nlewycky added a comment.
For the IR level, I think this has got to be valid:
declare void @bar(i32* byval %p)
define void @foo(i32* byval %p) {
tail call void @bar(i32* byval %p.tmp)
ret void
}
The `tail` is an aliasing property which indicates that the callee doesn't
touch any of the alloca's in the caller, a rough proxy for "stack" since there
is no other stack in LLVM IR. That doesn't mean that we're going to actually
lower it to a tail call in the final assembly, but if we don't put `tail` on a
call, we won't even consider it for becoming a tail call. I think the backend
can sort out whether the byval is going to lead to a stack allocation in @foo
and emit a non-tail call if so.
https://reviews.llvm.org/D22900
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