fix: support masked_scatter by lowering path and corner case of masked_scatter

Author: chohk88

py/torch_tensorrt/dynamo/conversion/converter_utils.py

@@ -344,6 +344,10 @@ def create_constant(
     with unset_fake_temporarily():
 
         torch_value = to_torch(value, dtype)
+        if torch_value is None:
+            raise ValueError(
+                f"Cannot convert tensor '{name}' to a TensorRT constant because its value is None."
+            )
         if torch_value.dtype == torch.float64:
             raise ValueError(
                 "TensorRT does not support float64 (double) precision. To resolve this, please set truncate_double=True in your compilation settings and re-run the model."
@@ -1065,3 +1069,42 @@ def load_tensorrt_llm() -> bool:
             )
             return False
     return False
+
+
+def promote_trt_tensors_to_same_dtype(
+    ctx: ConversionContext, lhs: TRTTensor, rhs: TRTTensor, name_prefix: str
+) -> tuple[TRTTensor, TRTTensor]:
+    """
+    Promotes two TensorRT tensors to a common data type to ensure type compatibility
+    during operations (e.g., select, where, etc.), following simplified PyTorch promotion rules.
+
+    Args:
+        ctx: Conversion context containing the TRT network definition.
+        lhs: The left-hand-side TensorRT tensor.
+        rhs: The right-hand-side TensorRT tensor.
+        name_prefix: A prefix string used to name any cast operations.
+
+    Returns:
+        A tuple of (lhs_cast, rhs_cast) TensorRT tensors, both cast to the promoted dtype.
+    """
+
+    # Define supported float types (TensorRT supports float16 and float32)
+    float_types = {trt.float16, trt.float32}
+
+    # Case 1: If either tensor is a float, promote to the wider float type
+    if lhs.dtype in float_types or rhs.dtype in float_types:
+        # Prefer float32 if either tensor is float32
+        if lhs.dtype == trt.float32 or rhs.dtype == trt.float32:
+            promoted_dtype = trt.float32
+        else:
+            promoted_dtype = trt.float16
+    else:
+        # Case 2: If both tensors are int types (e.g., int32, int64), promote to int32
+        # (Note: TensorRT does not support int64 for many ops like select/where)
+        promoted_dtype = trt.int32
+
+    # Cast both tensors to the promoted dtype
+    lhs_cast = cast_trt_tensor(ctx, lhs, promoted_dtype, f"{name_prefix}lhs_cast")
+    rhs_cast = cast_trt_tensor(ctx, rhs, promoted_dtype, f"{name_prefix}rhs_cast")
+
+    return lhs_cast, rhs_cast

py/torch_tensorrt/dynamo/conversion/impl/condition/ops.py

@@ -11,6 +11,7 @@
     cast_trt_tensor,
     get_trt_tensor,
     prepend_ones,
+    promote_trt_tensors_to_same_dtype,
     set_layer_name,
 )
 from torch_tensorrt.dynamo.conversion.impl.elementwise import ne
@@ -57,6 +58,9 @@ def where(
     if diff > 0:
         other = prepend_ones(ctx, other, f"{name}_other_broadcast", diff)
 
+    # Ensure that input and other have the same TRT dtype
+    input, other = promote_trt_tensors_to_same_dtype(ctx, input, other, name)
+
     return select(ctx, target, source_ir, name, input, other, condition)
 
 

py/torch_tensorrt/dynamo/conversion/impl/select.py

@@ -462,6 +462,7 @@ def gather(
 ) -> TRTTensor:
     input_shape = input.shape
     dim = get_positive_dim(dim, len(input_shape))
+    index = cast_trt_tensor(ctx, index, trt.int32, name + "_cast_index_tensor")
     gather_layer = ctx.net.add_gather(input, index, axis=dim)
     gather_layer.mode = trt.GatherMode.ELEMENT
     set_layer_name(gather_layer, target, name + "_gather_layer_element", source_ir)

py/torch_tensorrt/dynamo/lowering/_decompositions.py

@@ -196,8 +196,10 @@ def slice_scatter_decomposition(
 ) -> torch.Tensor:
     dim_size = input_tensor.shape[dim]
     device_input_tensor = input_tensor.device
+
+    start = 0 if start is None else start  # Ensure start is int
     start = get_positive_dim(start, input_tensor.shape[dim])
-    if end is None:
+    if end is None:  # Ensure end is int
         end = dim_size
     end = get_positive_dim(end, input_tensor.shape[dim])
     if step is None:
@@ -575,6 +577,53 @@ def cudnn_grid_sampler_decomposition(
     return torch.grid_sampler_2d(x, grid, 0, 0, True)
 
 
+@register_torch_trt_decomposition(
+    aten.masked_scatter, registry=TORCH_TRT_DECOMPOSITIONS
+)
+def masked_scatter_decomposition(
+    input: torch.Tensor,
+    mask: torch.Tensor,
+    source: torch.Tensor,
+) -> torch.Tensor:
+    """
+    Decomposition of `aten.masked_scatter` for TensorRT.
+
+    Emulates the behavior of `input[mask] = source` using only TensorRT-compatible ops.
+
+    Steps:
+      1) Broadcast `input` and `mask` to a common shape.
+      2) Flatten all tensors for uniform indexing.
+      3) Compute gather indices for `source` by applying cumsum to the boolean mask.
+         - Use `masked_fill` to avoid invalid indices in positions where `mask` is False.
+      4) Gather values from `source` at valid positions.
+      5) Use `torch.where` to insert gathered values into `input` where `mask` is True.
+      6) Reshape the result back to the original broadcasted shape.
+    """
+
+    # 1) Broadcast input and mask to the same shape
+    input_b, mask_b = aten.broadcast_tensors([input, mask])
+
+    # 2) Flatten tensors for element-wise operations
+    input_flat = input_b.flatten()
+    mask_flat = mask_b.flatten()
+    source_flat = source.flatten()
+
+    # 3) Compute gather indices from cumsum of the mask
+    # Subtract 1 so that the first True position maps to index 0 in source
+    source_idx = mask_flat.cumsum(0) - 1
+    # Set gather index to 0 where mask is False (these will be ignored later)
+    safe_idx = source_idx.masked_fill(~mask_flat, 0)
+
+    # 4) Gather values from source using computed indices
+    gathered = source_flat.gather(0, safe_idx)
+
+    # 5) Replace masked positions in input with gathered values
+    replaced = torch.where(mask_flat, gathered, input_flat)
+
+    # 6) Reshape the result to match the original broadcasted shape
+    return replaced.view(input_b.shape)
+
+
 def get_decompositions(
     enable_experimental_decompositions: bool = False,
 ) -> Dict[OpOverload, Callable[[Any], Any]]:

tests/py/dynamo/lowering/test_decompositions.py

@@ -2167,6 +2167,86 @@ def forward(self, x, grid):
             msg="Cudnn_grid_sampler TRT outputs don't match with the original model.",
         )
 
+    @parameterized.expand(
+        [
+            ("float32_2d", torch.float32, (4, 4)),
+            ("float16_3d", torch.float16, (2, 3, 4)),
+        ]
+    )
+    def test_masked_scatter(self, _, dtype, shape):
+        """
+        Test that masked_scatter.default is correctly decomposed into
+        (cumsum, gather, where, etc.) and that final TRT results match PyTorch.
+        """
+
+        class TestModule(torch.nn.Module):
+            def __init__(self):
+                super().__init__()
+
+            def forward(self, x, mask, source):
+                return torch.ops.aten.masked_scatter.default(x, mask, source)
+
+        x = torch.randn(*shape, dtype=dtype, device="cuda")
+
+        mask = torch.rand(*shape, device="cuda") > 0.5
+        num_trues = mask.sum().item()
+        if num_trues == 0:
+            mask[0] = True
+            num_trues = 1
+        source = torch.arange(num_trues, dtype=dtype, device="cuda")
+
+        inputs = [x, mask, source]
+
+        fx_graph = torch.fx.symbolic_trace(TestModule())
+
+        expected_ops = {
+            torch.ops.aten.where.self,
+            torch.ops.aten.gather.default,
+            torch.ops.aten.cumsum.default,
+        }
+        unexpected_ops = {torch.ops.aten.masked_scatter.default}
+
+        unexpected_ops_seen, expected_ops_unseen = lower_graph_testing(
+            fx_graph,
+            inputs,
+            expected_ops=expected_ops,
+            unexpected_ops=unexpected_ops,
+            min_block_size=1,
+        )
+
+        self.assertEqual(
+            len(unexpected_ops_seen),
+            0,
+            f"The following unexpected ops were encountered: {unexpected_ops_seen}",
+        )
+
+        self.assertEqual(
+            len(expected_ops_unseen),
+            0,
+            f"The following expected ops were not encountered: {expected_ops_unseen}",
+        )
+
+        torch._dynamo.reset()
+
+        trt_model = torch_tensorrt.compile(
+            fx_graph,
+            "torch_compile",
+            inputs,
+            min_block_size=1,
+            pass_through_build_failures=True,
+        )
+        with torch.no_grad():
+            trt_results = trt_model(*inputs).detach().cpu()
+            torch_results = fx_graph(*inputs).detach().cpu()
+
+        max_diff = float(torch.max(torch.abs(trt_results - torch_results)))
+        self.assertAlmostEqual(
+            max_diff,
+            0,
+            DECIMALS_OF_AGREEMENT,
+            f"Masked_scatter TRT outputs don't match with the original model. (diff={max_diff})",
+        )
+
 
 if __name__ == "__main__":
     run_tests()