[clang] [llvm] [BPF] introduce `__attribute__((bpf_fastcall))` (PR #101228)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Is there some reason this is an attribute, and not a calling convention, at the IR level?

And why would it not be named `__attribute__((fastcall))` when targeting BPF? (e.g., do we need a new calling convention at all?)

https://github.com/llvm/llvm-project/pull/101228