[mips] Fix return lowering

[sdardis]

Summary

Fix a machine verifier issue where a instruction was using a invalid
register. The return pseudo is expanded and has the return address
register added to it. The return register may have been spuriously
mark as killed earlier.

This partially resolves PR/27458

Thanks to Quentin Colombet for reporting the issue!

[slthakur]

LGTM

[sdardis]

Thanks for the review. rL297372.

lib/Target/Mips/MipsSEInstrInfo.cpp

return (Opc == Mips::BEQ    || Opc == Mips::BEQ_MM || Opc == Mips::BNE    ||
           Opc == Mips::BGEC64 || Opc == Mips::BGEUC64 || Opc == Mips::BLTC64 ||
           Opc == Mips::BLTUC64 || Opc == Mips::BGTZC64 ||
           Opc == Mips::BGEZC64 || Opc == Mips::BLTZC64 ||
           Opc == Mips::BLEZC64 || Opc == Mips::BC) ? Opc : 0;
 }
 
 void MipsSEInstrInfo::expandRetRA(MachineBasicBlock &MBB,
                                   MachineBasicBlock::iterator I) const {
+
+  MachineInstrBuilder MIB;
   if (Subtarget.isGP64bit())
-    BuildMI(MBB, I, I->getDebugLoc(), get(Mips::PseudoReturn64))
-        .addReg(Mips::RA_64);
+    MIB = BuildMI(MBB, I, I->getDebugLoc(), get(Mips::PseudoReturn64))
+              .addReg(Mips::RA_64, RegState::Undef);
   else
-    BuildMI(MBB, I, I->getDebugLoc(), get(Mips::PseudoReturn)).addReg(Mips::RA);
+    MIB = BuildMI(MBB, I, I->getDebugLoc(), get(Mips::PseudoReturn))
+              .addReg(Mips::RA, RegState::Undef);
+
+  // Retain any imp-use flags.
+  for (auto & MO : I->operands()) {
+    if (MO.isImplicit())
+      MIB.add(MO);
+  }
 }
 
 void MipsSEInstrInfo::expandERet(MachineBasicBlock &MBB,
                                  MachineBasicBlock::iterator I) const {
   BuildMI(MBB, I, I->getDebugLoc(), get(Mips::ERET));
 }
 
 std::pair<bool, bool>

test/CodeGen/Mips/return_address.ll

-; RUN: llc -march=mipsel < %s | FileCheck %s
+; RUN: llc -march=mipsel -verify-machineinstrs < %s | FileCheck %s
 
 define i8* @f1() nounwind {
 entry:
   %0 = call i8* @llvm.returnaddress(i32 0)
   ret i8* %0
 
 ; CHECK:    move  $2, $ra
 }

test/CodeGen/Mips/tnaked.ll

-; RUN: llc -march=mipsel < %s | FileCheck %s
+; RUN: llc -march=mipsel < %s -verify-machineinstrs | FileCheck %s
 
 
 define void @tnaked() #0 {
 entry:
   ret void
 }
 
 ; CHECK:  .ent  tnaked