Enable output padding when only outermost dim is dynamic (#159404)

nandesuka · facebook-github-bot · commit d3bea2e33959 · 2025-08-07T06:26:55.000-07:00
Summary:

When the shape of the output tensor has a dynamic outer most dim, the stride can still be padded to conform to configured alignment if specified by padding config.

Test Plan:
CI

Rollback Plan:

Reviewed By: blaine-rister, eellison

Differential Revision: D79146886
diff --git a/test/inductor/test_padding.py b/test/inductor/test_padding.py
@@ -49,6 +49,18 @@ def geninp():
     return input_dict
 
 
+def get_padded_stride(shape, alignment_bytes, pad_output, itemsize):
+    align = alignment_bytes // itemsize
+    new_strides = [0 for _ in range(len(shape))]
+    new_strides[len(shape) - 1] = 1
+    for i in range(len(shape) - 1, 0, -1):
+        stride = shape[i] * new_strides[i]
+        if pad_output and stride % align != 0:
+            stride = (stride + align - 1) // align * align
+        new_strides[i - 1] = stride
+    return tuple(new_strides)
+
+
 class LinearAndSoftmax(nn.Module):
     """
     It's very common that a transformer model will do a matmul and then
@@ -745,20 +757,11 @@ def get_input(size: tuple[int], alignment_bytes: int) -> torch.Tensor:
         input_tensors = [get_input(shape, alignment_bytes) for _ in range(num_inputs)]
 
         config_patches = {
-            "compile_threads": 1,
             "comprehensive_padding": pad_output,
             "cpu_backend": "triton",
-            "disable_padding_cpu": False,
-            "implicit_fallbacks": False,
-            "inplace_buffers": False,
             "padding_alignment_bytes": alignment_bytes,
-            "pad_channels_last": True,
             "pad_outputs": True,
             "padding_stride_threshold": 0,
-            "triton.prefer_nd_tiling": True,
-            "triton.use_block_ptr": True,
-            "triton.codegen_upcast_to_fp32": False,
-            "unroll_reductions_threshold": 1,
         }
         with config.patch(config_patches):
             compiled = torch.compile(torch.cat)
@@ -767,7 +770,89 @@ def get_input(size: tuple[int], alignment_bytes: int) -> torch.Tensor:
         output_shape = (shape[0] * num_inputs, shape[1])
         output_stride = input_tensors[0].stride()
         output_line = f"buf12 = empty_strided_{GPU_TYPE}({output_shape}, {output_stride}, torch.float32)"
-        self.assertTrue(any(output_line in line for line in code))
+        self.assertTrue(output_line in code[0])
+
+    @parametrize(
+        "shape,alignment_bytes,pad_output",
+        [
+            ((512, 1), 32, False),
+            ((512, 1), 32, True),
+            ((32, 30), 64, False),
+            ((32, 30), 64, True),
+            ((512, 100, 1), 32, False),
+            ((512, 100, 1), 32, True),
+            ((32, 50, 30), 64, False),
+            ((32, 50, 30), 64, True),
+        ],
+    )
+    def test_outer_dynamic_shape_padding(self, shape, alignment_bytes, pad_output):
+        """
+        When only the outter most dim is dynamic shape, the output can still be padded up
+        based on padding configuration.
+        """
+        num_inputs = 2
+        input_tensors = [
+            torch.randn(shape, dtype=torch.float32) for _ in range(num_inputs)
+        ]
+
+        config_patches = {
+            "comprehensive_padding": pad_output,
+            "cpu_backend": "triton",
+            "padding_alignment_bytes": alignment_bytes,
+            "pad_outputs": True,
+            "padding_stride_threshold": 0,
+        }
+        with config.patch(config_patches):
+            torch._dynamo.mark_dynamic(input_tensors[0], 0)
+            torch._dynamo.mark_dynamic(input_tensors[1], 0)
+            compiled = torch.compile(torch.add)
+            result, _ = run_and_get_code(compiled, *input_tensors)
+
+        expected_stride = get_padded_stride(
+            result.shape, alignment_bytes, pad_output, result.dtype.itemsize
+        )
+        self.assertEqual(result.stride(), expected_stride)
+
+    @parametrize(
+        "shape,alignment_bytes,pad_output",
+        [
+            ((500, 10, 1), 32, False),
+            ((500, 20, 1), 32, True),
+            ((30, 10, 20), 64, True),
+            ((30, 10, 20), 64, False),
+        ],
+    )
+    def test_perm_outer_dynamic_shape_padding(self, shape, alignment_bytes, pad_output):
+        """
+        When only the outter most dim is dynamic shape, the output can still be padded up
+        based on padding configuration. Test when this occurs after a permute op.
+        """
+
+        def permute_contig(x):
+            return torch.transpose(x, 0, 2).contiguous()
+
+        num_inputs = 1
+        input_tensors = [
+            torch.randn(shape, dtype=torch.float32) for _ in range(num_inputs)
+        ]
+
+        config_patches = {
+            "comprehensive_padding": pad_output,
+            "cpu_backend": "triton",
+            "padding_alignment_bytes": alignment_bytes,
+            "pad_outputs": True,
+            "padding_stride_threshold": 0,
+            "triton.use_block_ptr": True,
+        }
+        with config.patch(config_patches):
+            torch._dynamo.mark_dynamic(input_tensors[0], 2)
+            compiled = torch.compile(permute_contig)
+            result, _ = run_and_get_code(compiled, *input_tensors)
+
+        expected_stride = get_padded_stride(
+            result.shape, alignment_bytes, pad_output, result.dtype.itemsize
+        )
+        self.assertEqual(result.stride(), expected_stride)
 
 
 if __name__ == "__main__":
diff --git a/torch/_inductor/ir.py b/torch/_inductor/ir.py
@@ -3707,10 +3707,8 @@ def _pad_strides(
         # do for dynamic shape.
         #
         # Skip padding the strides for dynamic shape for now.
-        if not all(
-            isinstance(s, (int, sympy.Integer))
-            for s in itertools.chain(in_strides, size)
-        ):
+        # If outermost dim is dynamic, stride still can be fully static
+        if not all(isinstance(s, (int, sympy.Integer)) for s in in_strides):
             return in_strides
 
         stride_order = get_stride_order(in_strides)
@@ -3725,11 +3723,11 @@ def _pad_strides(
         for rank, idx in enumerate(fill_order[1:], start=1):
             prev_idx = fill_order[rank - 1]
             stride = new_strides[prev_idx] * size[prev_idx]
-
-            if stride > config.padding_stride_threshold and stride % align != 0:
-                stride = ceildiv(stride, align) * align
-                padded = True
-            new_strides[idx] = stride
+            if isinstance(stride, (int, sympy.Integer)):
+                if stride > config.padding_stride_threshold and stride % align != 0:
+                    stride = ceildiv(stride, align) * align
+                    padded = True
+                new_strides[idx] = stride
 
         if not padded:
             # Consider a tensor with shape [256, 1, 5, 5]
