diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index 20ad6445344d8..7c108ce2c8323 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1325,11 +1325,6 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   const MCInstrDesc &MCID = DefMI->getDesc();
   if (MCID.getNumDefs() != 1)
     return false;
-  // Only support subregister destinations when the def is read-undef.
-  MachineOperand &DstOperand = CopyMI->getOperand(0);
-  Register CopyDstReg = DstOperand.getReg();
-  if (DstOperand.getSubReg() && !DstOperand.isUndef())
-    return false;
 
   // If both SrcIdx and DstIdx are set, correct rematerialization would widen
   // the register substantially (beyond both source and dest size). This is bad
@@ -1339,6 +1334,32 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   if (SrcIdx && DstIdx)
     return false;
 
+  // Only support subregister destinations when the def is read-undef.
+  MachineOperand &DstOperand = CopyMI->getOperand(0);
+  Register CopyDstReg = DstOperand.getReg();
+  if (DstOperand.getSubReg() && !DstOperand.isUndef())
+    return false;
+
+  // In the physical register case, checking that the def is read-undef is not
+  // enough. We're widening the def and need to avoid clobbering other live
+  // values in the unused register pieces.
+  //
+  // TODO: Targets may support rewriting the rematerialized instruction to only
+  // touch relevant lanes, in which case we don't need any liveness check.
+  if (CopyDstReg.isPhysical() && CP.isPartial()) {
+    for (MCRegUnit Unit : TRI->regunits(DstReg)) {
+      // Ignore the register units we are writing anyway.
+      if (is_contained(TRI->regunits(CopyDstReg), Unit))
+        continue;
+
+      // Check if the other lanes we are defining are live at the
+      // rematerialization point.
+      LiveRange &LR = LIS->getRegUnit(Unit);
+      if (LR.liveAt(CopyIdx))
+        return false;
+    }
+  }
+
   const unsigned DefSubIdx = DefMI->getOperand(0).getSubReg();
   const TargetRegisterClass *DefRC = TII->getRegClass(MCID, 0, TRI, *MF);
   if (!DefMI->isImplicitDef()) {
diff --git a/llvm/test/CodeGen/AMDGPU/remat-physreg-copy-subreg-extract-already-live-at-def-issue120970.mir b/llvm/test/CodeGen/AMDGPU/remat-physreg-copy-subreg-extract-already-live-at-def-issue120970.mir
new file mode 100644
index 0000000000000..3879f6dccf9d6
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/remat-physreg-copy-subreg-extract-already-live-at-def-issue120970.mir
@@ -0,0 +1,85 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=register-coalescer -o - %s | FileCheck %s
+
+# This used to assert due to trying to rematerialize V_MOV_B64_PSEUDO
+# at copy to $vgpr1. This would assert since this would clobber the
+# live value in $vgpr0.
+
+---
+name: rematerialize_physreg_sub_def_already_live_at_def_assert
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0
+
+    ; CHECK-LABEL: name: rematerialize_physreg_sub_def_already_live_at_def_assert
+    ; CHECK: liveins: $vgpr0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[V_MOV_B:%[0-9]+]]:vreg_64 = V_MOV_B64_PSEUDO 1, implicit $exec
+    ; CHECK-NEXT: $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+    ; CHECK-NEXT: $vgpr1 = COPY [[V_MOV_B]].sub1
+    ; CHECK-NEXT: SI_RETURN implicit $vgpr0, implicit killed $vgpr1
+    %0:vreg_64 = V_MOV_B64_PSEUDO 1, implicit $exec
+    %1:vgpr_32 = COPY %0.sub1
+    $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+    $vgpr1 = COPY %1
+    SI_RETURN implicit $vgpr0, implicit killed $vgpr1
+...
+
+# Same as previous, except with an IMPLICIT_DEF
+---
+name: rematerialize_physreg_sub_def_already_live_at_def_assert_implicit_def
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0
+
+    ; CHECK-LABEL: name: rematerialize_physreg_sub_def_already_live_at_def_assert_implicit_def
+    ; CHECK: liveins: $vgpr0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+    ; CHECK-NEXT: $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+    ; CHECK-NEXT: $vgpr1 = COPY [[DEF]].sub1
+    ; CHECK-NEXT: SI_RETURN implicit $vgpr0, implicit killed $vgpr1
+    %0:vreg_64 = IMPLICIT_DEF
+    %1:vgpr_32 = COPY %0.sub1
+    $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+    $vgpr1 = COPY %1
+    SI_RETURN implicit $vgpr0, implicit killed $vgpr1
+...
+
+---
+name: rematerialize_physreg_sub_def_no_live_sub_def_0
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0
+
+    ; CHECK-LABEL: name: rematerialize_physreg_sub_def_no_live_sub_def_0
+    ; CHECK: liveins: $vgpr0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: dead $vgpr0_vgpr1 = V_MOV_B64_PSEUDO 1, implicit $exec, implicit-def $vgpr1
+    ; CHECK-NEXT: SI_RETURN implicit killed $vgpr1
+    %0:vreg_64 = V_MOV_B64_PSEUDO 1, implicit $exec
+    %1:vgpr_32 = COPY %0.sub1
+    $vgpr1 = COPY %1
+    SI_RETURN implicit killed $vgpr1
+...
+
+---
+name: rematerialize_physreg_sub_def_no_live_sub_def_1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0
+
+    ; CHECK-LABEL: name: rematerialize_physreg_sub_def_no_live_sub_def_1
+    ; CHECK: liveins: $vgpr0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: dead $vgpr1_vgpr2 = V_MOV_B64_PSEUDO 1, implicit $exec, implicit-def $vgpr1
+    ; CHECK-NEXT: SI_RETURN implicit killed $vgpr1
+    %0:vreg_64 = V_MOV_B64_PSEUDO 1, implicit $exec
+    %1:vgpr_32 = COPY %0.sub0
+    $vgpr1 = COPY %1
+    SI_RETURN implicit killed $vgpr1
+...