Register upper bound inference for index.Tensor (#1018)

Summary:
Pull Request resolved: #1018

Register upper bound inference for index.Tensor for HintBasedSymShapeEvalPass

Reviewed By: guangy10

Differential Revision: D49394788

fbshipit-source-id: 3820054c10448a9a9099e49b4f830abd56d872aa

Author: manuelcandales

exir/passes/sym_shape_eval_pass.py

@@ -32,6 +32,137 @@ def nonzero(args, kwargs) -> List[Optional[int]]:
     return [eval_expr(args[0].numel()), len(args[0].shape)]
 
 
+@register_upper_bound_inference(exir_ops.edge.aten.index.Tensor)
+@register_upper_bound_inference(torch.ops.aten.index.Tensor)
+def index_Tensor(args, kwargs) -> List[Optional[int]]:
+    tensor = args[0]
+    indices = args[1]
+
+    # Compute numbers of contiguous blocks of non-null indices.
+    # For example, if A, B, C, D, E are non-null tensors, then
+    # [None, None, A, B, None, C, D, E, None] has 2 blocks.
+    index_blocks = 0
+    in_block = False
+    for index in indices:
+        if index is not None:
+            if not in_block:
+                in_block = True
+                index_blocks += 1
+        else:
+            in_block = False
+
+    if index_blocks == 0:
+        # If no dimensions are actually being indexed, either because the indices list is empty
+        # or all indices are null, then the result is just the same as the input tensor.
+        return tensor.shape
+
+    adjacent = index_blocks == 1
+
+    # Number of leading null indices in the indices list.
+    num_leading_null_indices = 0
+    for index in indices:
+        if index is None:
+            num_leading_null_indices += 1
+        else:
+            break
+
+    # Number of null indices in total in the indices list.
+    num_null_indices = sum([ix is None for ix in indices])
+
+    # Number of dimensions being indexed (bool/byte tensors are treated as masks, and index as
+    # many input dimensions as their number of dimensions.
+    num_indexed_dims = 0
+    mask_indices = []
+    int_indices = []
+    for index in indices:
+        if index is not None:
+            if index.dtype in [torch.bool, torch.uint8]:
+                num_indexed_dims += index.dim()
+                mask_indices.append(index)
+            else:
+                num_indexed_dims += 1
+                int_indices.append(index)
+
+    broadcast_sizes = []
+    if len(int_indices) > 0:
+        # All of the integer index tensors (non-mask & non-null index tensors) need to broadcast.
+        # We need to compute the resulting shape.
+        curr_ndim = 0
+        rev_shape = []
+        for index in int_indices:
+            for j in range(index.dim()):
+                rev_j_size = eval_expr(index.size(index.dim() - j - 1))
+                if j >= curr_ndim:
+                    curr_ndim += 1
+                    rev_shape.append(rev_j_size)
+                elif rev_shape[j] == 1:
+                    rev_shape[j] = rev_j_size
+        broadcast_sizes = list(reversed(rev_shape))
+
+    # The number of True elements in the mask indices must broadcast (i.e some might be 1
+    # but the others must all be equal). They also need to broadcast with broadcast_sizes[0]
+    # Therefore, if broadcast_sizes[0] != 1, we don't need to worry about the mask indices,
+    # since we are assuming that the inputs are valid. However, if broadcast_sizes[0] = 1,
+    # we do need to consider them. We can't know how many True elements there are in each mask,
+    # but we know that the broadcasted size, can't be larger than the minimum number of True
+    # elements across all mask indices with a number of elements other than 1.
+    if len(mask_indices) > 0 and (len(broadcast_sizes) == 0 or broadcast_sizes[0] == 1):
+        upper_bound_broadcast_size = 1
+        intialized = False
+        for mask in mask_indices:
+            mask_numel = eval_expr(mask.numel())
+            if mask_numel != 1:
+                if intialized:
+                    assert isinstance(
+                        mask_numel, int
+                    ), "Expect mask_numel to be a concrete int"
+                    assert isinstance(
+                        upper_bound_broadcast_size, int
+                    ), "Expect upper_bound_broadcast_size to be a concrete int"
+                    if upper_bound_broadcast_size > mask_numel:
+                        upper_bound_broadcast_size = mask_numel
+                else:
+                    upper_bound_broadcast_size = mask_numel
+                    intialized = True
+        if len(broadcast_sizes) == 0:
+            broadcast_sizes.append(upper_bound_broadcast_size)
+        else:
+            broadcast_sizes[0] = upper_bound_broadcast_size
+
+    broadcast_ndim = len(broadcast_sizes)
+
+    out_ndim = tensor.dim() + broadcast_ndim - num_indexed_dims
+    out_sizes: List[Optional[int]] = [0 for _ in range(out_ndim)]
+
+    if adjacent:
+        for i in range(num_leading_null_indices):
+            out_sizes[i] = eval_expr(tensor.size(i))
+        for i in range(broadcast_ndim):
+            out_sizes[i + num_leading_null_indices] = broadcast_sizes[i]
+        for i in range(num_indexed_dims + num_leading_null_indices, tensor.dim()):
+            out_sizes[i + broadcast_ndim - num_indexed_dims] = eval_expr(tensor.size(i))
+    else:
+        for i in range(broadcast_ndim):
+            out_sizes[i] = broadcast_sizes[i]
+        in_i = 0
+        out_i = broadcast_ndim
+        for index in indices:
+            if index is None:
+                out_sizes[out_i] = eval_expr(tensor.size(in_i))
+                out_i += 1
+                in_i += 1
+            else:
+                if index.dtype in [torch.bool, torch.uint8]:
+                    in_i += index.dim()
+                else:
+                    in_i += 1
+
+        for i in range(num_indexed_dims + num_null_indices, tensor.dim()):
+            out_sizes[i + broadcast_ndim - num_indexed_dims] = eval_expr(tensor.size(i))
+
+    return out_sizes
+
+
 class HintBasedSymShapeEvalPass(PassBase):
     """
     If we enable dynamic shape tracing, a tensor's shape may become a symbolic