diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index 79c42b886fbbe..a6a36ff254029 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -311,8 +311,6 @@ PassBuilder::buildO1FunctionSimplificationPipeline(OptimizationLevel Level,
 
   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
@@ -337,6 +335,8 @@ PassBuilder::buildO1FunctionSimplificationPipeline(OptimizationLevel Level,
                                               /*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),
@@ -485,8 +485,6 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   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
@@ -511,6 +509,8 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
                                               /*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.
@@ -1623,6 +1623,9 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   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());
@@ -1630,8 +1633,6 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   LoopPassManager LPM;
   LPM.addPass(IndVarSimplifyPass());
   LPM.addPass(LoopDeletionPass());
-  if (EnableLoopFlatten && Level.getSpeedupLevel() > 1)
-    LPM.addPass(LoopFlattenPass());
   // FIXME: Add loop interchange.
 
   // Unroll small loops and perform peeling.