Update comments

theano/gof/cc.py

@@ -282,6 +282,11 @@ def get_nothing(r, name, sub):
 
 def get_c_declare(r, name, sub):
     """Wrapper around c_declare that declares py_name"""
+    # The declaration will be used by the Apply node that
+    # is computing it (`r.owner`), and by each of the clients.
+    # If some of these have `check_input=True` in their `.op`,
+    # it means they need `r`'s dtype to be declared, so
+    # we have to pass `check_input=True` to `c_declare`.
     if ((any([getattr(c.op, 'check_input', config.check_input)
               for (c, _) in r.clients
               if not isinstance(c, string_types)]) or
@@ -307,6 +312,12 @@ def get_c_init(r, name, sub):
 
 def get_c_extract(r, name, sub):
     """Wrapper around c_extract that initializes py_name from storage."""
+    # `c_extract` is called when getting the value of an apply node's
+    # input from the compute map, before being used by its clients.
+    # If one of the clients has `check_input=True`, we need to perform
+    # checks on the variable.
+    # However that code is not used by C code of the apply node creating
+    # this variable, so there is no need to check `r.owner.op.check_input`.
     if any([getattr(c.op, 'check_input', config.check_input)
             for (c, _) in r.clients
             if not isinstance(c, string_types)]):
@@ -336,10 +347,18 @@ def get_c_extract(r, name, sub):
 
 def get_c_extract_out(r, name, sub):
     """Wrapper around c_extract_out that initializes py_name from storage."""
+    # `c_extract_out` is used to extract an output variable from
+    # the compute map, to be used as pre-allocated memory for `r`
+    # before its value gets computed.
+    # If the node producing `r` has `check_inputs=True`, it may
+    # also perform type checks on the initial value of the output,
+    # so we need to pass `check_input=True` to `c_extract_out`.
+    # However, that code is not used by potential clients of `r`,
+    # so we do not need to check them.
+    check_input = getattr(r.owner.op, 'check_input', config.check_input)
     # check_broadcast is just an hack to easily remove just the
     # broadcast check on the old GPU back-end. This check isn't
     # done in the new GPU back-end or on the CPU.
-    check_input = getattr(r.owner.op, 'check_input', config.check_input)
     if getattr(r.owner.op, 'check_broadcast', True):
         c_extract = r.type.c_extract_out(name, sub, check_input)
     else: