MLBEDSW-7373: Vela sometimes write empty buffers in incorrect format

 - Fixed an issue whereby a zero length buffer was written out instead
of an empty buffer
 - Added a warning message to highlight when this type of semantically
incorrect empty buffer is read from an input network

Change-Id: Iac3bc71a2dbfda53737bbeb6e7f895552f0f13d0
Signed-off-by: Tim Hall <tim.hall@arm.com>

2 files changed, 18 insertions, 7 deletions

diff --git a/ethosu/vela/tflite_reader.py b/ethosu/vela/tflite_reader.py
index 2f3192b7..18b61e75 100644
--- a/ethosu/vela/tflite_reader.py
+++ b/ethosu/vela/tflite_reader.py
@@ -255,9 +255,23 @@ class TFLiteGraph:
 
             parsing_step = "parsing buffers length"
             self.buffers = []
-            for idx in range(model.BuffersLength()):
-                parsing_step = f"parsing buffer {idx}"
-                self.buffers.append(self.parse_buffer(model.Buffers(idx)))
+            if not model.BuffersIsNone():
+                for idx in range(model.BuffersLength()):
+                    parsing_step = f"parsing buffer {idx}"
+                    buffer = model.Buffers(idx)
+                    buffer_data = self.parse_buffer(buffer)
+                    # buffers can be either; empty, or contain no data (zero length), or contain data (non-zero length).
+                    # when a buffer is None it means that it is either empty or zero length, and an empty buffer
+                    # will have DataIsNone() equal to true.
+                    # we should detect zero length buffers and report a warning because the TFLite semantics for these
+                    # types of buffers changed in TensorFlow 2.11, whereby they could result in runtime errors
+                    if buffer_data is None and not buffer.DataIsNone():
+                        print(
+                            f"Warning: Input TensorFlow Lite network contains a zero length buffer (index = {idx})"
+                            f" which is semantically not empty. However, it will be treated as an empty buffer."
+                        )
+
+                    self.buffers.append(buffer_data)
 
             parsing_step = "parsing operator codes length"
             self.operator_codes = []
diff --git a/ethosu/vela/tflite_writer.py b/ethosu/vela/tflite_writer.py
index 8d44774b..625cf7cc 100644
--- a/ethosu/vela/tflite_writer.py
+++ b/ethosu/vela/tflite_writer.py
@@ -259,16 +259,13 @@ class TFLiteSerialiser:
             tens_shape = tens.original_shape
         values = tens.values
 
-        if values is None:
-            values = np.empty(shape=(0), dtype=np.uint8)
-
         if tens in self.tensors_to_reshape:
             reorder = self.tensors_to_reshape[tens]
             tens_shape = [tens_shape[idx] for idx in reorder]
             values = values.transpose(reorder)
 
         buf_id = self.buffer_map[tens]
-        self.buffers_to_write[buf_id] = values.flatten().view(np.uint8)
+        self.buffers_to_write[buf_id] = None if values is None else values.flatten().view(np.uint8)
 
         shape = self.write_int_vector(tens_shape)