llvm dev - Aug 2019 - how to generate select-free LLVM IR?

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.

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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

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Robert
Henry via llvm-dev
Sent: Thursday, August 1, 2019 4:54 PM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] how to generate select-free LLVM IR?

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.

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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