diff --git a/lib/SILOptimizer/SILCombiner/SILCombine.cpp b/lib/SILOptimizer/SILCombiner/SILCombine.cpp
index 4481b188a43c5..df91fb8e13cf9 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombine.cpp
+++ b/lib/SILOptimizer/SILCombiner/SILCombine.cpp
@@ -172,7 +172,7 @@ bool SILCombiner::trySinkOwnedForwardingInst(SingleValueInstruction *svi) {
     svi->moveBefore(consumingUser);
     MadeChange = true;
 
-    // NOTE: We return nullptr here so that our caller doesn't delete the
+    // NOTE: We return false here so that our caller doesn't delete the
     // instruction and instead tries to simplify it.
     return false;
   }
diff --git a/lib/SILOptimizer/SILCombiner/SILCombiner.h b/lib/SILOptimizer/SILCombiner/SILCombiner.h
index 741e60e70bf86..8fcc715e0d734 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombiner.h
+++ b/lib/SILOptimizer/SILCombiner/SILCombiner.h
@@ -353,10 +353,6 @@ class SILCombiner :
   bool optimizeIdentityCastComposition(ApplyInst *FInverse,
                                        StringRef FInverseName, StringRef FName);
 
-  /// Let \p user and \p value be two forwarding single value instructions with
-  /// the property that \p user, through potentially a chain of forwarding
-  /// instructions.
-  ///
   /// Let \p user and \p value be two forwarding single value instructions  with
   /// the property that \p value is the value that \p user forwards. In this
   /// case, this helper routine will eliminate \p value if it can rewrite user
diff --git a/lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp b/lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp
index d8e03c625c2e4..2e0fcbcc6a3cd 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp
+++ b/lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp
@@ -141,6 +141,7 @@ class SingleBlockOwnedForwardingInstFolder {
     if (!hasOneNonDebugUse(next))
       return false;
 
+    assert(getSingleNonDebugUser(rest.back()) == next);
     rest.push_back(next);
     return true;
   }
@@ -166,14 +167,13 @@ class SingleBlockOwnedForwardingInstFolder {
   }
 
 private:
-  /// From backwards -> forwards, for each instruction in rest, delete all of
-  /// its debug uses and then set its single remaining use to be SILUndef.
+  /// Processing from def->use by walking rest backwards, delete all of its
+  /// debug uses and then set its single remaining use to be SILUndef.
   ///
   /// This means that after this runs front's forwarding operand is now
   /// SILUndef.
   void cleanupRest() & {
-    // We process backwards -> forwards. This cleans up everything but the front
-    // value.
+    // We process from def->use. This cleans up everything but the front value.
     while (!rest.empty()) {
       auto *inst = rest.pop_back_val();
       deleteAllDebugUses(inst, SC.getInstModCallbacks());