[PATCH] D99502: [InstructionCost] Don't conflate Invalid costs with Unknown costs.

[Sander de Smalen via Phabricator via llvm-commits]

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We previously made a change to getUserCost to return a Invalid cost
when one of the TTI costs returned '-1' (meaning 'unknown' or
'infinitely expensive'). It makes no sense to say that:

  shufflevector <2 x i8> %x, <2 x i8> %y, <4 x i32> <i32 0, i32 1, i32 2, i32 3>

has an invalid cost. Perhaps the cost is not known, but the IR is valid
and can be code-generated. Invalid should only be used for IR that
cannot possibly be code-generated and where a cost is nonsensical.

With more passes now asserting that the cost must be valid, it is possible
that those assertions will fail for perfectly valid IR. An incomplete
cost-model probably shouldn't be a reason for the compiler to break.

It's better to consider these costs as 'very expensive' and ignore them
for other reasons. At some point, we should consider replacing -1 with
some other mechanism.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D99502

Files:
  llvm/include/llvm/Analysis/TargetTransformInfo.h
  llvm/lib/Analysis/CostModel.cpp
  llvm/test/Analysis/CostModel/AMDGPU/br.ll
  llvm/test/Analysis/CostModel/AMDGPU/shufflevector.ll
  llvm/test/Analysis/CostModel/ARM/shuffle.ll
  llvm/test/Analysis/CostModel/X86/shuffle-extract_subvector.ll
  llvm/test/Analysis/CostModel/X86/shuffle-insert_subvector.ll

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