[LLVMdev] How to prevent LLVM from undoing a custom lowering

[Greg McGary]

My target has only logical shifts and lacks an arithmetic right shift 
instruction.
I have a custom LowerSRA function that rewrites SRA as SHL + SIGN_EXTEND
when the shift width is either constant 16 or 24.  Unfortunately, I 
observe that a
later pass combines the SHL + SIGN_EXTEND back into SRA so we crash.

The idea I had for defeating this behavior is lower to a target-specific
version of SHL and/or SIGN_EXTEND, rather than using the generic ISD::SHL
and/or ISD::SIGN_EXTEND.  Is that the recommended way to do it?

G

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