Index: llvm/lib/CodeGen/PHIElimination.cpp
===================================================================
--- llvm/lib/CodeGen/PHIElimination.cpp
+++ llvm/lib/CodeGen/PHIElimination.cpp
@@ -85,7 +85,7 @@
     bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
 
     void LowerPHINode(MachineBasicBlock &MBB,
-                      MachineBasicBlock::iterator LastPHIIt);
+                      MachineBasicBlock::iterator AfterPHIsIt);
 
     /// analyzePHINodes - Gather information about the PHI nodes in
     /// here. In particular, we want to map the number of uses of a virtual
@@ -206,12 +206,13 @@
   if (MBB.empty() || !MBB.front().isPHI())
     return false;   // Quick exit for basic blocks without PHIs.
 
-  // Get an iterator to the last PHI node.
-  MachineBasicBlock::iterator LastPHIIt =
-    std::prev(MBB.SkipPHIsAndLabels(MBB.begin()));
+  // Get an iterator to the first non-PHI node.
+  // Skip debug instruction to avoid impacting on PHI lowering.
+  MachineBasicBlock::iterator AfterPHIsIt =
+    MBB.SkipPHIsLabelsAndDebug(MBB.begin());
 
   while (MBB.front().isPHI())
-    LowerPHINode(MBB, LastPHIIt);
+    LowerPHINode(MBB, AfterPHIsIt);
 
   return true;
 }
@@ -236,13 +237,12 @@
   }
   return true;
 }
+
 /// LowerPHINode - Lower the PHI node at the top of the specified block.
 void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
-                                  MachineBasicBlock::iterator LastPHIIt) {
+                                  MachineBasicBlock::iterator AfterPHIsIt) {
   ++NumLowered;
 
-  MachineBasicBlock::iterator AfterPHIsIt = std::next(LastPHIIt);
-
   // Unlink the PHI node from the basic block, but don't delete the PHI yet.
   MachineInstr *MPhi = MBB.remove(&*MBB.begin());
 
Index: llvm/test/CodeGen/X86/phi-node-elimination-dbg-invariant.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/phi-node-elimination-dbg-invariant.mir
@@ -0,0 +1,52 @@
+# RUN: llc -mtriple=x86_64-- -run-pass phi-node-elimination -o - %s | FileCheck %s
+# Debug instruction should not impact PHI node lowering with
+# a debug instruction between PHI and LABEL.
+# Fix issue: https://bugs.llvm.org/show_bug.cgi?id=43859
+
+---
+name:            test1
+tracksRegLiveness: true
+registers:
+body:             |
+  ; Verify PHI lowering without Debug instruction.
+  ; CHECK-LABEL: name: test1
+  ; CHECK: bb.2:
+  ; CHECK: EH_LABEL 0
+  ; CHECK: %1:gr32 = COPY %3
+  ; CHECK: %2:gr32 = ADD32ri8
+  bb.0:
+    %0:gr32 = IMPLICIT_DEF
+    JMP_1 %bb.2
+
+  bb.1:
+
+  bb.2:
+    %1:gr32 = PHI %0, %bb.0, %2, %bb.1
+    EH_LABEL 0
+    %2:gr32 = ADD32ri8 killed %1, 1, implicit-def $eflags
+...
+
+---
+name:            test2
+tracksRegLiveness: true
+body:             |
+  ; Verify PHI lowering with a debug instruction between PHI and LABEL,
+  ; skip debug instruction and insert copy instruction after LABEL.
+  ; CHECK-LABEL: name: test2
+  ; CHECK: bb.2:
+  ; CHECK: DBG_VALUE %1
+  ; CHECK: EH_LABEL 0
+  ; CHECK: %1:gr32 = COPY %3
+  ; CHECK: %2:gr32 = ADD32ri8
+  bb.0:
+    %0:gr32 = IMPLICIT_DEF
+    JMP_1 %bb.2
+
+  bb.1:
+
+  bb.2:
+    %1:gr32 = PHI %0, %bb.0, %2, %bb.1
+    DBG_VALUE %1
+    EH_LABEL 0
+    %2:gr32 = ADD32ri8 killed %1, 1, implicit-def $eflags
+...