[CodeGen] Don't attempt a tail-call with a non-forwarded explicit sret.

Tailcalls are only OK with forwarded sret pointers. With explicit sret,
one approximation is to check that the pointer isn't an Instruction, as
in that case it might point into some local memory (alloca). That's not
OK with tailcalls.

Explicit sret counterpart to r233409.
Differential Revison: http://reviews.llvm.org/D8510


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233410 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -5556,6 +5556,11 @@ void SelectionDAGBuilder::LowerCallTo(ImmutableCallSite CS, SDValue Callee,
     // Skip the first return-type Attribute to get to params.
     Entry.setAttributes(&CS, i - CS.arg_begin() + 1);
     Args.push_back(Entry);
+
+    // If we have an explicit sret argument that is an Instruction, (i.e., it
+    // might point to function-local memory), we can't meaningfully tail-call.
+    if (Entry.isSRet && isa<Instruction>(V))
+      isTailCall = false;
   }
 
   // Check if target-independent constraints permit a tail call here.