diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -436,16 +436,13 @@
             MO.setReg(VI->second.Reg);
             // We have Reg -> VI.Reg:VI.SubReg, so if Reg is used with a
             // sub-register, we need to compose the sub-register indices.
-            MO.setSubReg(TRI->composeSubRegIndices(MO.getSubReg(),
-                                                   VI->second.SubReg));
+            MO.setSubReg(
+                TRI->composeSubRegIndices(VI->second.SubReg, MO.getSubReg()));
           } else {
             // The direct replacement is not possible, due to failing register
             // class constraints. An explicit COPY is necessary. Create one
-            // that can be reused
-            auto *NewRC = MI->getRegClassConstraint(i, TII, TRI);
-            if (NewRC == nullptr)
-              NewRC = OrigRC;
-            Register NewReg = MRI->createVirtualRegister(NewRC);
+            // that can be reused.
+            Register NewReg = MRI->createVirtualRegister(OrigRC);
             BuildMI(*PredBB, NewMI, NewMI.getDebugLoc(),
                     TII->get(TargetOpcode::COPY), NewReg)
                 .addReg(VI->second.Reg, 0, VI->second.SubReg);
diff --git a/llvm/test/CodeGen/AArch64/taildup-subreg-compose.mir b/llvm/test/CodeGen/AArch64/taildup-subreg-compose.mir
--- a/llvm/test/CodeGen/AArch64/taildup-subreg-compose.mir
+++ b/llvm/test/CodeGen/AArch64/taildup-subreg-compose.mir
@@ -23,7 +23,7 @@
   ; CHECK-NEXT:   liveins: $q1
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   [[COPY:%[0-9]+]]:fpr128 = COPY $q1
-  ; CHECK-NEXT:   $s1 = COPY [[COPY]].dsub
+  ; CHECK-NEXT:   $s1 = COPY [[COPY]].ssub
   ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:fpr64 = COPY [[COPY]].dsub
   ; CHECK-NEXT:   B %bb.4
   ; CHECK-NEXT: {{  $}}
@@ -31,7 +31,7 @@
   ; CHECK-NEXT:   successors: %bb.4(0x80000000)
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   [[DEF:%[0-9]+]]:fpr128 = IMPLICIT_DEF
-  ; CHECK-NEXT:   $s1 = COPY [[DEF]].dsub
+  ; CHECK-NEXT:   $s1 = COPY [[DEF]].ssub
   ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:fpr64 = COPY [[DEF]].dsub
   ; CHECK-NEXT:   B %bb.4
   ; CHECK-NEXT: {{  $}}