[SelectionDAGBuilder] Make sure DemoteReg ends up in right reg-class in visitRet().

[jonpa]

Summary

The virtual register containing the address for returned value on
stack should in the DAG be represented with a CopyFromReg node and not
a Register node. Otherwise, InstrEmitter will not make sure that it
ends up in the right register class for the target instruction.

SystemZ needs this, because the reg class for address registers is a
subset of the general 64 bit register class.
test/SystemZ/CodeGen/args-07.ll and args-04.ll updated to run with
-verify-machineinstrs.

Note: InstrEmitter will not check for proper regclass for a Register node, as it
does for a CopyFromReg. It seems that the Register nodes as they are handled
in InstrEmitter are intended for call / return phys reg operands which need to be
flagged as implicit. But since the DemoteReg is a virtual register, it therefore
seems it doesn't really need to be a Register node for any particular reason.

[hfinkel]

LGTM.

[jonpa]

commited as r253461.

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

if (!FuncInfo.CanLowerReturn) {
 
     // Emit a store of the return value through the virtual register.
     // Leave Outs empty so that LowerReturn won't try to load return
     // registers the usual way.
     SmallVector<EVT, 1> PtrValueVTs;
     ComputeValueVTs(TLI, DL, PointerType::getUnqual(F->getReturnType()),
                     PtrValueVTs);
 
-    SDValue RetPtr = DAG.getRegister(DemoteReg, PtrValueVTs[0]);
+    SDValue RetPtr = DAG.getCopyFromReg(DAG.getEntryNode(), getCurSDLoc(),
+                                        DemoteReg, PtrValueVTs[0]);
     SDValue RetOp = getValue(I.getOperand(0));
 
     SmallVector<EVT, 4> ValueVTs;
     SmallVector<uint64_t, 4> Offsets;
     ComputeValueVTs(TLI, DL, I.getOperand(0)->getType(), ValueVTs, &Offsets);
     unsigned NumValues = ValueVTs.size();
 
     SmallVector<SDValue, 4> Chains(NumValues);

test/CodeGen/SystemZ/args-04.ll

 ; Test incoming GPR, FPR and stack arguments when no extension type is given.
 ; This type of argument is used for passing structures, etc.
 ;
-; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+; RUN: llc < %s -mtriple=s390x-linux-gnu -verify-machineinstrs | FileCheck %s
 
 ; Do some arithmetic so that we can see the register being used.
 define i8 @f1(i8 %r2) {
 ; CHECK-LABEL: f1:
 ; CHECK: ahi %r2, 1
 ; CHECK: br %r14
   %y = add i8 %r2, 1
   ret i8 %y

test/CodeGen/SystemZ/args-07.ll

 ; Test multiple return values (LLVM ABI extension)
 ;
-; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+; RUN: llc < %s -mtriple=s390x-linux-gnu -verify-machineinstrs| FileCheck %s
 
 ; Up to four integer return values fit into GPRs.
 define { i64, i64, i64, i64 } @f1() {
 ; CHECK-LABEL: f1:
 ; CHECK: lghi %r2, 0
 ; CHECK: lghi %r3, 1
 ; CHECK: lghi %r4, 2
 ; CHECK: lghi %r5, 3