[ARM] Fix CPSR liveness in tMOVCCr_pseudo lowering.

The lowering was missing live-ins in certain cases, like a sequence of
multiple tMOVCCr_pseudo instructions.  This would lead to a verifier
failure, and on pre-v6 Thumb CPSR would be incorrectly clobbered.

For reasons I don't completely understand, it's hard to get a sequence
of multiple tMOVCCr_pseudo instructions; the issue only seems to show up
with 64-bit comparisons where the result is zero-extended. I added some
extra testcases in case that changes in the future. Probably some
optimization opportunities here if anyone is interested. (@test_slt_not
is the case that was getting miscompiled.)

The code to check the liveness of CPSR was stolen from
X86ISelLowering.cpp; maybe it could be refactored into common helper,
but I have no idea where to put it.

Differential Revision: https://reviews.llvm.org/D54192

llvm-svn: 346355

1 files changed, 44 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 0f68fb0287c..56d2e510cb7 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -9160,6 +9160,42 @@ ARMTargetLowering::EmitLowered__dbzchk(MachineInstr &MI,
   return ContBB;
 }
 
+// The CPSR operand of SelectItr might be missing a kill marker
+// because there were multiple uses of CPSR, and ISel didn't know
+// which to mark. Figure out whether SelectItr should have had a
+// kill marker, and set it if it should. Returns the correct kill
+// marker value.
+static bool checkAndUpdateCPSRKill(MachineBasicBlock::iterator SelectItr,
+                                   MachineBasicBlock* BB,
+                                   const TargetRegisterInfo* TRI) {
+  // Scan forward through BB for a use/def of CPSR.
+  MachineBasicBlock::iterator miI(std::next(SelectItr));
+  for (MachineBasicBlock::iterator miE = BB->end(); miI != miE; ++miI) {
+    const MachineInstr& mi = *miI;
+    if (mi.readsRegister(ARM::CPSR))
+      return false;
+    if (mi.definesRegister(ARM::CPSR))
+      break; // Should have kill-flag - update below.
+  }
+
+  // If we hit the end of the block, check whether CPSR is live into a
+  // successor.
+  if (miI == BB->end()) {
+    for (MachineBasicBlock::succ_iterator sItr = BB->succ_begin(),
+                                          sEnd = BB->succ_end();
+         sItr != sEnd; ++sItr) {
+      MachineBasicBlock* succ = *sItr;
+      if (succ->isLiveIn(ARM::CPSR))
+        return false;
+    }
+  }
+
+  // We found a def, or hit the end of the basic block and CPSR wasn't live
+  // out. SelectMI should have a kill flag on CPSR.
+  SelectItr->addRegisterKilled(ARM::CPSR, TRI);
+  return true;
+}
+
 MachineBasicBlock *
 ARMTargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
                                                MachineBasicBlock *BB) const {
@@ -9259,6 +9295,14 @@ ARMTargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
     F->insert(It, copy0MBB);
     F->insert(It, sinkMBB);
 
+    // Check whether CPSR is live past the tMOVCCr_pseudo.
+    const TargetRegisterInfo *TRI = Subtarget->getRegisterInfo();
+    if (!MI.killsRegister(ARM::CPSR) &&
+        !checkAndUpdateCPSRKill(MI, thisMBB, TRI)) {
+      copy0MBB->addLiveIn(ARM::CPSR);
+      sinkMBB->addLiveIn(ARM::CPSR);
+    }
+
     // Transfer the remainder of BB and its successor edges to sinkMBB.
     sinkMBB->splice(sinkMBB->begin(), BB,
                     std::next(MachineBasicBlock::iterator(MI)), BB->end());