Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -290,6 +290,8 @@
 
   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
@@ -314,8 +316,6 @@
                                               /*UseBlockFrequencyInfo=*/true));
   FPM.addPass(SimplifyCFGPass());
   FPM.addPass(InstCombinePass());
-  if (EnableLoopFlatten)
-    FPM.addPass(createFunctionToLoopPassAdaptor(LoopFlattenPass()));
   // The loop passes in LPM2 (LoopFullUnrollPass) do not preserve MemorySSA.
   // *All* loop passes must preserve it, in order to be able to use it.
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM2),
@@ -464,6 +464,8 @@
 
   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
@@ -488,8 +490,6 @@
                                               /*UseBlockFrequencyInfo=*/true));
   FPM.addPass(SimplifyCFGPass());
   FPM.addPass(InstCombinePass());
-  if (EnableLoopFlatten)
-    FPM.addPass(createFunctionToLoopPassAdaptor(LoopFlattenPass()));
   // The loop passes in LPM2 (LoopIdiomRecognizePass, IndVarSimplifyPass,
   // LoopDeletionPass and LoopFullUnrollPass) do not preserve MemorySSA.
   // *All* loop passes must preserve it, in order to be able to use it.
@@ -1538,15 +1538,14 @@
   MainFPM.addPass(DSEPass());
   MainFPM.addPass(MergedLoadStoreMotionPass());
 
-  // More loops are countable; try to optimize them.
-  if (EnableLoopFlatten && Level.getSpeedupLevel() > 1)
-    MainFPM.addPass(createFunctionToLoopPassAdaptor(LoopFlattenPass()));
 
   if (EnableConstraintElimination)
     MainFPM.addPass(ConstraintEliminationPass());
 
   LoopPassManager LPM;
   LPM.addPass(IndVarSimplifyPass());
+  if (EnableLoopFlatten && Level.getSpeedupLevel() > 1)
+    LPM.addPass(LoopFlattenPass());
   LPM.addPass(LoopDeletionPass());
   // FIXME: Add loop interchange.
 
Index: llvm/test/Other/new-pm-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-defaults.ll
+++ llvm/test/Other/new-pm-defaults.ll
@@ -163,13 +163,11 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-EP-LOOP-LATE-NEXT: Running pass: NoOpLoopPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: LoopFullUnrollPass
 ; CHECK-EP-LOOP-END-NEXT: Running pass: NoOpLoopPass
 ; CHECK-O-NEXT: Running pass: SROA on foo
Index: llvm/test/Other/new-pm-lto-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-lto-defaults.ll
+++ llvm/test/Other/new-pm-lto-defaults.ll
@@ -102,10 +102,8 @@
 ; CHECK-O23SZ-NEXT: Running pass: MergedLoadStoreMotionPass on foo
 ; CHECK-O23SZ-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O23SZ-NEXT: Running pass: LCSSAPass
-; CHECK-O23SZ-NEXT: Running pass: LoopFlatten
-; CHECK-O23SZ-NEXT: Running pass: LoopSimplifyPass on foo
-; CHECK-O23SZ-NEXT: Running pass: LCSSAPass on foo
 ; CHECK-O23SZ-NEXT: Running pass: IndVarSimplifyPass on Loop
+; CHECK-O23SZ-NEXT: Running pass: LoopFlatten
 ; CHECK-O23SZ-NEXT: Running pass: LoopDeletionPass on Loop
 ; CHECK-O23SZ-NEXT: Running pass: LoopFullUnrollPass on Loop
 ; CHECK-O23SZ-NEXT: Running pass: LoopDistributePass on foo
Index: llvm/test/Other/new-pm-thinlto-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-defaults.ll
@@ -141,12 +141,10 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: LoopFullUnrollPass
 ; CHECK-O-NEXT: Running pass: SROA on foo
 ; CHECK-Os-NEXT: Running pass: MergedLoadStoreMotionPass
Index: llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
@@ -114,12 +114,10 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: LoopFullUnrollPass
 ; CHECK-O-NEXT: Running pass: SROA on foo
 ; CHECK-Os-NEXT: Running pass: MergedLoadStoreMotionPass
Index: llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
@@ -123,12 +123,10 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: LoopFullUnrollPass
 ; CHECK-O-NEXT: Running pass: SROA on foo
 ; CHECK-Os-NEXT: Running pass: MergedLoadStoreMotionPass
Index: llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
@@ -154,12 +154,10 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: LoopFullUnrollPass
 ; CHECK-O-NEXT: Running pass: SROA on foo
 ; CHECK-Os-NEXT: Running pass: MergedLoadStoreMotionPass
Index: llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
@@ -118,12 +118,10 @@
 ; CHECK-O-NEXT: Running pass: InstCombinePass
 ; CHECK-O-NEXT: Running pass: LoopSimplifyPass
 ; CHECK-O-NEXT: Running pass: LCSSAPass
-; CHECK-O-NEXT: Running pass: LoopFlatten
-; CHECK-O-NEXT: Running pass: LoopSimplifyPass
-; CHECK-O-NEXT: Running pass: LCSSAPass
 ; CHECK-O-NEXT: Running pass: LoopIdiomRecognizePass
 ; CHECK-O-NEXT: Running pass: IndVarSimplifyPass
 ; CHECK-O-NEXT: Running pass: LoopDeletionPass
+; CHECK-O-NEXT: Running pass: LoopFlatten
 ; CHECK-O-NEXT: Running pass: SROA on foo
 ; CHECK-Os-NEXT: Running pass: MergedLoadStoreMotionPass
 ; CHECK-Os-NEXT: Running pass: GVN