[SelectionDAG][FIX] Allow "returned" arguments to be bit-casted

[jdoerfert]

Summary

An argument that is return by a function but bit-casted before can still
be annotated as "returned". Make sure we do not crash for this case.

[arsenm]

llvm/test/CodeGen/Generic/arg_returned_bitcast.ll

1–2

test/CodeGen/Generic should not be used. This should be moved to some specific target, and test something in the output

[arsenm]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

8948–8949

Maybe this should also assert that the address spaces match? I don't know if the verifier checks this or not, but it should

[jdoerfert]

Update according to comments

[arsenm]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

for (unsigned Value = 0, NumValues = ValueVTs.size(); Value != NumValues;
         ExtendKind = ISD::SIGN_EXTEND;
       else if (Args[i].IsZExt)
         ExtendKind = ISD::ZERO_EXTEND;
 
       // Conservatively only handle 'returned' on non-vectors that can be lowered,
       // for now.
       if (Args[i].IsReturned && !Op.getValueType().isVector() &&
           CanLowerReturn) {
-        assert(CLI.RetTy == Args[i].Ty && RetTys.size() == NumValues &&
-               "unexpected use of 'returned'");
+        assert((CLI.RetTy == Args[i].Ty ||
+                CLI.RetTy->isPointerTy() && Args[i].Ty->isPointerTy()) &&
+               RetTys.size() == NumValues && "unexpected use of 'returned'");
         // Before passing 'returned' to the target lowering code, ensure that
         // either the register MVT and the actual EVT are the same size or that
         // the return value and argument are extended in the same way; in these
         // cases it's safe to pass the argument register value unchanged as the
         // return register value (although it's at the target's option whether
         // to do so)
         // TODO: allow code generation to take advantage of partially preserved
         // registers rather than clobbering the entire register when the

llvm/test/CodeGen/Generic/arg_returned_bitcast.ll

+; RUN: llc < %s -asm-verbose=false
+
+; Test that the "returned" attribute "works" even if there is a bitcast between
+; the argument and return value.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+
+declare double* @bar(i8* returned)
+
+define double* @foo(i8*) {
+  %r = tail call double* @bar(i8* %0)
+  ret double* %r
+}