提交 73ac6dba authored 作者: James Bergstra's avatar James Bergstra

corrections to aliasing.txt

上级 0f7e6fad
...@@ -73,18 +73,19 @@ subsequently make to ``np_array`` have no effect on our shared variable. ...@@ -73,18 +73,19 @@ subsequently make to ``np_array`` have no effect on our shared variable.
np_array += 1 # now it is an array of 2.0 s np_array += 1 # now it is an array of 2.0 s
s_default.value # -> array([1.0, 1.0]) s_default.value # -> array([1.0, 1.0])
s_false.value # -> array([2.0, 2.0]) s_false.value # -> array([1.0, 1.0])
s_true.value # -> array([2.0, 2.0])
If we are running this with the CPU as the device, If we are running this with the CPU as the device,
then changes we make to np_array *right away* will show up in ``s_false.value`` then changes we make to np_array *right away* will show up in ``s_true.value``
because numpy arrays are mutable, and ``s_false`` is using the ``np_array`` because numpy arrays are mutable, and ``s_true`` is using the ``np_array``
object as it's internal buffer. object as it's internal buffer.
However, this aliasing of ``np_array`` and ``s_false`` is *inconsistent and fragile*! However, this aliasing of ``np_array`` and ``s_true`` is *inconsistent and fragile*!
It is inconsistent because if Theano is using a GPU device, then the borrow flag It is inconsistent because if Theano is using a GPU device, then the borrow flag
has no effect. has no effect.
It is fragile because It is fragile because
if we call a theano function that updates the value of ``s_false`` the aliasing if we call a theano function that updates the value of ``s_true`` the aliasing
relationship *may* or *may not* be broken (it depends on what the Theano relationship *may* or *may not* be broken (it depends on what the Theano
function does). function does).
...@@ -135,7 +136,7 @@ factors (e.g. the compute device, the dtype of the numpy array). ...@@ -135,7 +136,7 @@ factors (e.g. the compute device, the dtype of the numpy array).
.. code-block:: python .. code-block:: python
v_internal = s.get_internal_value(borrow=True, return_internal_type=True) v_internal = s.get_value(borrow=True, return_internal_type=True)
It is possible to use ``borrow=False`` in conjunction with It is possible to use ``borrow=False`` in conjunction with
``return_internal_type=True``, which will return a deep copy of the internal object. ``return_internal_type=True``, which will return a deep copy of the internal object.
...@@ -188,12 +189,17 @@ that control how ``theano.function`` handles its arguments and return value[s]. ...@@ -188,12 +189,17 @@ that control how ``theano.function`` handles its arguments and return value[s].
Borrowing an input means that Theano will treat the argument you provide as if Borrowing an input means that Theano will treat the argument you provide as if
it were part of Theano's pool of temporaries. Consequently, your input it were part of Theano's pool of temporaries. Consequently, your input
may be reused as a buffer (and overwritten!) during the computation of other variables in the may be reused as a buffer (and overwritten!) during the computation of other variables in the
course of evaluating that function (e.g. ``f``). course of evaluating that function (e.g. ``f``).
Borrowing an output means that Theano will not insist on allocating a fresh Borrowing an output means that Theano will not insist on allocating a fresh
output buffer every time you call the function. It will possibly reuse the same one as output buffer every time you call the function. It will possibly reuse the same one as
a previous call, and overwrite the old contents. Consequently, it may overwrite a previous call, and overwrite the old contents. Consequently, it may overwrite
old return values by side effect. old return values by side effect.
Those return values may also be overwritten in
the course of evaluating *another compiled function* (for example, the output
may be aliased to a shared variable). So be careful to use a borrowed return
value right away before calling any more Theano functions.
It is also possible to pass an ``return_internal_type=True`` flag to the ``Out`` It is also possible to pass an ``return_internal_type=True`` flag to the ``Out``
variable which has the same interpretation as the ``return_internal_type`` flag variable which has the same interpretation as the ``return_internal_type`` flag
......
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