diff --git a/include/llvm/Analysis/AliasSetTracker.h b/include/llvm/Analysis/AliasSetTracker.h
index 6117d91ec65..e32b6d628b7 100644
--- a/include/llvm/Analysis/AliasSetTracker.h
+++ b/include/llvm/Analysis/AliasSetTracker.h
@@ -253,13 +253,16 @@ private:
                   const MDNode *TBAAInfo,
                   bool KnownMustAlias = false);
   void addUnknownInst(Instruction *I, AliasAnalysis &AA);
-  void removeUnknownInst(Instruction *I) {
+  void removeUnknownInst(AliasSetTracker &AST, Instruction *I) {
+    bool WasEmpty = UnknownInsts.empty();
     for (size_t i = 0, e = UnknownInsts.size(); i != e; ++i)
       if (UnknownInsts[i] == I) {
         UnknownInsts[i] = UnknownInsts.back();
         UnknownInsts.pop_back();
         --i; --e;  // Revisit the moved entry.
       }
+    if (!WasEmpty && UnknownInsts.empty())
+      dropRef(AST);
   }
   void setVolatile() { Volatile = true; }
 
diff --git a/lib/Analysis/AliasSetTracker.cpp b/lib/Analysis/AliasSetTracker.cpp
index a45fe2389ee..cf3aecfd010 100644
--- a/lib/Analysis/AliasSetTracker.cpp
+++ b/lib/Analysis/AliasSetTracker.cpp
@@ -55,12 +55,16 @@ void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
       AliasTy = MayAlias;
   }
 
+  bool ASHadUnknownInsts = false;
   if (UnknownInsts.empty()) {            // Merge call sites...
-    if (!AS.UnknownInsts.empty())
+    if (!AS.UnknownInsts.empty()) {
       std::swap(UnknownInsts, AS.UnknownInsts);
+      addRef();
+    }
   } else if (!AS.UnknownInsts.empty()) {
     UnknownInsts.insert(UnknownInsts.end(), AS.UnknownInsts.begin(), AS.UnknownInsts.end());
     AS.UnknownInsts.clear();
+    ASHadUnknownInsts = true;
   }
 
   AS.Forward = this;  // Forward across AS now...
@@ -76,6 +80,8 @@ void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
     AS.PtrListEnd = &AS.PtrList;
     assert(*AS.PtrListEnd == nullptr && "End of list is not null?");
   }
+  if (ASHadUnknownInsts)
+    AS.dropRef(AST);
 }
 
 void AliasSetTracker::removeAliasSet(AliasSet *AS) {
@@ -123,6 +129,8 @@ void AliasSet::addPointer(AliasSetTracker &AST, PointerRec &Entry,
 }
 
 void AliasSet::addUnknownInst(Instruction *I, AliasAnalysis &AA) {
+  if (UnknownInsts.empty())
+    addRef();
   UnknownInsts.push_back(I);
 
   if (!I->mayWriteToMemory()) {
@@ -218,13 +226,14 @@ AliasSet *AliasSetTracker::findAliasSetForPointer(const Value *Ptr,
                                                   uint64_t Size,
                                                   const MDNode *TBAAInfo) {
   AliasSet *FoundSet = nullptr;
-  for (iterator I = begin(), E = end(); I != E; ++I) {
-    if (I->Forward || !I->aliasesPointer(Ptr, Size, TBAAInfo, AA)) continue;
+  for (iterator I = begin(), E = end(); I != E;) {
+    iterator Cur = I++;
+    if (Cur->Forward || !Cur->aliasesPointer(Ptr, Size, TBAAInfo, AA)) continue;
     
     if (!FoundSet) {      // If this is the first alias set ptr can go into.
-      FoundSet = I;       // Remember it.
+      FoundSet = Cur;     // Remember it.
     } else {              // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*I, *this);     // Merge in contents.
+      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
     }
   }
 
@@ -246,14 +255,14 @@ bool AliasSetTracker::containsPointer(Value *Ptr, uint64_t Size,
 
 AliasSet *AliasSetTracker::findAliasSetForUnknownInst(Instruction *Inst) {
   AliasSet *FoundSet = nullptr;
-  for (iterator I = begin(), E = end(); I != E; ++I) {
-    if (I->Forward || !I->aliasesUnknownInst(Inst, AA))
+  for (iterator I = begin(), E = end(); I != E;) {
+    iterator Cur = I++;
+    if (Cur->Forward || !Cur->aliasesUnknownInst(Inst, AA))
       continue;
-    
     if (!FoundSet)            // If this is the first alias set ptr can go into.
-      FoundSet = I;           // Remember it.
-    else if (!I->Forward)     // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*I, *this);     // Merge in contents.
+      FoundSet = Cur;         // Remember it.
+    else if (!Cur->Forward)   // Otherwise, we must merge the sets.
+      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
   }
   return FoundSet;
 }
@@ -393,6 +402,8 @@ void AliasSetTracker::add(const AliasSetTracker &AST) {
 /// tracker.
 void AliasSetTracker::remove(AliasSet &AS) {
   // Drop all call sites.
+  if (!AS.UnknownInsts.empty())
+    AS.dropRef(*this);
   AS.UnknownInsts.clear();
   
   // Clear the alias set.
@@ -489,10 +500,10 @@ void AliasSetTracker::deleteValue(Value *PtrVal) {
   if (Instruction *Inst = dyn_cast<Instruction>(PtrVal)) {
     if (Inst->mayReadOrWriteMemory()) {
       // Scan all the alias sets to see if this call site is contained.
-      for (iterator I = begin(), E = end(); I != E; ++I) {
-        if (I->Forward) continue;
-        
-        I->removeUnknownInst(Inst);
+      for (iterator I = begin(), E = end(); I != E;) {
+        iterator Cur = I++;
+        if (!Cur->Forward)
+          Cur->removeUnknownInst(*this, Inst);
       }
     }
   }
diff --git a/test/Transforms/LICM/PR21582.ll b/test/Transforms/LICM/PR21582.ll
new file mode 100644
index 00000000000..c068c2f8e32
--- /dev/null
+++ b/test/Transforms/LICM/PR21582.ll
@@ -0,0 +1,40 @@
+; RUN: opt < %s -basicaa -licm -S | FileCheck %s
+@b = external global i32, align 4
+@fn3.i = external global i32, align 4
+
+declare i32 @g() nounwind
+
+define i32 @f() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.end, %entry
+; CHECK-LABEL: for.cond:
+; CHECK: store i32 0, i32* @b
+  store i32 0, i32* @b, align 4
+  br i1 true, label %for.body.preheader, label %for.end
+
+for.body.preheader:                               ; preds = %for.cond
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %for.body.preheader
+  %g.15 = phi i32 [ undef, %for.body ], [ 0, %for.body.preheader ]
+  %arrayidx2 = getelementptr inbounds i32* @fn3.i, i64 0
+  %0 = load i32* %arrayidx2, align 4
+  %call = call i32 @g()
+  br i1 false, label %for.body, label %for.end.loopexit
+
+for.end.loopexit:                                 ; preds = %for.body
+  br label %for.end
+
+for.end:                                          ; preds = %for.end.loopexit, %for.cond
+  %whatever = phi i32 [ %call, %for.end.loopexit ], [ undef, %for.cond ]
+  br i1 false, label %for.cond, label %if.then
+
+if.then:                                          ; preds = %for.end
+; CHECK-LABEL: if.then:
+; CHECK: phi i32 [ {{.*}}, %for.end ]
+; CHECK-NOT: store i32 0, i32* @b
+; CHECK: ret i32
+  ret i32 %whatever
+}