[llvm-dev] Prevent LLVM optimizations from erasing unused basic blocks (original) (raw)

Gleb Popov via llvm-dev llvm-dev at lists.llvm.org
Tue Oct 9 11:31:59 PDT 2018

Hello LLVM Devs.

In my compiler I attach some arbitrary data to functions by creating BBs with inline assembly. However, these blocks are "unused" from LLVM point of view and get erased from the function.

To counter that I started adding checks for conditions that are guaranteed to be true or false. I ended up with calling @llvm.returnaddress(i32 0) intrinsic and comparing the result with 0. It worked well until in one function I had two such calls and SROA replaced one of checks with constant 1 and erased the BB.

I should probably stop trying to fool LLVM and "do it right", but don't have any idea how. Note that I can't use global variables for a reason, so the data has to be encoded in a BB using inline assembly. All I need is just prevent optimizations from erasing it.

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