FlatTensor alignment tests (#16317)

Summary:

1. Pass segment_alignment from ExecutorchBackendConfig to flat tensor serializer
2. Set segment_alignment=128 as default to match ExecutorchBackendConfig
3. Remove tensor_alignment from the config (do not have multiple tensors per segment)
4. Additional tests for varying segment alignment.

Differential Revision: D89422691

Author: lucylq

exir/program/_program.py

@@ -596,7 +596,9 @@ def __init__(
             FlatTensorSerializer,
         )
 
-        self._data_serializer: DataSerializer = FlatTensorSerializer(FlatTensorConfig(tensor_alignment=4096, segment_alignment=4096))
+        self._data_serializer: DataSerializer = FlatTensorSerializer(
+            FlatTensorConfig(self._segment_alignment)
+        )
 
     def _get_emitter_output(self) -> EmitterOutput:
         if self._emitter_output is None:
@@ -1855,7 +1857,9 @@ def __init__(
             FlatTensorSerializer,
         )
 
-        self._data_serializer = FlatTensorSerializer()
+        self._data_serializer = FlatTensorSerializer(
+            FlatTensorConfig(segment_alignment=backend_config.segment_alignment)
+        )
         self._pte_data, self._tensor_data = serialize_for_executorch(
             self._emitter_output,
             backend_config,

extension/flat_tensor/serialize/serialize.py

@@ -39,6 +39,9 @@
 # endian.
 _HEADER_BYTEORDER: Literal["little"] = "little"
 
+# Alignment of the flatbuffer (after the header).
+_FLATBUFFER_ALIGNMENT: int = 16
+
 # Current version. Keep in sync with c++ version number in serialize.
 _FLAT_TENSOR_VERSION: int = 0
 
@@ -95,8 +98,7 @@ def _deserialize_to_flat_tensor(flatbuffer: bytes) -> FlatTensor:
 
 @dataclass
 class FlatTensorConfig:
-    tensor_alignment: int = 16
-    segment_alignment: int = 16
+    segment_alignment: int = 128
 
 
 @dataclass
@@ -334,18 +336,13 @@ def serialize(
         )
 
         flatbuffer_payload = _serialize_to_flatbuffer(flat_tensor)
-        padded_flatbuffer_length: int = aligned_size(
-            input_size=len(flatbuffer_payload),
-            alignment=self.config.tensor_alignment,
-        )
-
         padded_header_length: int = aligned_size(
             input_size=FlatTensorHeader.EXPECTED_LENGTH,
-            alignment=self.config.tensor_alignment,
+            alignment=_FLATBUFFER_ALIGNMENT,
         )
 
         segment_base_offset = aligned_size(
-            padded_flatbuffer_length + padded_header_length,
+            len(flatbuffer_payload) + padded_header_length,
             self.config.segment_alignment,
         )
 
@@ -360,19 +357,16 @@ def serialize(
 
         # Pad header and payload to segment alignment.
         header_data = pad_to(header_data, padded_header_length)
-        original_flatbuffer_payload_size = len(flatbuffer_payload)
-        flatbuffer_payload.append(
-            b"\x00" * (padded_flatbuffer_length - len(flatbuffer_payload))
-        )
         injected_flatbuffer_data: bytes = _insert_flatbuffer_header(
             flatbuffer_data=flatbuffer_payload.__bytes__(),
             magic_regex=r"FT[0-9a-zA-Z][0-9a-zA-Z]",
             header_data=header_data,
         )
+        injected_flatbuffer_data = pad_to(injected_flatbuffer_data, segment_base_offset)
 
         eh = _get_extended_header(injected_flatbuffer_data)
         assert eh is not None
-        assert eh.flatbuffer_size == original_flatbuffer_payload_size
+        assert eh.flatbuffer_size == len(flatbuffer_payload)
         assert eh.segment_base_offset == segment_base_offset
         assert eh.flatbuffer_offset == padded_header_length
         assert eh.segment_data_size == len(aggregated_segment_data)

extension/flat_tensor/test/test_serialize.py

@@ -29,6 +29,7 @@
 
 from executorch.extension.flat_tensor.serialize.serialize import (
     _deserialize_to_flat_tensor,
+    _FLATBUFFER_ALIGNMENT,
     FlatTensorConfig,
     FlatTensorHeader,
     FlatTensorSerializer,
@@ -109,8 +110,7 @@ def _check_named_data_entries(
                         f"Named data record {key}.{field.name} does not match.",
                     )
 
-    def test_serialize(self) -> None:
-        config = FlatTensorConfig()
+    def _serialize_with_alignment(self, config: FlatTensorConfig) -> None:
         serializer: DataSerializer = FlatTensorSerializer(config)
         serialized_data = bytes(serializer.serialize(TEST_DATA_PAYLOAD))
 
@@ -120,15 +120,15 @@ def test_serialize(self) -> None:
         )
         self.assertTrue(header.is_valid())
 
-        # Header is aligned to config.segment_alignment, which is where the flatbuffer starts.
+        # Flatbuffer is non-empty.
+        self.assertTrue(header.flatbuffer_size > 0)
+
+        # Align the flatbuffer to _FLATBUFFER_ALIGNMENT.
         self.assertEqual(
             header.flatbuffer_offset,
-            aligned_size(FlatTensorHeader.EXPECTED_LENGTH, config.segment_alignment),
+            aligned_size(FlatTensorHeader.EXPECTED_LENGTH, _FLATBUFFER_ALIGNMENT),
         )
 
-        # Flatbuffer is non-empty.
-        self.assertTrue(header.flatbuffer_size > 0)
-
         # Segment base offset is aligned to config.segment_alignment.
         expected_segment_base_offset = aligned_size(
             header.flatbuffer_offset + header.flatbuffer_size, config.segment_alignment
@@ -180,12 +180,12 @@ def test_serialize(self) -> None:
         segments = flat_tensor.segments
         self.assertEqual(len(segments), 3)
 
-        # Segment 0 contains fqn1, fqn2; 4 bytes, aligned to config.tensor_alignment.
+        # Segment 0 contains fqn1, fqn2; 4 bytes, aligned to config.segment_alignment.
         self.assertEqual(segments[0].offset, 0)
         self.assertEqual(segments[0].size, len(TEST_BUFFER[0]))
 
-        # Segment 1 contains fqn3; 32 bytes, aligned to config.tensor_alignment.
-        self.assertEqual(segments[1].offset, config.tensor_alignment)
+        # Segment 1 contains fqn3; 32 bytes, aligned to config.segment_alignment.
+        self.assertEqual(segments[1].offset, config.segment_alignment)
         self.assertEqual(segments[1].size, len(TEST_BUFFER[1]))
 
         # Segment 2 contains key0; 17 bytes, aligned to 64.
@@ -194,7 +194,7 @@ def test_serialize(self) -> None:
         )
         self.assertEqual(
             segments[2].offset,
-            aligned_size(config.tensor_alignment * 3, custom_alignment),
+            aligned_size(config.segment_alignment * 2, custom_alignment),
         )
         self.assertEqual(segments[2].size, len(TEST_BUFFER[2]))
 
@@ -245,6 +245,18 @@ def test_serialize(self) -> None:
 
         self.assertEqual(segments[2].offset + segments[2].size, len(segment_data))
 
+    def test_serialize_default_alignment(self) -> None:
+        config = FlatTensorConfig()
+        self._serialize_with_alignment(config)
+
+    def test_serialize_align_4096(self) -> None:
+        config = FlatTensorConfig(segment_alignment=4096)
+        self._serialize_with_alignment(config)
+
+    def test_serialize_align_1024(self) -> None:
+        config = FlatTensorConfig(segment_alignment=1024)
+        self._serialize_with_alignment(config)
+
     def test_round_trip(self) -> None:
         # Serialize and then deserialize the test payload. Make sure it's reconstructed
         # properly.