1 files changed, 85 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index f747b5d682f..0dc5ae188eb 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -265,6 +265,14 @@ namespace {
     /// Return true if a copy involving a physreg should be joined.
     bool canJoinPhys(const CoalescerPair &CP);
 
+    /// When merging SrcReg and DstReg together, and the operand of the
+    /// specified DBG_VALUE refers to one of them, would the def that a
+    /// DBG_VALUE refers to change? This can happen when the DBG_VALUEs
+    /// operand is dead and it's merged into a different live value,
+    /// meaning the DBG_VALUE operands must be updated.
+    bool mergingChangesDbgValue(MachineInstr *DbgV, unsigned SrcReg,
+                                unsigned DstReg) const;
+
     /// Replace all defs and uses of SrcReg to DstReg and update the subregister
     /// number if it is not zero. If DstReg is a physical register and the
     /// existing subregister number of the def / use being updated is not zero,
@@ -1641,8 +1649,59 @@ void RegisterCoalescer::addUndefFlag(const LiveInterval &Int, SlotIndex UseIdx,
   }
 }
 
-void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
-                                          unsigned DstReg,
+bool RegisterCoalescer::mergingChangesDbgValue(MachineInstr *DbgV,
+                                               unsigned SrcReg,
+                                               unsigned DstReg) const {
+  assert(DbgV->isDebugValue());
+  assert(DbgV->getParent() && "DbgValue with no parent");
+  assert(DbgV->getOperand(0).isReg());
+  unsigned DbgReg = DbgV->getOperand(0).getReg();
+
+  SlotIndex MIIdx = LIS->getSlotIndexes()->getIndexAfter(*DbgV);
+  const LiveInterval &SrcLI = LIS->getInterval(SrcReg);
+
+  // Is the source register live across the DBG_VALUE?
+  bool SrcLive = false;
+  auto LII = SrcLI.find(MIIdx);
+  if (LII != SrcLI.end() && LII->contains(MIIdx))
+    SrcLive = true;
+
+  bool DstLive = false;
+  // Destination register can be physical or virtual.
+  if (TargetRegisterInfo::isVirtualRegister(DstReg)) {
+    // Is DstReg live across the DBG_VALUE?
+    const LiveInterval &DstLI = LIS->getInterval(DstReg);
+    LII = DstLI.find(MIIdx);
+    DstLive = (LII != DstLI.end() && LII->contains(MIIdx));
+  } else if (MRI->isConstantPhysReg(DstReg)) {
+    // Constant physical registers are always live.
+    DstLive = true;
+  } else {
+    // For physical registers, see if any register unit containing DstReg
+    // is live across the DBG_VALUE.
+    for (MCRegUnitIterator UI(DstReg, TRI); UI.isValid(); ++UI) {
+      const LiveRange &DstLI = LIS->getRegUnit(*UI);
+      auto DstLII = DstLI.find(MIIdx);
+      if (DstLII != DstLI.end() && DstLII->contains(MIIdx)) {
+        DstLive = true;
+        break;
+      }
+    }
+  }
+
+  // We now know whether src and dst are live. Best case: we have a DBG_VALUE
+  // of a live register, coalesing won't change its value.
+  if ((DstLive && DbgReg == DstReg) || (SrcLive && DbgReg == SrcReg))
+    return false;
+  // If neither register are live, no damage done.
+  if (!DstLive && !SrcLive)
+    return false;
+  // Otherwise, we will end up resurrecting the DBG_VALUE with a different
+  // register, which is unsafe.
+  return true;
+}
+
+void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg, unsigned DstReg,
                                           unsigned SubIdx) {
   bool DstIsPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
   LiveInterval *DstInt = DstIsPhys ? nullptr : &LIS->getInterval(DstReg);
@@ -1854,6 +1913,20 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
   ShrinkMask = LaneBitmask::getNone();
   ShrinkMainRange = false;
 
+  // Although we can update the DBG_VALUEs to the merged register, as debug uses
+  // do not contribute to liveness it might not be a sound update. Collect
+  // DBG_VALUEs that would change value were this interval merging to succeed.
+  SmallVector<MachineInstr *, 4> DbgValuesToChange;
+  auto CheckForDbgUser = [this, &CP, &DbgValuesToChange](MachineInstr &MI) {
+    if (MI.isDebugValue() && MI.getOperand(0).isReg() &&
+        mergingChangesDbgValue(&MI, CP.getSrcReg(), CP.getDstReg()))
+      DbgValuesToChange.push_back(&MI);
+  };
+  for (auto &RegIt : MRI->reg_instructions(CP.getSrcReg()))
+    CheckForDbgUser(RegIt);
+  for (auto &RegIt : MRI->reg_instructions(CP.getDstReg()))
+    CheckForDbgUser(RegIt);
+
   // Okay, attempt to join these two intervals.  On failure, this returns false.
   // Otherwise, if one of the intervals being joined is a physreg, this method
   // always canonicalizes DstInt to be it.  The output "SrcInt" will not have
@@ -1922,6 +1995,16 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
     updateRegDefsUses(CP.getDstReg(), CP.getDstReg(), CP.getDstIdx());
   updateRegDefsUses(CP.getSrcReg(), CP.getDstReg(), CP.getSrcIdx());
 
+  // The updates to these DBG_VALUEs are not sound -- mark them undef.
+  // FIXME: further analysis might recover them, this is the minimal sound
+  // solution.
+  for (MachineInstr *MI : DbgValuesToChange) {
+    assert(MI->getOperand(0).isReg());
+    LLVM_DEBUG(dbgs() << "Update of " << MI->getOperand(0) << " would be "
+                      << "unsound, setting undef\n");
+    MI->getOperand(0).setReg(0);
+  }
+
   // Shrink subregister ranges if necessary.
   if (ShrinkMask.any()) {
     LiveInterval &LI = LIS->getInterval(CP.getDstReg());