[all-commits] [llvm/llvm-project] d764aa: [RISCV] Add cost model for scalable scatter and ga...
Philip Reames via All-commits
all-commits at lists.llvm.org
Thu Jun 16 14:22:46 PDT 2022
Author: Philip Reames <preames at rivosinc.com>
Date: 2022-06-16 (Thu, 16 Jun 2022)
[RISCV] Add cost model for scalable scatter and gather
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
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