diff --git a/llvm/lib/CodeGen/DetectDeadLanes.cpp b/llvm/lib/CodeGen/DetectDeadLanes.cpp
--- a/llvm/lib/CodeGen/DetectDeadLanes.cpp
+++ b/llvm/lib/CodeGen/DetectDeadLanes.cpp
@@ -340,8 +340,8 @@
     llvm_unreachable("function must be called with COPY-like instruction");
   }
 
-  assert(Def.getSubReg() == 0 &&
-         "Should not have subregister defs in machine SSA phase");
+  unsigned SubIdx = Def.getSubReg();
+  DefinedLanes = TRI->composeSubRegIndexLaneMask(SubIdx, DefinedLanes);
   DefinedLanes &= MRI->getMaxLaneMaskForVReg(Def.getReg());
   return DefinedLanes;
 }
@@ -406,9 +406,9 @@
   if (DefMI.isImplicitDef() || Def.isDead())
     return LaneBitmask::getNone();
 
-  assert(Def.getSubReg() == 0 &&
-         "Should not have subregister defs in machine SSA phase");
-  return MRI->getMaxLaneMaskForVReg(Reg);
+  unsigned SubReg = Def.getSubReg();
+  return SubReg != 0 ? TRI->getSubRegIndexLaneMask(SubReg)
+                     : MRI->getMaxLaneMaskForVReg(Reg);
 }
 
 LaneBitmask DetectDeadLanes::determineInitialUsedLanes(unsigned Reg) {