[converter] fuse bn & conv (#319)

* [converter] fuse bn & conv

* fix lint

tinynn/converter/operators/optimize.py

@@ -167,6 +167,68 @@ def fuse_conv_fc_bn(self):
             assert vertex['node_type'] == ExtendedOperator.BATCH_NORM
         self.graph.graph.delete_vertices(remove_ids)
 
+    @class_conditional(lambda self: self.level >= GraphOptimizer.FUSE_BN)
+    def fuse_bn_conv(self):
+        edges = self.graph.graph.es.select(functools.partial(is_rev_bn_fusable_edge, graph_converter=self.graph.graph))
+        filtered_pairs = ((self.graph.graph.vs[x.source], self.graph.graph.vs[x.target]) for x in edges)
+
+        def _remove_last_pred(seq):
+            bn = seq[0]
+            conv = seq[1]
+
+            # Collect the arguments of the conv and batch-norm nodes
+            weight = conv['op'].inputs[1]
+            bias = conv['op'].inputs[2] if len(conv['op'].inputs) > 2 else None
+            bn_w, bn_b, bn_mean, bn_var = bn['op'].inputs[1:]
+            bn_w, bn_b, bn_mean, bn_var = (
+                bn_w.tensor.copy(),
+                bn_b.tensor.copy(),
+                bn_mean.tensor.copy(),
+                bn_var.tensor.copy(),
+            )
+            activ_w = weight.tensor.copy()
+            activ_b = bias.tensor.copy() if bias is not None else None
+            eps = bn['op'].eps
+
+            new_weight = fuse_rev_bn_weight(eps, bn_w, bn_var, activ_w)
+            new_bias = fuse_rev_bn_bias(eps, bn_w, bn_var, bn_mean, bn_b, activ_b, activ_w)
+
+            return False, (conv, bias, new_weight, new_bias)
+
+        def _remove_last_action(first_node, last_node, custom_data):
+            conv, bias, new_weight, new_bias = custom_data
+
+            new_w = self.create_attr_tensor(new_weight)
+            new_b = self.create_attr_tensor(new_bias)
+
+            actions = []
+            actions.append((self.graph.replace_operator_input, (conv, 1, new_w)))
+            if bias is not None:
+                actions.append((self.graph.replace_operator_input, (conv, 2, new_b)))
+            else:
+                actions.append((self.graph.append_operator_input, (conv, new_b)))
+            return actions
+
+        def _skip_pred(seq):
+            bn = seq[0]['op']
+            conv = seq[1]['op']
+
+            skip = bn.inputs[0].quantization is not None or (
+                conv.inputs[1].shape[1] == 1 and conv.inputs[1].shape[0] == conv.groups and conv.groups > 1
+            )
+            return skip
+
+        elinimate_sequences(
+            self.graph,
+            filtered_pairs,
+            True,
+            None,
+            _remove_last_pred,
+            _remove_last_action,
+            _skip_pred,
+            force_forward_input=True,
+        )
+
     @class_conditional(lambda self: self.level >= GraphOptimizer.COMMON_OPTIMIZE)
     def fuse_activation(self):
         # Find fusable ops
@@ -3325,6 +3387,7 @@ def optimize(self):
         self.fuse_conv_fc_bn()
         self.fuse_activation()
         self.fuse_requantize()
+        self.fuse_bn_conv()
 
         # Convert TinyNeuralNetwork ops to TFLite ops
         self.transform_graph()
@@ -3433,6 +3496,20 @@ def is_bn_fusable_edge(edge: ig.Edge, graph_converter: ig.Graph):
     )
 
 
+def is_rev_bn_fusable_edge(edge: ig.Edge, graph_converter: ig.Graph):
+    source_vertex = graph_converter.vs[edge.source]
+    target_vertex = graph_converter.vs[edge.target]
+    return (
+        target_vertex['node_type'] == ExtendedOperator.GENERIC_CONV
+        and source_vertex['node_type'] == ExtendedOperator.BATCH_NORM
+        and source_vertex.outdegree() == 1
+        and source_vertex['op'].inputs[1].buffer is not None
+        and source_vertex['op'].inputs[2].buffer is not None
+        and target_vertex['op'].inputs[1].buffer is not None
+        and source_vertex['op'].fusedActivationFunction == ActivationFunctionType.NONE
+    )
+
+
 def is_padding_fusable_edge(edge: ig.Edge, graph_converter: ig.Graph):
     source_vertex = graph_converter.vs[edge.source]
     target_vertex = graph_converter.vs[edge.target]
@@ -4123,6 +4200,33 @@ def fuse_bn_bias(eps, scale, var, mean, bn_b, activ_b):
         return (-mean) * inv * scale + bn_b
 
 
+def fuse_rev_bn_weight(eps, scale, var, weight):
+    shape = [1, -1] + [1] * (len(weight.shape) - 2)
+
+    inv = 1 / np.sqrt(var + eps)
+
+    return weight * (scale * inv).reshape(shape)
+
+
+def fuse_rev_bn_bias(eps, scale, var, mean, bn_b, activ_b, weight):
+    reduced_dims = tuple([i for i in range(len(weight.shape)) if i > 1])
+
+    inv = 1 / np.sqrt(var + eps)
+    fused_b = bn_b - mean * inv * scale
+
+    if weight.shape[1] == 1 and mean.shape[0] > 1:
+        offset_b = (weight.sum(reduced_dims) * fused_b.reshape(-1, 1)).reshape(-1)
+    else:
+        offset_b = np.matmul(weight.sum(reduced_dims), fused_b.reshape(-1, 1)).reshape(-1)
+
+    if activ_b is not None:
+        if activ_b.shape != mean.shape and activ_b.ndim == 1 and activ_b.size == 1:
+            activ_b = activ_b.repeat(mean.size)
+        return activ_b + offset_b
+    else:
+        return offset_b
+
+
 def fuse_slices(seq: typing.Iterable[ig.Vertex]):
     cur_start = None
     cur_end = None