This directory contains the low-level tensor libraries for PyTorch,
as well as the new ATen C++ bindings.

The low-level libraries trace their lineage from the original Torch.  There are
multiple variants of the library, summarized here:

* TH = TorcH
* THC = TorcH Cuda
* THCS = TorcH Cuda Sparse (now defunct)
* THNN = TorcH Neural Network (now defunct)
* THS = TorcH Sparse (now defunct)

(You'll also see these abbreviations show up in symbol names.)

## Reference counting

PyTorch employs reference counting in order to permit tensors to provide
differing views on a common underlying storage.  For example, when you call
view() on a Tensor, a new THTensor is allocated with differing dimensions,
but it shares the same c10::StorageImpl with the original tensor.

Unfortunately, this means we are in the business of manually tracking reference
counts inside our C library code.  Fortunately, for most of our library code implementing
tensor operations, there is only one rule you have to remember:

> **Golden Rule of Reference Counting:** You must either FREE or RETURN
> a pointer which was returned by a function whose name begins with
> `new` or which you called `retain` on.
> If you return this pointer, your function name must begin with `new`.

In a long function, there may be many invocations of functions with `new` in
their name.  Your responsibility is to go through each of them and ensure
that there is a matching `free` for it for EACH exit point of the function.

### Examples

Suppose you want to get a reference to the indices of a sparse tensor.  This
function is called `newIndices`.  The `new` means you MUST free it when you're
done (usually at the end of your function.)  (It's worth noting that
`newIndices` doesn't actually allocate a fresh indices tensor; it just gives
you a pointer to the existing one.)  DO NOT directly access the member
variables of the struct.

```
THIndexTensor *indices = THSTensor_(newIndices)(state, sparse);
// ... do some stuff ...
THIndexTensor_(free)(state, indices);
```

Let's take a look at the implementation of `newIndices`.  This doesn't free the
return result of `newNarrow`, but returns it.  This justifies the `new` in its
name.

```
THIndexTensor *THSTensor_(newIndices)(const THSTensor *self) {
  // ...
  return THIndexTensor_(newNarrow)(self->indices, 1, 0, self->nnz);
}
```

Passing an object to another function does NOT absolve you of responsibility
of freeing it.  If that function holds on to a pointer to the object, it
will `retain` it itself.

```
  THByteStorage *inferred_size = THByteStorage_newInferSize(size, numel);
  THTensor_(setStorage)(self, tensor->storage, tensor->storageOffset, inferred_size, NULL);
  c10::raw::intrusive_ptr::decref(inferred_size);
```

Sometimes, you have a tensor in hand which you'd like to use directly, but
under some conditions you have to have to call, e.g., `newContiguous`, to get
it into the correct form:

```
  if (!(k_->stride(3) == 1) || !(k_->stride[2] == k_->size(3))) {
    kernel = THTensor_(newContiguous)(k_);
  } else {
    THTensor_(retain)(k_);
    kernel = k_;
  }
  ...
  c10::raw::intrusive_ptr::decref(kernel);
```

In this case, we have (redundantly) called `retain` on `k_`, so that we can
unconditionally free `kernel` at the end of the function; intuitively, you
want it to be possible to replace the conditional expression with an equivalent
function call, e.g., `kernel = THTensor_(newContiguous2D)(k_)`.

### Tips

* If you have an early exit in a function (via a `return`), don't forget to
  `free` any pointers which you allocated up to this point.  If at all possible,
  move early exits prior to these allocations, so that you don't have to clean up.

* Very occasionally, you may be able to implement an algorithm more efficiently
  if you "destroy" its input.  This is a `move`; after moving an object away,
  you must NOT `free` it.  This is the one exception to the rule, and at the
  moment there is only one instance of `move` in the code base.

* We use `THError` to signal error cases, and fortunately,
  you do NOT need to make sure you've freed everything before calling `THError`,
  because by default, it aborts the entire process.  However, it's good style
  to call `THError` before performing any allocations, since in some cases we
  sketchily throw a C++ exception and try to recover (in particular, the test
  suite does this.)
