[PATCH] D92760: [SelectionDAG] Implement SplitVecOp_INSERT_SUBVECTOR

[Paul Walker via Phabricator via llvm-commits]

paulwalker-arm added a comment.

In D92760#2442179 <https://reviews.llvm.org/D92760#2442179>, @david-arm wrote:

> If you can find a test that exposes the need for this split function with generic IR that would be really helpful I think - LLVM has managed for a long time without needing this split function so something has changed. I suspect it's probably just the new intrinsic that now exposes this code path.

Personally I think using the intrinsic as the test is better than trying to rely on an exact sequence of unluckiness to generate the failure.

For information I believe scalable vectors is what has changed.  Specifically the changes we've made to INSERT_SUBVECTOR to allow a fixed-length vector to be extracted from/inserted into a scalable vector.  Before this you'd only ever insert "fixed into fixed" or "scalable into scalable".  Given the nature of INSERT_SUBVECTOR the operand is always smaller than the result and thus if the operand type is illegal then the result type must also be illegal, in which case SplitVecRes_INSERT_SUBVECTOR will handle type legalisation.  However by allowing mixed vector types we've opened up the possibility for extracting an illegal vector type, that requires splitting to be made legal, from a legal one.


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https://reviews.llvm.org/D92760