Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -95,7 +95,7 @@
                                  cl::Hidden,
                                  cl::desc("Enable Unroll And Jam Pass"));
 
-cl::opt<bool> EnableLoopFlatten("enable-loop-flatten", cl::init(false),
+cl::opt<bool> EnableLoopFlatten("enable-loop-flatten", cl::init(true),
                                 cl::Hidden,
                                 cl::desc("Enable the LoopFlatten Pass"));
 
Index: llvm/test/Other/new-pm-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-defaults.ll
+++ llvm/test/Other/new-pm-defaults.ll
@@ -163,6 +163,9 @@
 ; 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
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
@@ -100,6 +100,9 @@
 ; CHECK-O23SZ-NEXT: Running pass: DSEPass on foo
 ; CHECK-O23SZ-NEXT: Running analysis: PostDominatorTreeAnalysis on foo
 ; 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
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
@@ -141,6 +141,9 @@
 ; 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
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
@@ -114,6 +114,9 @@
 ; 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
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
@@ -123,6 +123,9 @@
 ; 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
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
@@ -154,6 +154,9 @@
 ; 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
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
@@ -118,6 +118,9 @@
 ; 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