Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -300,6 +300,8 @@
   // TODO: Investigate promotion cap for O1.
   LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap));
   LPM1.addPass(SimpleLoopUnswitchPass());
+  if (EnableLoopFlatten)
+    LPM1.addPass(LoopFlattenPass());
 
   LPM2.addPass(LoopIdiomRecognizePass());
   LPM2.addPass(IndVarSimplifyPass());
@@ -311,8 +313,6 @@
 
   if (EnableLoopInterchange)
     LPM2.addPass(LoopInterchangePass());
-  if (EnableLoopFlatten)
-    LPM2.addPass(LoopFlattenPass());
 
   // Do not enable unrolling in PreLinkThinLTO phase during sample PGO
   // because it changes IR to makes profile annotation in back compile
@@ -475,6 +475,9 @@
   LPM1.addPass(
       SimpleLoopUnswitchPass(/* NonTrivial */ Level == OptimizationLevel::O3 &&
                              EnableO3NonTrivialUnswitching));
+  if (EnableLoopFlatten)
+    LPM1.addPass(LoopFlattenPass());
+
   LPM2.addPass(LoopIdiomRecognizePass());
   LPM2.addPass(IndVarSimplifyPass());
 
@@ -485,8 +488,6 @@
 
   if (EnableLoopInterchange)
     LPM2.addPass(LoopInterchangePass());
-  if (EnableLoopFlatten)
-    LPM2.addPass(LoopFlattenPass());
 
   // Do not enable unrolling in PreLinkThinLTO phase during sample PGO
   // because it changes IR to makes profile annotation in back compile
@@ -1628,10 +1629,10 @@
     MainFPM.addPass(ConstraintEliminationPass());
 
   LoopPassManager LPM;
-  LPM.addPass(IndVarSimplifyPass());
-  LPM.addPass(LoopDeletionPass());
   if (EnableLoopFlatten && Level.getSpeedupLevel() > 1)
     LPM.addPass(LoopFlattenPass());
+  LPM.addPass(IndVarSimplifyPass());
+  LPM.addPass(LoopDeletionPass());
   // FIXME: Add loop interchange.
 
   // Unroll small loops and perform peeling.
Index: llvm/lib/Transforms/Scalar/LoopFlatten.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopFlatten.cpp
+++ llvm/lib/Transforms/Scalar/LoopFlatten.cpp
@@ -31,6 +31,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/MemorySSA.h"
 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
@@ -822,7 +823,9 @@
   if (!Changed)
     return PreservedAnalyses::all();
 
-  return getLoopPassPreservedAnalyses();
+  auto PA = getLoopPassPreservedAnalyses();
+  PA.preserve<MemorySSAAnalysis>();
+  return PA;
 }
 
 namespace {