This was a bug introduced in d764aa. A pointer type is not a primitive type, and thus we were ending up dividing by zero when computing VLMax.
Differential Revision: https://reviews.llvm.org/D128219
The costing we use for fixed length vector gather and scatter is to simply count up the memory ops, and multiply by a fixed memory op cost. For scalable vectors, we don't actually know how many lanes are active. Instead, we have to end up making a worst case assumption on how many lanes could be active. In the generic +V case, this results in very high costs, but we can do better when we know an upper bound on the VLEN.
There's some obvious ways to improve this - e.g. using information about VL and mask bits from the instruction to reduce the upper bound - but this seems like a reasonable starting point.
The resulting costs do bias us pretty strongly away from generating scatter/gather for generic +V. Without this, we'd be returning an invalid cost and thus definitely not vectorizing, so no major change in practical behavior expected.
Differential Revision: https://reviews.llvm.org/D127541
Simon Pilgrim