[PATCH] D132185: [TTI][AArch64] Update vector extract cost for Neoverse-N1.

[Dave Green via Phabricator via llvm-commits]

dmgreen added reviewers: david-arm, fhahn, peterwaller-arm.
dmgreen added a comment.

I've tried setting the default VectorInsertExtractBaseCost to 2 in the past, but only ever seen performance regressions. From what I remember they were large and persuasive enough to discourage me from considering it any further.

The VectorInsertExtractBaseCost (the getVectorInstrCost) to an extent is not really a measure of the throughput of a vector<>gpr moves. It is more importantly a control of how much vector shuffling you are will to accept at the expense of just using scalar code. On a machine like N1 with 4 scalar pipelines and 2 SIMD, I'm not sure that it makes a lot of sense to more aggressively target SIMD.

The costmodel under AArch64 is pretty rough in places, and codegen is slowly getting better over time, so there is certainly room for improvement, but I think it would take quite a lot to convince me that this is the right way forward.


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https://reviews.llvm.org/D132185