Comment by fuhsnn
1 year ago
>[[musttail]] also ends the lifetime of local objects AFAICS.
That's good to know. I had this github issue [0] in the back of my mind, as well as witnessing occasions of clang turning [[musttail]] into inner loops, and concluded clang's implementation must be more sophisticated than simply replacing calls with jumps. Just a little paranoia from trying to be serious with compiler dev[1]: fulfilling a laid-out spec feels more sound versus imitating something out there.
>this constraint is unnecessarily strict
I would agree, at least for x86 psABI, it can be pretty elaborative as long as the return value is the same register and argument stack don't exceed what's provided. Debug/profiling side might hate it, though.
[0] https://github.com/llvm/llvm-project/issues/72555 [1] https://github.com/fuhsnn/slimcc/
I certainly understand your caution. I don't have intimate expertise with the implementation of musttail in the backend -- when I implemented musttail in Clang, it was piggybacking on an existing attribute from the LLVM IR: https://llvm.org/docs/LangRef.html#call-instruction
That said, my rough understanding is that a tail call ends the lifetime of all objects in the old stack frame. It follows that it is UB to access any objects from the previous stack frame after a tail call, and that would include Gerben's first example in https://github.com/llvm/llvm-project/issues/72555
Your slimcc project looks really interesting, thanks for the pointer.