Document float16 issue with GpuElemwise

theano/sandbox/gpuarray/opt.py

@@ -274,6 +274,12 @@ def local_gpu_elemwise(node):
     if isinstance(op.scalar_op, Pow):
         old_out_dtype = node.outputs[0].dtype
         old_inp_dtypes = [inp.dtype for inp in node.inputs]
+
+        # Upcast the input dtypes with 'float32' to obtain a floating-point
+        # dtype in which to do the computation.
+        # TODO : Currently, a bug in GpuElemwise prevents support for float16.
+        # It should be fixed and then the upcast below can use 'float16'
+        # instead of 'float32'
         new_out_dtype = upcast("float32", *old_inp_dtypes)
 
         # Transfer the inputs on the GPU and cast them to the right dtype