[InstCombine] Optimize invalid `addrspacecast` to `poison`

[justinfargnoli]

I think a good follow up would be using InstCombiner::isValidAddrSpaceCast in InstCombine to convert invalid casts to poison, though there may be something I'm missing about the semantics here.

Originally posted by @AlexMaclean in #125607 (review)

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Justin Fargnoli (justinfargnoli)

I think a good follow up would be using `InstCombiner::isValidAddrSpaceCast` in InstCombine to convert invalid casts to poison, though there may be something I'm missing about the semantics here.

Originally posted by @AlexMaclean in #125607 (review)

[llvmbot]

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Justin Fargnoli (justinfargnoli)

I think a good follow up would be using `InstCombiner::isValidAddrSpaceCast` in InstCombine to convert invalid casts to poison, though there may be something I'm missing about the semantics here.

Originally posted by @AlexMaclean in #125607 (review)

[justinfargnoli]

To be precise, the request is to implement:

visitAddrSpaceCast(AddrSpaceCastInst &CI) {
  if (!isValidAddrSpaceCast(FromAS, ToAS)) {
    Value *PoisonPtr = PoisonValue::get(PointerType::get(Context, ToAS));
    return PoisonPtr
  }
  ...

Links:

• isValidAddrSpaceCast()
• visitAddrSpaceCast()

[arsenm]

Is there a practical reason to try optimizing these? It's certainly a bug in the program or compiler if these appear

[nikic]

!isValidAddrSpaceCast != isInvalidAddrSpaceCast. The default implementation is literally "return false", i.e. don't assume anything about which addrspacecasts are valid.