Force accumulator to float32.

theano/gpuarray/opt.py

@@ -1112,10 +1112,20 @@ def local_gpua_careduce(op, context_name, inputs, outputs):
         else:
             return False
         x, = inputs
+        idtype = x.dtype
+        adtype = getattr(op, 'acc_dtype', None)
+        odtype = getattr(op, 'dtype', outputs[0].dtype)
+
+        # Force accumulator to float32 for float32 inputs since tree
+        # reduction will not loose as much precision as linear
+        # accumulation and float64 is much slower on GPU.
+        if idtype == 'float32' and odtype == 'float32':
+            adtype = 'float32'
+
         greduce = op2(
             op.scalar_op, axis=op.axis,
-            dtype=getattr(op, 'dtype', outputs[0].dtype),
-            acc_dtype=getattr(op, 'acc_dtype', None))
+            dtype=odtype,
+            acc_dtype=adtype)
         gvar = greduce(x)
         # We need to have the make node called, otherwise the mask can
         # be None