[Relax][ONNX] Handle empty Slice with negative step#19537
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ONNX Slice can produce an empty tensor when a negative step moves away from the end bound. For example, starts=[0], ends=[5], and steps=[-1] should produce a zero-length result instead of being rejected by Relax strided_slice legalization. Normalize statically empty constant Slice ranges before emitting Relax strided_slice, and add a frontend regression test matching ONNX Runtime behavior.
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This pull request introduces logic to handle statically empty ONNX slice ranges in the Relax frontend. It adds helper functions to extract static dimensions and canonicalize slice indices according to ONNX rules, ensuring that empty slices are correctly normalized for Relax's strided_slice. A new test case was added to verify this behavior. I have no feedback to provide.
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This PR fixes #19532.
ONNX
Slicecan produce an empty tensor when a negative step moves away from the end bound. For example,starts=[0],ends=[5], andsteps=[-1]should produce a zero-length result, matching ONNX Runtime.Previously, the Relax ONNX frontend passed those bounds directly to
relax.strided_slice. During legalization, TOPI rejected the range because the canonicalized end is greater than the begin for a negative stride.This PR normalizes statically empty constant
Sliceranges before emittingrelax.strided_slice, so Relax can represent the same zero-length result. It also adds a regression test for the negative-step empty result case.Testing:
pytest tests/python/relax/test_frontend_onnx.py::test_slice -qpytest tests/python/relax/test_frontend_onnx.py -k slice -qpytest tests/python/relax/test_op_index.py tests/python/relax/test_transform_legalize_ops_index_linear_algebra.py -k strided_slice -qpytest tests/python/relax/test_frontend_onnx.py -k "not test_all_reduce_funcs_axes_input and not test_topk" -qgit diff --checkpython -m py_compile python/tvm/relax/frontend/onnx/onnx_frontend.py tests/python/relax/test_frontend_onnx.pyNote: Full
test_frontend_onnx.py -qcurrently has 14 failures intest_all_reduce_funcs_axes_inputandtest_topk; the same failures reproduce on the cached baseline source, so they are unrelated to this patch.