Summary:
Calls marked 'tail' cannot read or write allocas from the current frame
because the current frame might be destroyed by the time they run.
However, a tail call may use an alloca with byval. Calling with byval
copies the contents of the alloca into argument registers or stack
slots, so there is no lifetime issue. Tail calls never modify allocas,
so we can return just ModRefInfo::Ref.
Fixes PR38466, a longstanding bug.
Reviewers: hfinkel, nlewycky, gbiv, george.burgess.iv
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D50679
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339636 91177308-0d34-0410-b5e6-96231b3b80d8
3ea7b0a0b1