[llvm-dev] how to generate select-free LLVM IR?

[Robert Henry via llvm-dev]

I run "clang ... --emit-llvm ....".  I get LLVM IR select instructions out. I don't want select instructions, but want explicit control flow.

Is there a way to do this?

I'm scared of disabling the SimplifyCFG pass since it appears to do much more than just rewrite to use select statements.

[Eli Friedman via llvm-dev]

Probably you want to a pass to transform select instructions to control flow late, after optimizations have run.  Trying to prevent clang IR generation and IR optimizations from forming selects is way too difficult.

 

I don’t think this transform exists as an independent pass, but you can borrow code from CodeGenPrepare::optimizeSelectInst.

 

-Eli

[Maxime Arthaud via llvm-dev]

Hi Robert,

You could write your own LLVM pass that lowers select instructions.

Many static analyzers based on LLVM actually do that. See for instance
in IKOS:
https://github.com/NASA-SW-VnV/ikos/blob/master/frontend/llvm/src/pass/lower_select.cpp

Best,

--
Maxime Arthaud

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