diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -10353,12 +10353,28 @@
   case Intrinsic::ppc_vsx_disassemble_pair: {
     int NumVecs = 2;
     SDValue WideVec = Op.getOperand(1);
+
+    bool FromAssemble = false;
+    if (auto *VecIntrinsic = dyn_cast<ConstantSDNode>(WideVec.getOperand(0)))
+      if (VecIntrinsic->getZExtValue() == Intrinsic::ppc_mma_assemble_acc ||
+          VecIntrinsic->getZExtValue() == Intrinsic::ppc_vsx_assemble_pair)
+        FromAssemble = true;
+
     if (IntrinsicID == Intrinsic::ppc_mma_disassemble_acc) {
       NumVecs = 4;
       WideVec = DAG.getNode(PPCISD::XXMFACC, dl, MVT::v512i1, WideVec);
     }
     SmallVector<SDValue, 4> RetOps;
     for (int VecNo = 0; VecNo < NumVecs; VecNo++) {
+      // Propagate undef values if the source is just an assemble, otherwise
+      // we'll get incorrect code for subreg tracking.
+      if (FromAssemble) {
+        SDValue Assembler = (NumVecs == 4) ? WideVec.getOperand(0) : WideVec;
+        if (Assembler.getOperand(VecNo + 1).isUndef()) {
+          RetOps.push_back(DAG.getUNDEF(MVT::v16i8));
+          continue;
+        }
+      }
       SDValue Extract = DAG.getNode(
           PPCISD::EXTRACT_VSX_REG, dl, MVT::v16i8, WideVec,
           DAG.getConstant(Subtarget.isLittleEndian() ? NumVecs - 1 - VecNo
diff --git a/llvm/test/CodeGen/PowerPC/ppc64-acc-regalloc-bugfix.ll b/llvm/test/CodeGen/PowerPC/ppc64-acc-regalloc-bugfix.ll
--- a/llvm/test/CodeGen/PowerPC/ppc64-acc-regalloc-bugfix.ll
+++ b/llvm/test/CodeGen/PowerPC/ppc64-acc-regalloc-bugfix.ll
@@ -1,20 +1,21 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -verify-machineinstrs -mtriple powerpc64le-unknown-linux-gnu \
 ; RUN:     -mcpu=pwr10 -ppc-vsr-nums-as-vr -ppc-asm-full-reg-names < %s \
-; RUN:    | FileCheck %s
+; RUN:     -ppc-track-subreg-liveness | FileCheck %s
 
 define void @copy_novsrp() local_unnamed_addr {
 ; CHECK-LABEL: copy_novsrp:
 ; CHECK:       # %bb.0: # %dmblvi_entry
-; CHECK-NEXT:    xxlxor v2, v2, v2
+; CHECK-NEXT:    xxlxor vs3, vs3, vs3
 ; CHECK-NEXT:    xxlxor vs0, vs0, vs0
-; CHECK-NEXT:    xxlor vs3, v2, v2
-; CHECK-NEXT:    stxv vs1, 0(0)
+; CHECK-NEXT:    stxv vs3, 0(0)
 dmblvi_entry:
   %0 = tail call <512 x i1> @llvm.ppc.mma.assemble.acc(<16 x i8> zeroinitializer, <16 x i8> undef, <16 x i8> undef, <16 x i8> zeroinitializer)
   %1 = tail call { <16 x i8>, <16 x i8>, <16 x i8>, <16 x i8> } @llvm.ppc.mma.disassemble.acc(<512 x i1> %0)
   %2 = extractvalue { <16 x i8>, <16 x i8>, <16 x i8>, <16 x i8> } %1, 2
+  %3 = extractvalue { <16 x i8>, <16 x i8>, <16 x i8>, <16 x i8> } %1, 0
   store <16 x i8> %2, <16 x i8>* null, align 1
+  store <16 x i8> %3, <16 x i8>* null, align 1
   unreachable
 }