[CodeGen][LLVM] Lower if_then_else() with select

[wrongtest-intellif]

Hi, there. It seems better to use select directly for if_then_else() expr. Since if_then_else() is common in padding representation, it could hopefully save lots of cross-bb overheads.

My two remaining questions:
(1) Are there any places to test certain codegen features?
(2) Are there known llvm-based target which do not handle select instruction well?

[junrushao]

Quick question: in TIR, T.Select means both branches are executed no matter which value the conditional is evaluated to - which brings benefits that make it vectorizable in certain cases. However, if the conditional is a safeguard to avoid access out of buffer boundary, replacing it with T.Select will lead the program to crash. Therefore, I would love to learn if our implementation here in LLVM codegen will lead to the same effect? Thanks a lot!

[wrongtest-intellif]

the conditional is a safeguard

Thanks for the reply! So is the if_then_else also take short-circuit evaluation semantic like if stmt? If so I think my replacement is unsafe. What about add a check like SideEffect(e) < kReadState?

[wrongtest-intellif]

After some thinking, it seems to be better to make TIR transformation if_then_else() -> T.select if neccesary, thus do not complex the llvm codegen.

Thank you all the same! @junrushao1994

[junrushao]

@wrongtest Thank you so much for the discussion!