[llvm] [BPF] Allow libcalls behind a feature gate (PR #168442)

[Lucas Ste via llvm-commits]

LucasSte wrote:

> This is the wrong solution to the problem. BPF isn't correctly configuring its set of RuntimeLibcalls. If you removed the function from the default set, you should hit the software expansion without hitting the external call.

Are you suggesting that we create an entry in `RuntimeLibcalls.td` to specify which lib calls are not available in BPF?

> Whether or not the libcall is available should be treated as an ABI property, preferably directly implied by the triple without a function-level control like a subtarget feature

When building a Rust program for BPF we do build the compiler-builtins crate, so ultimately we link everything together. Should we have a specific setting to identify we are building a program for Rust?


https://github.com/llvm/llvm-project/pull/168442