diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 76f99848a823c..d503d7a2345fd 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -1330,9 +1330,12 @@ void TargetInstrInfo::reassociateOps(
   MachineOperand &OpC = Root.getOperand(0);
 
   Register RegA = OpA.getReg();
+  unsigned SubRegA = OpA.getSubReg();
   Register RegB = OpB.getReg();
   Register RegX = OpX.getReg();
+  unsigned SubRegX = OpX.getSubReg();
   Register RegY = OpY.getReg();
+  unsigned SubRegY = OpY.getSubReg();
   Register RegC = OpC.getReg();
 
   if (RegA.isVirtual())
@@ -1350,6 +1353,7 @@ void TargetInstrInfo::reassociateOps(
   // recycling RegB because the MachineCombiner's computation of the critical
   // path requires a new register definition rather than an existing one.
   Register NewVR = MRI.createVirtualRegister(RC);
+  unsigned SubRegNewVR = 0;
   InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
 
   auto [NewRootOpc, NewPrevOpc] = getReassociationOpcodes(Pattern, Root, Prev);
@@ -1362,6 +1366,7 @@ void TargetInstrInfo::reassociateOps(
 
   if (SwapPrevOperands) {
     std::swap(RegX, RegY);
+    std::swap(SubRegX, SubRegY);
     std::swap(KillX, KillY);
   }
 
@@ -1414,9 +1419,9 @@ void TargetInstrInfo::reassociateOps(
     if (Idx == 0)
       continue;
     if (Idx == PrevFirstOpIdx)
-      MIB1.addReg(RegX, getKillRegState(KillX));
+      MIB1.addReg(RegX, getKillRegState(KillX), SubRegX);
     else if (Idx == PrevSecondOpIdx)
-      MIB1.addReg(RegY, getKillRegState(KillY));
+      MIB1.addReg(RegY, getKillRegState(KillY), SubRegY);
     else
       MIB1.add(MO);
   }
@@ -1424,6 +1429,7 @@ void TargetInstrInfo::reassociateOps(
 
   if (SwapRootOperands) {
     std::swap(RegA, NewVR);
+    std::swap(SubRegA, SubRegNewVR);
     std::swap(KillA, KillNewVR);
   }
 
@@ -1435,9 +1441,9 @@ void TargetInstrInfo::reassociateOps(
     if (Idx == 0)
       continue;
     if (Idx == RootFirstOpIdx)
-      MIB2 = MIB2.addReg(RegA, getKillRegState(KillA));
+      MIB2 = MIB2.addReg(RegA, getKillRegState(KillA), SubRegA);
     else if (Idx == RootSecondOpIdx)
-      MIB2 = MIB2.addReg(NewVR, getKillRegState(KillNewVR));
+      MIB2 = MIB2.addReg(NewVR, getKillRegState(KillNewVR), SubRegNewVR);
     else
       MIB2 = MIB2.add(MO);
   }
@@ -1525,6 +1531,7 @@ void TargetInstrInfo::genAlternativeCodeSequence(
       if (IndexedReg.index() == 0)
         continue;
 
+      // FIXME: Losing subregisters
       MachineInstr *Instr = MRI.getUniqueVRegDef(IndexedReg.value());
       MachineInstrBuilder MIB;
       Register AccReg;
diff --git a/llvm/test/CodeGen/AArch64/machine-combiner-subregs.mir b/llvm/test/CodeGen/AArch64/machine-combiner-subregs.mir
new file mode 100644
index 0000000000000..c96a0385c3a4e
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/machine-combiner-subregs.mir
@@ -0,0 +1,35 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=aarch64-gnu-linux -mcpu=neoverse-n2 -run-pass=machine-combiner -o - %s | FileCheck %s
+
+# Make sure machine combiner doesn't drop subregister indexes.
+
+---
+name:            reassociate_adds2_reassoc
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $q0, $q1, $q2, $q3
+
+    ; CHECK-LABEL: name: reassociate_adds2_reassoc
+    ; CHECK: liveins: $q0, $q1, $q2, $q3
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:fpr128 = COPY $q0
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:fpr128 = COPY $q1
+    ; CHECK-NEXT: [[COPY2:%[0-9]+]]:fpr128 = COPY $q2
+    ; CHECK-NEXT: [[COPY3:%[0-9]+]]:fpr128 = COPY $q3
+    ; CHECK-NEXT: [[FADDSrr:%[0-9]+]]:fpr32 = nsz reassoc nofpexcept FADDSrr [[COPY]].ssub, [[COPY1]].ssub, implicit $fpcr
+    ; CHECK-NEXT: [[FADDSrr1:%[0-9]+]]:fpr32 = nsz reassoc nofpexcept FADDSrr [[COPY2]].ssub, [[COPY3]].ssub, implicit $fpcr
+    ; CHECK-NEXT: [[FADDSrr2:%[0-9]+]]:fpr32 = nsz reassoc nofpexcept FADDSrr killed [[FADDSrr1]], killed [[FADDSrr]], implicit $fpcr
+    ; CHECK-NEXT: $s0 = COPY [[FADDSrr2]]
+    ; CHECK-NEXT: RET_ReallyLR implicit $s0
+    %0:fpr128 = COPY $q0
+    %1:fpr128 = COPY $q1
+    %2:fpr128 = COPY $q2
+    %3:fpr128 = COPY $q3
+    %4:fpr32 = nsz reassoc nofpexcept FADDSrr %0.ssub, %1.ssub, implicit $fpcr
+    %5:fpr32 = nsz reassoc nofpexcept FADDSrr %2.ssub, killed %4, implicit $fpcr
+    %6:fpr32 = nsz reassoc nofpexcept FADDSrr killed %5, %3.ssub, implicit $fpcr
+    $s0 = COPY %6
+    RET_ReallyLR implicit $s0
+
+...
diff --git a/llvm/test/CodeGen/RISCV/rvv/machine-combiner-subreg-verifier-error.mir b/llvm/test/CodeGen/RISCV/rvv/machine-combiner-subreg-verifier-error.mir
new file mode 100644
index 0000000000000..76dfd4e746bea
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/rvv/machine-combiner-subreg-verifier-error.mir
@@ -0,0 +1,39 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=riscv64 -mattr=+v -verify-machineinstrs -run-pass=machine-combiner -o - %s | FileCheck %s
+
+# Make sure the verifier doesn't fail due to dropping subregister
+# uses.
+
+---
+name:            machine_combiner_subreg_verifier_error
+tracksRegLiveness: true
+isSSA:           true
+body:             |
+  bb.0:
+    liveins: $v8m4, $v12m4
+
+    ; CHECK-LABEL: name: machine_combiner_subreg_verifier_error
+    ; CHECK: liveins: $v8m4, $v12m4
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:vrm4 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF1:%[0-9]+]]:gprnox0 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF2:%[0-9]+]]:vrm8 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF3:%[0-9]+]]:vr = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF4:%[0-9]+]]:vrm2 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[DEF5:%[0-9]+]]:vr = IMPLICIT_DEF
+    ; CHECK-NEXT: [[PseudoVSLIDEDOWN_VI_M8_:%[0-9]+]]:vrm8 = PseudoVSLIDEDOWN_VI_M8 $noreg, [[DEF2]], 26, 2, 5 /* e32 */, 3 /* ta, ma */
+    ; CHECK-NEXT: [[PseudoVADD_VV_MF2_:%[0-9]+]]:vr = PseudoVADD_VV_MF2 $noreg, [[DEF2]].sub_vrm1_0, killed [[DEF3]], 2, 5 /* e32 */, 1 /* ta, mu */
+    ; CHECK-NEXT: [[PseudoVADD_VV_MF2_1:%[0-9]+]]:vr = PseudoVADD_VV_MF2 $noreg, [[PseudoVSLIDEDOWN_VI_M8_]].sub_vrm1_0, killed [[PseudoVADD_VV_MF2_]], 2, 5 /* e32 */, 1 /* ta, mu */
+    ; CHECK-NEXT: PseudoRET implicit $v8
+    %0:vrm4 = IMPLICIT_DEF
+    %1:gprnox0 = IMPLICIT_DEF
+    %2:vrm8 = IMPLICIT_DEF
+    %3:vr = IMPLICIT_DEF
+    %4:vrm2 = IMPLICIT_DEF
+    %5:vr = IMPLICIT_DEF
+    %6:vrm8 = PseudoVSLIDEDOWN_VI_M8 $noreg, %2, 26, 2, 5 /* e32 */, 3 /* ta, ma */
+    %7:vr = PseudoVADD_VV_MF2 $noreg, %6.sub_vrm1_0, %2.sub_vrm1_0, 2, 5 /* e32 */, 1 /* ta, mu */
+    %8:vr = PseudoVADD_VV_MF2 $noreg, killed %7, killed %3, 2, 5 /* e32 */, 1 /* ta, mu */
+    PseudoRET implicit $v8
+
+...