[llvm] [BPF] Define empty set of BPF libcalls (PR #169537)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I mean sure in an ideal world, but that's not how it works today, and this is going to break things that use the bpf target and bring their own libcalls (eg aya).

I don't understand how anyone could be relying on libcalls now with BPF. The current behavior is to emit a hard error on any case that would require a call.

> I don't understand why libcalls shouldn't be allowed tho. 

They could be, but they should be a defined ABI and not just emergent behavior from LLVM's poor defaults. They weren't actually supported by the backend previously, they were just an implementation detail of the error path

> sounds like we should expand if the libcall isn't there?

That is what the legalizer is supposed to 




https://github.com/llvm/llvm-project/pull/169537