[DAG] isGuaranteedNotToBeUndefOrPoison - add ISD::BITCAST handling with DemandedElts

[RKSimon]

Currently ISD::BITCAST nodes fallback to the default implementation (which tests all elements), but we should be rescaling the input demanded elts to match the vector element count of the input operand, typically with APIntOps::ScaleBitMask but there will need to be special case handling of src/dst scalar types as well.

Test coverage will be the main issue

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Currently ISD::BITCAST nodes fallback to the default implementation (which tests all elements), but we should be rescaling the input demanded elts to match the vector element count of the input operand, typically with APIntOps::ScaleBitMask but there will need to be special case handling of src/dst scalar types as well.

Test coverage will be the main issue

[yichi170]

Hi @RKSimon, I would like to give it a try. Can you assign it to me?

[yichi170]

Hi @RKSimon, I've finished the implementation; however, I'm unsure about how to properly test the change. Right now, I’m only verifying the change in DemandedElts by printing it out for debugging.

Is there a way to test this through codegen so that the effect shows up in the output and can be checked with a lit test?

[RKSimon]

I did say tests would be the problem 😕

Are there no changes in 'ninja check-llvm-codegen' at all?

[yichi170]

Got it. I'll try to write a new test for it!

Are there no changes in 'ninja check-llvm-codegen' at all?

I checked all targets, and there is no change in codegen.