[llvm] [LangRef] Disallow accessing byval arguments from tail-called functions (PR #110093)

[Eli Friedman via llvm-commits]

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@@ -12658,10 +12658,55 @@ This instruction requires several arguments:
       the return value of the callee is returned to the caller's caller, even
       if a void return type is in use.
 
-   Both markers imply that the callee does not access allocas from the caller.
-   The ``tail`` marker additionally implies that the callee does not access
-   varargs from the caller. Calls marked ``musttail`` must obey the following
-   additional  rules:
+   Both markers imply that the callee does not access allocas or varargs from
+   the caller. They also imply that the callee does not access the memory
+   pointed to by a byval argument to the caller, unless that pointer is passed
+   to the callee as a byval argument. For example:
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efriedma-quic wrote:

Phrasing here is a little unclear; the "unless that pointer is passed to the callee as a byval argument" part applies to allocas as well.

https://github.com/llvm/llvm-project/pull/110093