diff --git a/lib/CodeGen/LiveIntervalAnalysis.cpp b/lib/CodeGen/LiveIntervalAnalysis.cpp
index 3d95827226a..b1e202a273d 100644
--- a/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -304,6 +304,10 @@ void LiveIntervals::handleVirtualRegisterDef(MachineBasicBlock *mbb,
 
     // Make sure the first definition is not a partial redefinition. Add an
     // <imp-def> of the full register.
+    // FIXME: LiveIntervals shouldn't modify the code like this.  Whoever
+    // created the machine instruction should annotate it with <undef> flags
+    // as needed.  Then we can simply assert here.  The REG_SEQUENCE lowering
+    // is the main suspect.
     if (MO.getSubReg()) {
       mi->addRegisterDefined(interval.reg);
       // Mark all defs of interval.reg on this instruction as reading <undef>.