Add pass to merge consecutive view ops (#768)

Summary:
A sequence of view ops may break many of our transformations, where we assume a single view op.

Pull Request resolved: #768

Differential Revision: D46696966

Pulled By: int3

fbshipit-source-id: 92bdda891c2feb53821d51b8b6808e3d3544330c

python/aitemplate/compiler/transform/optimize_graph.py

@@ -48,6 +48,7 @@
 )
 from aitemplate.compiler.transform.split_large_split_ops import split_large_split_ops
 from aitemplate.compiler.transform.transform_memory_ops import transform_memory_ops
+from aitemplate.compiler.transform.transform_merge_view_ops import merge_view_ops
 from aitemplate.compiler.transform.transform_odd_alignment import (
     transform_odd_alignment,
 )
@@ -105,6 +106,7 @@ def optimize_graph(
         fuse_mm_reshape_permute,
         # make sure we run move_view_op_before_concat before transform_memory_ops
         move_view_op_before_concat,
+        merge_view_ops,
         transform_memory_ops,
         fuse_ops,
         fuse_elementwise,

python/aitemplate/compiler/transform/transform_merge_view_ops.py

@@ -0,0 +1,98 @@
+#  Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+"""
+This file implements a pass that merges consecutive view ops if possible.
+"""
+from typing import List, Set
+
+from aitemplate.compiler import ops
+from aitemplate.compiler.base import Operator, Tensor
+from aitemplate.compiler.transform import transform_utils
+from aitemplate.compiler.transform.toposort import toposort
+
+
+_VIEW_OPS = {"reshape", "flatten", "squeeze", "unsqueeze"}
+
+
+def _is_inout(t: Tensor):
+    return t._attrs["is_input"] or t._attrs["is_output"]
+
+
+def _merge_view_ops_for(graph: List[Tensor], tensor: Tensor) -> List[Tensor]:
+    """
+    `tensor` should have exactly 1 src op, and that op must be a view op. We
+    will look for view ops in the dst ops and merge them with the src view op
+    by creating a new reshape op.
+    """
+    src_op = tensor._attrs["src_ops"][0]
+    in_tensor = src_op._attrs["inputs"][0]
+    dst_ops = tensor._attrs["dst_ops"]
+    removed_ops: Set[Operator] = set()
+    for op in dst_ops:
+        if op._attrs["op"] not in _VIEW_OPS:
+            continue
+        out_tensor = op._attrs["outputs"][0]
+        in_shape = in_tensor._attrs["shape"]
+        out_shape = out_tensor._attrs["shape"]
+        if out_shape == in_shape and not (
+            _is_inout(in_tensor) and _is_inout(out_tensor)
+        ):
+            # If the shapes are identical, we can eliminate both view ops
+            transform_utils.replace_tensor(out_tensor, in_tensor)
+        else:
+            # Otherwise, create a new reshape op to replace the two view ops
+            new_out_tensor = ops.reshape()(in_tensor, out_shape)
+            if out_tensor._attrs["is_output"]:
+                new_out_tensor._attrs["is_output"] = True
+                new_out_tensor._attrs["name"] = out_tensor._attrs["name"]
+            transform_utils.replace_tensor(out_tensor, new_out_tensor)
+            graph.append(new_out_tensor)
+        graph.remove(out_tensor)
+        removed_ops.add(op)
+    for op in removed_ops:
+        transform_utils.remove_view_op_from_sorted_graph(op)
+    return graph
+
+
+def merge_view_ops(sorted_graph: List[Tensor], workdir: str = None) -> List[Tensor]:
+    """
+    Merge consecutive view ops.
+    """
+    changed = False
+    # Find pairs of consecutive view ops and merge them, iterating to a
+    # fixpoint.
+    # TODO: Instead of merging pairs of view ops, we should look for entire
+    # chains of view ops and merge them all at once.
+    while True:
+        for tensor in sorted_graph:
+            src_ops = tensor._attrs["src_ops"]
+            if len(src_ops) != 1:
+                continue
+            src_op = list(src_ops)[0]
+            if src_op._attrs["op"] not in _VIEW_OPS:
+                continue
+            dst_ops = tensor._attrs["dst_ops"]
+            if any(op._attrs["op"] in _VIEW_OPS for op in dst_ops):
+                # NOTE: _merge_view_ops_for does *not* return a sorted graph
+                sorted_graph = _merge_view_ops_for(sorted_graph, tensor)
+                changed = True
+                break
+        else:
+            break
+
+    if changed:
+        # Prune tensors that may have become unused after view op merging
+        return transform_utils.sanitize_sorted_graph(toposort(sorted_graph))
+    return sorted_graph