CODEWITHSUNDEEP
rustllvmvolatilellvm-ir
Bharadwaj
Bharadwaj

Reputation: 745

Is there a way to prevent emitted LLVM IR from removing unused functions?

I am trying to analyse the LLVM-IR emitted by the rustc front end. The plan is to emit the IR for specific language elements. Is there such a list of elements and IR code template mapping or list?

The compiler is intelligent enough to remove the unused functions in the emitted IR as wel:, unless something is printed on to the console using println!, the compiler removes every function used.

This doesn't work as well, having said that x isn't used anywhere or also when x is overwritten.

let x = function();

Is there some sort of qualifier in Rust so that emitted IR retains all the functions?

Upvotes: 4

Views: 1015

Answers (1)

Matthieu M.
Matthieu M.

Reputation: 300429

Is there such a list of elements and IR code template mapping or list?

The rustc code.

It might seem tongue in the cheek, but it's actually the only answer available.

Rust's ABI is not stable specifically because the Rust developers wish to retain the ability to change these kinds of things whenever a better performing way of doing them appears.

This applies to the in-memory representation of structures, to the calling conventions, etc...

Is there some sort of qualifier in Rust so that emitted IR retains all the functions?

The simplest way to retain a function is:

  • compiling as a library and marking the function pub

It's also possible to use #[inline(never)] but this is more fragile as a smart linker could realize the function is never called. Making the symbol available externally forces the linker's hand into retaining it.

Upvotes: 5

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