain traceable pointer locations, independent of their type. Hence pointers located there may no longer be dereferenced. [Note: This may be used to inform a garbage collector or leak detector that this region of memory need not be traced.]

Throws:

Throws no exceptions. [Note: Under some conditions implementations may need to allocate memory. However the request can be ignored if memory allocation fails. -- end note]

Requires:

No bytes in the specified range may have been previously registered with declare_no_pointers(). If the specified range is in an allocated object, then it must be entirely within a single allocated object. The object must be live until the corresponding undeclare_no_pointers() call. [Note: In a garbage-collecting implementation, the fact that a region in an object is registered with declare_no_pointers() should not prevent the object from being collected. --end note]

void undeclare_no_pointers( char* p, size_t n ) throw()

Effects:

Prepares an object containing a range registered with declare_no_pointers() for destruction. It must be called before the lifetime of the object ends. It has no other effect. It does not recreate any traceable pointer locations in the object.

Requires:

The same range must previously have been passed to declare_no_pointers().

Add to 20.6.8, between paragraphs 4 and 5:

Storage allocated directly with malloc(), calloc(), or realloc() is implicitly declared reachable (see 3.7.3.3) on allocation, ceases to be declared reachable on deallocation, and may not cease to be declared reachable as the result of an undeclare_reachable() call. [Note: This allows existing C libraries to remain unaffected by restrictions on reconstituted pointers, at the expense of providing far fewer garbage collection and leak detection options for malloc()-allocated objects. It also allows malloc() to be implemented with a separate allocation arena, bypassing the normal declare_reachable() implementation. The above functions should never intentionally be used as a replacement for declare_reachable(), and newly written code is strongly encouraged to treat memory allocated with these functions as though it were allocated with operator new. --end note]

Observations and issues:

1. Currently undeclare_reachable is a template, while declare_reachable operates on a void *. This is intentional, since only the former returns a pointer. Having the latter return a pointer is far less useful, and doesn't quite fit the name, though it may still be the right option.
2. It is unclear whether null pointers, and pointers to memory not allocated with one of the system memory allocators should be allowed as arguments to declare_reachable. Disallowing the former seems benign, and simplifies matters a bit in this formulation. Disallowing the latter causes problems for clients who don't know where the memory came from.
3. By a similar argument,there are reasons to believe that declare_reachable and undeclare_reachable should nest properly. A client that is passed one and wants to put it on an xor-ed list may not know whether the caller has already done so. In this formulation, it's safe to do so again.
4. It is almost certainly safe to dereference a reconstituted pointer if the object will be correctly declared reachable later. This is probably too confusing and useless to guarantee.
5. There is some argument that declare_reachable / undeclare_reachable should be replaced by an RAII mechanism. But the simple version would fail to cover many use cases, e.g. an object declared reachable while a pointer to it resides in an xor-list container. This is designed to be a minimalist proposal, so we leave the RAII version to the programmer for now.
6. The current no_pointers is inconsistent with the usual STL conventions. But we probably want to preserve the possibility of invoking this on a single integer field that is not part of an array. And there is probably not a standard-conforming way to compute the "one past the end" pointer for that case.
7. Ideally declare_no_pointers should be invocable even on parts of stack allocated variables. That does complicate the implementation. Even initially, a garbage-collected implementation will have to recognize this case, even if just to ignore it. We concluded that, since a garbage collector will need to know about stack locations anyway, this is not an undue burden, though it may involve some cost.
8. It is not clear whether declare_no_pointers needs an inverse operation, or whether that should be implicit at the end of the object lifetime. Hans prefers the latter, but is not sure how to implement it if we allow stack-allocated objects to be used. We would need a dynamic check on function return, which seems highly undesirable.
9. One of the authors is not enthusiastic about the special treatment of malloc() allocated memory. It clearly handicaps garbage collectors or leak detectors, in that such objects cannot be any more reliably collected than they can now, and leaks involving such objects cannot be reliably identified. On the other hand, we expect that it will make straightforward implementations of garbage collection and leak detection of objects allocated using operator new (which is the normative C++ model implied by the standard) far more reliable with existing libraries. Furthermore, this is probably the only thing we can do in good conscience without an endorsement from the C committee for changing the semantics of malloc-allocated memory.
10. Sean Parent suggests adding operator new overloads that effectively perform a declare_no_pointers() call on the entire resulting objects. Aside from convenience, this has the added benefit that it may be significantly cheaper to implement than the two separate calls, since the object can be placed appropriately. However, it is clearly less general than the separate calls, since it cannot be applied to statically allocated storage, stack allocated storage, or a piece of a heap-allocated object. The authors strongly approve of this addition. However, it superficially makes the interface much wider than what had been discussed earlier, since it involves many new function overloads. Hence we look for guidance from LWG before adding these.

Observations on Implementations

1. It must ensure that for programs obeying the restriction on safely-derived pointers are compiled such that any safely derived pointer remains visible to the garbage collector so long as it might still be dereferenced or used in a deallocation. Existing compilers already have this property for nearly all programs. Existing conservative collectors rely on it. A more careful discussion, including some performance results from a simple implementation that guarantees this property, can be found in Boehm, "Simple Garbage-Collector-Safety", PLDI 1996.
2. declare_reachable() and undeclare_reachable() can be implemented by simply maintaining a multiset of declared reachable pointers, and making sure that this multiset is visible to the garbage collector.
3. Efficient implementations of declare_no_pointers() are more challenging, but feasible. Our existing conservative collectors instead implement a facility to provide similar information when an object is allocated or, through a separate API, for roots. The interface proposed here requires the collector to distinguish between various cases, including ranges within stack-allocated objects, and does not allow the collector to segregate objects based on this kind of information. Of course the API proposed here does have the major advantage that it allows pointer layout information to be specified in the constructor instead of at allocation time, which is much more consistent with the usual C++ model.

   Our current implementation strategy is to have declare_no_pointers() simply record its arguments in a data structure that allows efficient lookup of these ranges by address. When an address range is scanned, this data structure can be consulted. For large address ranges the added cost should be minimal, since it is amortized by other scanning overhead. We expect that when scanning small ranges during garbage collection in production code, it may be too expensive to always consult this data structure. In the initial implementation, we will ignore declare_no_pointers calls made on small objects. A better long term strategy would be, for example, to consider the information only on every nth garbage collection. This might cause objects accidentally "referenced" by pointers in such regions to be temporarily retained in spite of the declare_no_pointers calls. But they would not be retained for an unbounded period of time.