diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -2686,14 +2686,8 @@
     return CR_Replace;
 
   // Check for simple erasable conflicts.
-  if (DefMI->isImplicitDef()) {
-    // We need the def for the subregister if there is nothing else live at the
-    // subrange at this point.
-    if (TrackSubRegLiveness
-        && (V.WriteLanes & (OtherV.ValidLanes | OtherV.WriteLanes)).none())
-      return CR_Replace;
+  if (DefMI->isImplicitDef())
     return CR_Erase;
-  }
 
   // Include the non-conflict where DefMI is a coalescable copy that kills
   // OtherVNI. We still want the copy erased and value numbers merged.
diff --git a/llvm/test/CodeGen/AMDGPU/coalescing_makes_lanes_undef.mir b/llvm/test/CodeGen/AMDGPU/coalescing_makes_lanes_undef.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/coalescing_makes_lanes_undef.mir
@@ -0,0 +1,49 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=gfx803 -run-pass simple-register-coalescing -verify-machineinstrs -o - %s | FileCheck %s
+
+# Register coalescer is going to eliminate %2:sgpr_32 = COPY %1.sub0 from bb.1
+# by joining %2 and %1.sub0 into %0.sub0 register. Check that when this happen
+# the implicit intialization of %0.sub0 in the bb.2 have undef flag
+# for the MIR to be valid.
+
+---
+name: coalescing_makes_lane_undefined
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: coalescing_makes_lane_undefined
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x40000000), %bb.2(0x40000000)
+  ; CHECK:   S_CBRANCH_SCC0 %bb.2, implicit undef $scc
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.3(0x80000000)
+  ; CHECK:   undef %0.sub0:sgpr_64 = S_MOV_B32 1
+  ; CHECK:   %0.sub1:sgpr_64 = S_MOV_B32 2
+  ; CHECK:   S_BRANCH %bb.3
+  ; CHECK: bb.2:
+  ; CHECK:   successors: %bb.3(0x80000000)
+  ; CHECK:   undef %0.sub0:sgpr_64 = IMPLICIT_DEF
+  ; CHECK: bb.3:
+  ; CHECK:   S_NOP 0, implicit %0.sub0
+  ; CHECK:   S_NOP 0, implicit %0
+  bb.0:
+    successors: %bb.1, %bb.2
+    S_CBRANCH_SCC0 %bb.2, implicit undef $scc
+
+  bb.1:
+    successors: %bb.3
+    undef %1.sub0:sgpr_64 = S_MOV_B32 1
+    %1.sub1:sgpr_64 = S_MOV_B32 2
+    %2:sgpr_32 = COPY %1.sub0 ; copy to be joined
+    S_BRANCH %bb.3
+
+  bb.2:
+    successors: %bb.3
+    %2:sgpr_32 = IMPLICIT_DEF
+    undef %1.sub0:sgpr_64 = IMPLICIT_DEF
+    %1.sub1:sgpr_64 = IMPLICIT_DEF
+
+  bb.3:
+    S_NOP 0, implicit killed %2
+    S_NOP 0, implicit killed %1
+
+...