D Programming Language 2.0

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Last update Sun May 10 11:52:06 2009

# Garbage Collection

D is a fully garbage collected language. That means that it is never necessary to free memory. Just allocate as needed, and the garbage collector will periodically return all unused memory to the pool of available memory.

C and C++ programmers accustomed to explicitly managing memory allocation and deallocation will likely be skeptical of the benefits and efficacy of garbage collection. Experience both with new projects written with garbage collection in mind, and converting existing projects to garbage collection shows that:

- Garbage collected programs are faster. This is counterintuitive, but the reasons are:
  - Reference counting is a common solution to solve explicit memory allocation problems. The code to implement the increment and decrement operations whenever assignments are made is one source of slowdown. Hiding it behind smart pointer classes doesn't help the speed. (Reference counting methods are not a general solution anyway, as circular references never get deleted.)
  - Destructors are used to deallocate resources acquired by an object. For most classes, this resource is allocated memory. With garbage collection, most destructors then become empty and can be discarded entirely.
  - All those destructors freeing memory can become significant when objects are allocated on the stack. For each one, some mechanism must be established so that if an exception happens, the destructors all get called in each frame to release any memory they hold. If the destructors become irrelevant, then there's no need to set up special stack frames to handle exceptions, and the code runs faster.
  - All the code necessary to manage memory can add up to quite a bit. The larger a program is, the less in the cache it is, the more paging it does, and the slower it runs.
  - Garbage collection kicks in only when memory gets tight. When memory is not tight, the program runs at full speed and does not spend any time freeing memory.
  - Modern garbage collectors are far more advanced now than the older, slower ones. Generational, copying collectors eliminate much of the inefficiency of early mark and sweep algorithms.
  - Modern garbage collectors do heap compaction. Heap compaction tends to reduce the number of pages actively referenced by a program, which means that memory accesses are more likely to be cache hits and less swapping.
  - Garbage collected programs do not suffer from gradual deterioration due to an accumulation of memory leaks.
- Garbage collectors reclaim unused memory, therefore they do not suffer from "memory leaks" which can cause long running applications to gradually consume more and more memory until they bring down the system. GC programs have longer term stability.
- Garbage collected programs have fewer hard-to-find pointer bugs. This is because there are no dangling references to freed memory. There is no code to explicitly manage memory, hence no bugs in such code.
- Garbage collected programs are faster to develop and debug, because there's no need for developing, debugging, testing, or maintaining the explicit deallocation code.
- Garbage collected programs can be significantly smaller, because there is no code to manage deallocation, and there is no need for exception handlers to deallocate memory.

Garbage collection is not a panacea. There are some downsides:

- It is not predictable when a collection gets run, so the program can arbitrarily pause.
- The time it takes for a collection to run is not bounded. While in practice it is very quick, this cannot be guaranteed.
- All threads other than the collector thread must be halted while the collection is in progress.
- Garbage collectors can keep around some memory that an explicit deallocator would not. In practice, this is not much of an issue since explicit deallocators usually have memory leaks causing them to eventually use far more memory, and because explicit deallocators do not normally return deallocated memory to the operating system anyway, instead just returning it to its own internal pool.
- Garbage collection should be implemented as a basic operating system kernel service. But since they are not, garbage collecting programs must carry around with them the garbage collection implementation. While this can be a shared DLL, it is still there.

These constraints are addressed by techniques outlined in Memory Management.

## How Garbage Collection Works

The GC works by:

1. Looking for all the pointer 'roots' into GC allocated memory.
2. Recursively scanning all allocated memory pointed to by roots looking for more pointers into GC allocated memory.
3. Freeing all GC allocated memory that has no active pointers to it.
4. Possibly compacting the remaining used memory by copying the allocated objects (called a copying collector).

## Interfacing Garbage Collected Objects With Foreign Code

The garbage collector looks for roots in:

1. its static data segment
2. the stacks and register contents of each thread
3. any roots added by std.gc.addRoot() or std.gc.addRange()

If the only root of an object is held outside of this, then the collecter will miss it and free the memory.

To avoid this from happening,

- Maintain a root to the object in an area the collector does scan for roots.
- Add a root to the object using std.gc.addRoot() or std.gc.addRange().
- Reallocate and copy the object using the foreign code's storage allocator or using the C runtime library's malloc/free.

## Pointers and the Garbage Collector

Pointers in D can be broadly divided into two categories: those that point to garbage collected memory, and those that do not. Examples of the latter are pointers created by calls to C's malloc(), pointers received from C library routines, pointers to static data, pointers to objects on the stack, etc. For those pointers, anything that is legal in C can be done with them.

For garbage collected pointers and references, however, there are some restrictions. These restrictions are minor, but they are intended to enable the maximum flexibility in garbage collector design.

Undefined behavior:

- Do not xor pointers with other values, like the xor pointer linked list trick used in C.
- Do not use the xor trick to swap two pointer values.
- Do not store pointers into non-pointer variables using casts and other tricks.

      void* p;
      ...
      int x = cast(int)p;   // error: undefined behavior

  The garbage collector does not scan non-pointer types for roots.
- Do not take advantage of alignment of pointers to store bit flags in the low order bits:

      p = cast(void*)(cast(int)p | 1);  // error: undefined behavior

- Do not store into pointers values that may point into the garbage collected heap:

      p = cast(void*)12345678;   // error: undefined behavior

  A copying garbage collector may change this value.
- Do not store magic values into pointers, other than `null`.
- Do not write pointer values out to disk and read them back in again.
- Do not use pointer values to compute a hash function. A copying garbage collector can arbitrarily move objects around in memory, thus invalidating the computed hash value.
- Do not depend on the ordering of pointers:

      if (p1 < p2)		// error: undefined behavior
          ...

  since, again, the garbage collector can move objects around in memory.
- Do not add or subtract an offset to a pointer such that the result points outside of the bounds of the garbage collected object originally allocated.

      char* p = new char[10];
      char* q = p + 6;	// ok
      q = p + 11;		// error: undefined behavior
      q = p - 1;		// error: undefined behavior

- Do not misalign pointers if those pointers may point into the gc heap, such as:

      align (1) struct Foo
      {   byte b;
          char* p;	// misaligned pointer
      }

  Misaligned pointers may be used if the underlying hardware supports them and the pointer is never used to point into the gc heap.
- Do not use byte-by-byte memory copies to copy pointer values. This may result in intermediate conditions where there is not a valid pointer, and if the gc pauses the thread in such a condition, it can corrupt memory. Most implementations of `memcpy()` will work since the internal implementation of it does the copy in aligned chunks greater than or equal to a pointer size, but since this kind of implementation is not guaranteed by the C standard, use `memcpy()` only with extreme caution.

Things that are reliable and can be done:

- Use a union to share storage with a pointer:

      union U { void* ptr; int value }

- A pointer to the start of a garbage collected object need not be maintained if a pointer to the interior of the object exists.

      char[] p = new char[10];
      char[] q = p[3..6];
      // q is enough to hold on to the object, don't need to keep
      // p as well.

One can avoid using pointers anyway for most tasks. D provides features rendering most explicit pointer uses obsolete, such as reference objects, dynamic arrays, and garbage collection. Pointers are provided in order to interface successfully with C APIs and for some low level work.

## Working with the Garbage Collector

Garbage collection doesn't solve every memory deallocation problem. For example, if a root to a large data structure is kept, the garbage collector cannot reclaim it, even if it is never referred to again. To eliminate this problem, it is good practice to set a reference or pointer to an object to null when no longer needed.

This advice applies only to static references or references embedded inside other objects. There is not much point for such stored on the stack to be nulled, since the collector doesn't scan for roots past the top of the stack, and because new stack frames are initialized anyway.

## References

- Wikipedia
- GC FAQ
- Uniprocessor Garbage Collector Techniques
- Garbage Collection : Algorithms for Automatic Dynamic Memory Management

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