better implementation of pruning

python/aitemplate/compiler/transform/transform_merge_view_ops.py

@@ -30,7 +30,7 @@ def _is_inout(t: Tensor):
     return t._attrs["is_input"] or t._attrs["is_output"]
 
 
-def _merge_view_ops_for(sorted_graph: List[Tensor], tensor: Tensor) -> List[Tensor]:
+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
@@ -58,11 +58,11 @@ def _merge_view_ops_for(sorted_graph: List[Tensor], tensor: Tensor) -> List[Tens
                 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)
-            sorted_graph.append(new_out_tensor)
-        sorted_graph.remove(out_tensor)
+            graph.append(new_out_tensor)
+        graph.remove(out_tensor)
         removed_ops.add(op)
     dst_ops -= removed_ops
-    return sorted_graph
+    return graph
 
 
 def merge_view_ops(sorted_graph: List[Tensor], workdir: str = None) -> List[Tensor]:
@@ -84,6 +84,7 @@ def merge_view_ops(sorted_graph: List[Tensor], workdir: str = None) -> List[Tens
                 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
@@ -92,8 +93,6 @@ def merge_view_ops(sorted_graph: List[Tensor], workdir: str = None) -> List[Tens
 
     if changed:
         # Prune tensors that may have become unused after view op merging
-        for t in sorted_graph:
-            if len(t._attrs["dst_ops"]) == 0 and not t._attrs["is_output"]:
-                transform_utils.remove_tensor_from_sorted_graph(t)
-        return transform_utils.sanitize_sorted_graph(toposort(sorted_graph))
+        sorted_graph = toposort([t for t in sorted_graph if t._attrs["is_output"]])
+        return transform_utils.sanitize_sorted_graph(sorted_graph)
     return sorted_graph