Subtensor lift: reason about advanced indices jointly when gating#2251
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ricardoV94
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Jun 27, 2026
| continue | ||
| if not _index_provably_not_larger(idx, inp.type.shape[axis], fgraph): | ||
| return None | ||
| adv_indices = [ |
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this should be called adv_indices and input_static_dim
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Generalize the duplicate-free reasoning used to gate the advanced-index write rewrites from a per-axis check to a joint one over the whole advanced index group, sharing the logic across subtensor.py and subtensor_lift.py. - _index_provably_unique: per-axis uniqueness, now also proving single-signed aranges and views (Reshape/DimShuffle) that preserve the value multiset. - _indices_jointly_unique: distinct joint coordinate tuples via all-axes uniqueness, a single Nonzero (e.g. tril_indices), or jointly-unique constants where no single axis is unique on its own. - _indices_provably_not_larger: bound a gather by the indexed axes' size using static shapes, per-axis bounds, or joint uniqueness.
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jessegrabowski
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Jul 7, 2026
| if _is_provably_positive(step): # ascending | ||
| if _is_provably_non_negative(start): # all >= 0 | ||
| return True | ||
| if stop_c is not None and stop_c <= 0: # all <= -1 |
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stop_c can't be symbolic? why not _is_provably_negative(stop_c) here
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Cuz we don't have a _is_provably_negative helper :)
| and step_c is not None | ||
| and step_c != 0 | ||
| ): | ||
| n = max(0, -(-(stop_c - start_c) // step_c)) # length, via ceil division |
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push in some of these minus signs
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no bug ever came from simple algebra
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bedside reading: https://discuss.python.org/t/integer-ceiling-divide/91269/13
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We had heuristics that work per-axis, but we should reason about the advanced indices together since they interact. E.g., tril_indices, would fail the per-axis checks, as each index (row, col) is larger than the respective axis and contains duplicates. But jointly they index less than the full array and don't have duplicates, so we want to lift it through Elemwise and the like.
Shows up in pymc-devs/pymc#8297