Index: include/llvm/Analysis/AliasAnalysis.h
===================================================================
--- include/llvm/Analysis/AliasAnalysis.h
+++ include/llvm/Analysis/AliasAnalysis.h
@@ -690,6 +690,9 @@
     return canInstructionRangeModRef(I1, I2, MemoryLocation(Ptr, Size), Mode);
   }
 
+  void startKeepingCaches();
+  void stopKeepingCaches();
+
 private:
   class Concept;
 
@@ -771,6 +774,12 @@
   virtual ModRefInfo getModRefInfo(const CallBase *Call1,
                                    const CallBase *Call2) = 0;
 
+  // Don't clear internal caches. Only call when we know thare are no functional
+  // IR changes. Call its counterpart below to clear caches and make AA
+  // stateless again.
+  virtual void startKeepingCaches() = 0;
+  virtual void stopKeepingCaches() = 0;
+
   /// @}
 };
 
@@ -822,6 +831,9 @@
                            const CallBase *Call2) override {
     return Result.getModRefInfo(Call1, Call2);
   }
+
+  void startKeepingCaches() override { Result.startKeepingCaches(); }
+  void stopKeepingCaches() override { Result.stopKeepingCaches(); }
 };
 
 /// A CRTP-driven "mixin" base class to help implement the function alias
@@ -898,6 +910,14 @@
       return AAR ? AAR->getModRefInfo(Call1, Call2)
                  : CurrentResult.getModRefInfo(Call1, Call2);
     }
+
+    void startKeepingCaches() {
+      AAR ? AAR->startKeepingCaches() : CurrentResult.startKeepingCaches();
+    }
+
+    void stopKeepingCaches() {
+      AAR ? AAR->stopKeepingCaches() : CurrentResult.stopKeepingCaches();
+    }
   };
 
   explicit AAResultBase() = default;
@@ -947,6 +967,9 @@
   ModRefInfo getModRefInfo(const CallBase *Call1, const CallBase *Call2) {
     return ModRefInfo::ModRef;
   }
+
+  void startKeepingCaches() {}
+  void stopKeepingCaches() {}
 };
 
 /// Return true if this pointer is returned by a noalias function.
Index: include/llvm/Analysis/BasicAliasAnalysis.h
===================================================================
--- include/llvm/Analysis/BasicAliasAnalysis.h
+++ include/llvm/Analysis/BasicAliasAnalysis.h
@@ -100,7 +100,12 @@
   /// call site is not known.
   FunctionModRefBehavior getModRefBehavior(const Function *Fn);
 
+  void startKeepingCaches();
+  void stopKeepingCaches();
+
 private:
+  bool keepBasicAAInternalCaches = false;
+
   // A linear transformation of a Value; this class represents ZExt(SExt(V,
   // SExtBits), ZExtBits) * Scale + Offset.
   struct VariableGEPIndex {
Index: lib/Analysis/AliasAnalysis.cpp
===================================================================
--- lib/Analysis/AliasAnalysis.cpp
+++ lib/Analysis/AliasAnalysis.cpp
@@ -372,6 +372,16 @@
   return Result;
 }
 
+void AAResults::startKeepingCaches() {
+  for (const auto &AA : AAs)
+    AA->startKeepingCaches();
+}
+
+void AAResults::stopKeepingCaches() {
+  for (const auto &AA : AAs)
+    AA->stopKeepingCaches();
+}
+
 raw_ostream &llvm::operator<<(raw_ostream &OS, AliasResult AR) {
   switch (AR) {
   case NoAlias:
Index: lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- lib/Analysis/BasicAliasAnalysis.cpp
+++ lib/Analysis/BasicAliasAnalysis.cpp
@@ -816,6 +816,16 @@
 }
 #endif
 
+void BasicAAResult::startKeepingCaches() {
+  keepBasicAAInternalCaches = true;
+}
+
+void BasicAAResult::stopKeepingCaches() {
+  AliasCache.shrink_and_clear();
+  IsCapturedCache.shrink_and_clear();
+  keepBasicAAInternalCaches = false;
+}
+
 AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
                                  const MemoryLocation &LocB) {
   assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
@@ -834,8 +844,12 @@
   // shrink_and_clear so it quickly returns to the inline capacity of the
   // SmallDenseMap if it ever grows larger.
   // FIXME: This should really be shrink_to_inline_capacity_and_clear().
-  AliasCache.shrink_and_clear();
-  IsCapturedCache.shrink_and_clear();
+
+  if (!keepBasicAAInternalCaches) {
+    AliasCache.shrink_and_clear();
+    IsCapturedCache.shrink_and_clear();
+  }
+
   VisitedPhiBBs.clear();
   return Alias;
 }
Index: lib/Analysis/MemorySSA.cpp
===================================================================
--- lib/Analysis/MemorySSA.cpp
+++ lib/Analysis/MemorySSA.cpp
@@ -1155,7 +1155,9 @@
 MemorySSA::MemorySSA(Function &Func, AliasAnalysis *AA, DominatorTree *DT)
     : AA(AA), DT(DT), F(Func), LiveOnEntryDef(nullptr), Walker(nullptr),
       SkipWalker(nullptr), NextID(0) {
+  AA->startKeepingCaches();
   buildMemorySSA();
+  AA->stopKeepingCaches();
 }
 
 MemorySSA::~MemorySSA() {