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tlc-pack / relax

Apache License 2.0
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[Bug] Match Cast Missing After Legalization #437

Open zxybazh opened 1 year ago

zxybazh commented 1 year ago

When we were trying to get Expand operator supported, we tried to legalize broadcast_to op and found that the operator would fail well formed check after legalization. The cause is match cast went missing during legalization.

The reproducible script is here: https://gist.github.com/zxybazh/5b09bad2cf13ea7d15329a86b03a6abf

CC @MasterJH5574 @tqchen

MasterJH5574 commented 1 year ago

LegalizeOps does a step of RemoveAllUnused here. https://github.com/tlc-pack/relax/blob/ce5c7f4117e4fcc33894a1449f751f32f8e1460e/python/tvm/relax/transform/legalize_ops.py#L777

It looks to me that the RemoveAllUnused regards the MatchCast as an unused binding, and thereby removes it. While in fact, the MatchCast is a binding with side-effect (where the side-effect is “defining x_0 and x_1).

As a workaround, you can probably try removing the step of RemoveAllUnused. In the long term, I think RemoveAllUnused should be updated to skip the bindings with side-effects, or we introducing a better DCE pass.

tqchen commented 1 year ago

This should be a bug of remove unused not considering dep through symbolic variables and we should fix it. As a quick fix, we can avoid removing match cast for now. Then apply deeper analysis later