[llvm] [BPF] expand mem intrinsics (memcpy, memmove, memset) (PR #97648)

[Yingchi Long via llvm-commits]

inclyc wrote:

Hi @eddyz87,

Thanks for testing this PR in the kernel! :heart:
 
> Because of the way kernel BPF verifier works, the unrolled version is preferable to the loop (verifier traces execution, so processing a loop would take more instruction budget). Is it possible to make `llvm::expandMemMoveAsLoop()` respect the limits set by `MaxStoresPerMemset`?

I'd like to figure out how to, and I kindly ask if "memset" is unrolled then it is friendly to the verifier, what should we do if the memset is called with dynamic length? (Expand it, or report error?)

https://github.com/llvm/llvm-project/pull/97648