2 files changed, 20 insertions, 4 deletions
diff --git a/llvm/lib/CodeGen/MachineCSE.cpp b/llvm/lib/CodeGen/MachineCSE.cpp
index 58d73f06599..d9bd32b2fba 100644
--- a/llvm/lib/CodeGen/MachineCSE.cpp
+++ b/llvm/lib/CodeGen/MachineCSE.cpp
@@ -198,14 +198,16 @@ bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
     LLVM_DEBUG(dbgs() << "Coalescing: " << *DefMI);
     LLVM_DEBUG(dbgs() << "***     to: " << *MI);
 
-    // Update matching debug values.
-    DefMI->changeDebugValuesDefReg(SrcReg);
-
     // Propagate SrcReg of copies to MI.
     MO.setReg(SrcReg);
     MRI->clearKillFlags(SrcReg);
     // Coalesce single use copies.
     if (OnlyOneUse) {
+      // If (and only if) we've eliminated all uses of the copy, also
+      // copy-propagate to any debug-users of MI, or they'll be left using
+      // an undefined value.
+      DefMI->changeDebugValuesDefReg(SrcReg);
+
       DefMI->eraseFromParent();
       ++NumCoalesces;
     }
diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index 7316148e4c0..779f6086b8c 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -2119,7 +2119,21 @@ void MachineInstr::collectDebugValues(
 void MachineInstr::changeDebugValuesDefReg(Register Reg) {
   // Collect matching debug values.
   SmallVector<MachineInstr *, 2> DbgValues;
-  collectDebugValues(DbgValues);
+
+  if (!getOperand(0).isReg())
+    return;
+
+  unsigned DefReg = getOperand(0).getReg();
+  auto *MRI = getRegInfo();
+  for (auto &MO : MRI->use_operands(DefReg)) {
+    auto *DI = MO.getParent();
+    if (!DI->isDebugValue())
+      continue;
+    if (DI->getOperand(0).isReg() &&
+        DI->getOperand(0).getReg() == DefReg){
+      DbgValues.push_back(DI);
+    }
+  }
 
   // Propagate Reg to debug value instructions.
   for (auto *DBI : DbgValues)