diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/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.
diff --git a/llvm/test/Transforms/PhaseOrdering/AArch64/loopflatten.ll b/llvm/test/Transforms/PhaseOrdering/AArch64/loopflatten.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/PhaseOrdering/AArch64/loopflatten.ll
@@ -0,0 +1,64 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes='default<O3>' -enable-loop-flatten -loop-flatten-cost-threshold=3 -S %s | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64"
+
+define dso_local void @_Z3fooPiii(i32* %A, i32 %N, i32 %M) #0 {
+; CHECK-LABEL: @_Z3fooPiii(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP3:%.*]] = icmp sgt i32 [[N:%.*]], 0
+; CHECK-NEXT:    [[CMP21:%.*]] = icmp sgt i32 [[M:%.*]], 0
+; CHECK-NEXT:    [[OR_COND:%.*]] = select i1 [[CMP3]], i1 [[CMP21]], i1 false
+; CHECK-NEXT:    br i1 [[OR_COND]], label [[FOR_COND1_PREHEADER_LR_PH_SPLIT_US:%.*]], label [[FOR_COND_CLEANUP:%.*]]
+; CHECK:       for.cond1.preheader.lr.ph.split.us:
+; CHECK-NEXT:    [[TMP0:%.*]] = zext i32 [[M]] to i64
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i32 [[N]] to i64
+; CHECK-NEXT:    [[FLATTEN_TRIPCOUNT:%.*]] = mul nuw nsw i64 [[TMP0]], [[TMP1]]
+; CHECK-NEXT:    br label [[FOR_COND1_PREHEADER_US:%.*]]
+; CHECK:       for.cond1.preheader.us:
+; CHECK-NEXT:    [[INDVAR6:%.*]] = phi i64 [ [[INDVAR_NEXT7:%.*]], [[FOR_COND1_PREHEADER_US]] ], [ 0, [[FOR_COND1_PREHEADER_LR_PH_SPLIT_US]] ]
+; CHECK-NEXT:    [[ARRAYIDX_US:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[INDVAR6]]
+; CHECK-NEXT:    [[TMP2:%.*]] = load i32, i32* [[ARRAYIDX_US]], align 4
+; CHECK-NEXT:    tail call void @_Z1fi(i32 [[TMP2]])
+; CHECK-NEXT:    [[INDVAR_NEXT7]] = add nuw nsw i64 [[INDVAR6]], 1
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INDVAR_NEXT7]], [[FLATTEN_TRIPCOUNT]]
+; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label [[FOR_COND_CLEANUP]], label [[FOR_COND1_PREHEADER_US]]
+; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.cond
+
+for.cond:
+  %i.0 = phi i32 [ 0, %entry ], [ %inc6, %for.cond.cleanup3 ]
+  %cmp = icmp slt i32 %i.0, %N
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  br label %for.cond1
+
+for.cond1:
+  %j.0 = phi i32 [ 0, %for.body ], [ %inc, %for.body4 ]
+  %cmp2 = icmp slt i32 %j.0, %M
+  br i1 %cmp2, label %for.body4, label %for.cond.cleanup3
+
+for.cond.cleanup3:
+  %inc6 = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.body4:
+  %mul = mul nsw i32 %i.0, %M
+  %add = add nsw i32 %mul, %j.0
+  %idxprom = sext i32 %add to i64
+  %arrayidx = getelementptr inbounds i32, i32* %A, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  call void @_Z1fi(i32 %0)
+  %inc = add nsw i32 %j.0, 1
+  br label %for.cond1
+}
+
+declare dso_local void @_Z1fi(i32) #2