[Diffusion] rL361590: Clarify how musttail can be used to create forwarding thunks

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.

/llvm/trunk/docs/LangRef.rst:10013 The problem is that the "variadic" arguments to a thunk aren't necessarily passed the same way a true variadic argument list would be passed.  For example, on Windows, variadic functions never use floating-point registers for arguments.  So even if it's legal to call va_start, we can't promise the result is meaningful.
/llvm/trunk/docs/LangRef.rst:10018 The only reason the backend doesn't look at the "thunk" attribute is that it's using the presence of "musttail" as a proxy for whether the caller is a thunk.  The result of `getForwardedMustTailRegParms()` isn't meaningful in non-thunk functions.

Users:
  rnk (Author)

https://reviews.llvm.org/rL361590