MLBEDSW-6313 Static optimisation for Shape OP

*Shape OP value is available at compile time hence
it can be optimised
*Disconnected shape OP at compile time from parent
tensor
*Transformed shape OP tensor into constant

Change-Id: I0a024269e2b592c6146dd72e62d7a41951fb727a
Signed-off-by: Ayaan Masood <Ayaan.Masood@arm.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/ethosu/vela/tflite_graph_optimiser.py b/ethosu/vela/tflite_graph_optimiser.py
index 06395784..cf3985e4 100644
--- a/ethosu/vela/tflite_graph_optimiser.py
+++ b/ethosu/vela/tflite_graph_optimiser.py
@@ -1391,12 +1391,43 @@ def convert_mean_to_depthwise_conv_or_avgpool(op, arch, nng):
     return op
 
 
+def convert_shape_op_to_constant_tensor(op: Operation, arch, nng):
+    """Static optimisation for SHAPE operator output value known at compile time"""
+
+    # Disconnect SHAPE operator from its parent and transform SHAPE OP into constant
+
+    if op.type == Op.Shape and op.run_on_npu:
+
+        ifm, ofm = op.get_ifm_ofm()
+
+        if len(ifm.shape) != ofm.shape[0]:
+            return op
+
+        # Remove reference of the current shape op from the parent tensor's consumer list
+        ifm.consumer_list = [consumer for consumer in ifm.consumer_list if consumer.op_index != op.op_index]
+
+        # Clear any references to parent node
+        op.inputs = []
+
+        # Convert this SHAPE op to const
+        op.type = Op.Const
+
+        # Add size calculation to shape output tensors
+        ofm.values = np.array(ifm.shape)
+
+    return op
+
+
 def supported_operator_check(op, arch, nng):
     op.run_on_npu = arch.tflite_supported_operators.is_operator_supported(op)
     return op
 
 
 def tflite_optimise_graph(nng, arch):
+
+    # Compile time optimisations
+    optimisation_list = [convert_shape_op_to_constant_tensor]
+
     # Pre-processing step
     pre_process_list = [
         supported_operator_check,
@@ -1409,6 +1440,16 @@ def tflite_optimise_graph(nng, arch):
             sg,
             arch,
             [],
+            optimisation_list,
+            rewrite_unsupported=False,
+        )
+
+    for idx, sg in enumerate(nng.subgraphs):
+        nng.subgraphs[idx] = rewrite_graph.rewrite_graph_pre_order(
+            nng,
+            sg,
+            arch,
+            [],
             pre_process_list,
             rewrite_unsupported=False,
         )