[SystemZ] Add use of super-reg in splitMove()

If one of the subregs of the 128 bit reg is undefined when splitMove() splits
a store into two instructions, a use of an undefined physical register
results.

To remedy this, an implicit use of the super register is added onto both new
instructions, along with propagated kill and undef flags.

This was discovered with llvm-stress, and that test case is attached as
test/CodeGen/SystemZ/splitMove_undefReg_mverifier.ll

Thanks to Matthias Braun for helping with a nice explanation.

Review: Ulrich Weigand
llvm-svn: 298047

1 files changed, 14 insertions, 1 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index 16ffd04dcb0..a1ef3530f90 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -80,12 +80,25 @@ void SystemZInstrInfo::splitMove(MachineBasicBlock::iterator MI,
   MachineInstr *EarlierMI = MF.CloneMachineInstr(&*MI);
   MBB->insert(MI, EarlierMI);
 
-  // Set up the two 64-bit registers.
+  // Set up the two 64-bit registers and remember super reg and its flags.
   MachineOperand &HighRegOp = EarlierMI->getOperand(0);
   MachineOperand &LowRegOp = MI->getOperand(0);
+  unsigned Reg128 = LowRegOp.getReg();
+  unsigned Reg128Killed = getKillRegState(LowRegOp.isKill());
+  unsigned Reg128Undef  = getUndefRegState(LowRegOp.isUndef());
   HighRegOp.setReg(RI.getSubReg(HighRegOp.getReg(), SystemZ::subreg_h64));
   LowRegOp.setReg(RI.getSubReg(LowRegOp.getReg(), SystemZ::subreg_l64));
 
+  if (MI->mayStore()) {
+    // Add implicit uses of the super register in case one of the subregs is
+    // undefined. We could track liveness and skip storing an undefined
+    // subreg, but this is hopefully rare (discovered with llvm-stress).
+    // If Reg128 was killed, set kill flag on MI.
+    unsigned Reg128UndefImpl = (Reg128Undef | RegState::Implicit);
+    MachineInstrBuilder(MF, EarlierMI).addReg(Reg128, Reg128UndefImpl);
+    MachineInstrBuilder(MF, MI).addReg(Reg128, (Reg128UndefImpl | Reg128Killed));
+  }
+
   // The address in the first (high) instruction is already correct.
   // Adjust the offset in the second (low) instruction.
   MachineOperand &HighOffsetOp = EarlierMI->getOperand(2);