Index: lib/Transforms/Scalar/LoopInterchange.cpp
===================================================================
--- lib/Transforms/Scalar/LoopInterchange.cpp
+++ lib/Transforms/Scalar/LoopInterchange.cpp
@@ -22,6 +22,7 @@
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopIterator.h"
 #include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionExpander.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
@@ -323,9 +324,10 @@
 class LoopInterchangeLegality {
 public:
   LoopInterchangeLegality(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
-                          LoopInfo *LI, DominatorTree *DT, bool PreserveLCSSA)
+                          LoopInfo *LI, DominatorTree *DT, bool PreserveLCSSA,
+                          OptimizationRemarkEmitter *ORE)
       : OuterLoop(Outer), InnerLoop(Inner), SE(SE), LI(LI), DT(DT),
-        PreserveLCSSA(PreserveLCSSA), InnerLoopHasReduction(false) {}
+        PreserveLCSSA(PreserveLCSSA), ORE(ORE), InnerLoopHasReduction(false) {}
 
   /// Check if the loops can be interchanged.
   bool canInterchangeLoops(unsigned InnerLoopId, unsigned OuterLoopId,
@@ -353,6 +355,8 @@
   LoopInfo *LI;
   DominatorTree *DT;
   bool PreserveLCSSA;
+  /// Interface to emit optimization remarks.
+  OptimizationRemarkEmitter *ORE;
 
   bool InnerLoopHasReduction;
 };
@@ -422,6 +426,9 @@
   DependenceInfo *DI;
   DominatorTree *DT;
   bool PreserveLCSSA;
+  /// Interface to emit optimization remarks.
+  OptimizationRemarkEmitter *ORE;
+
   LoopInterchange()
       : FunctionPass(ID), SE(nullptr), LI(nullptr), DI(nullptr), DT(nullptr) {
     initializeLoopInterchangePass(*PassRegistry::getPassRegistry());
@@ -435,6 +442,7 @@
     AU.addRequired<DependenceAnalysisWrapperPass>();
     AU.addRequiredID(LoopSimplifyID);
     AU.addRequiredID(LCSSAID);
+    AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
   }
 
   bool runOnFunction(Function &F) override {
@@ -446,6 +454,7 @@
     DI = &getAnalysis<DependenceAnalysisWrapperPass>().getDI();
     auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
     DT = DTWP ? &DTWP->getDomTree() : nullptr;
+    ORE = &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
     PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
 
     // Build up a worklist of loop pairs to analyze.
@@ -575,7 +584,7 @@
     Loop *OuterLoop = LoopList[OuterLoopId];
 
     LoopInterchangeLegality LIL(OuterLoop, InnerLoop, SE, LI, DT,
-                                PreserveLCSSA);
+                                PreserveLCSSA, ORE);
     if (!LIL.canInterchangeLoops(InnerLoopId, OuterLoopId, DependencyMatrix)) {
       DEBUG(dbgs() << "Not interchanging Loops. Cannot prove legality\n");
       return false;
@@ -584,9 +593,21 @@
     LoopInterchangeProfitability LIP(OuterLoop, InnerLoop, SE);
     if (!LIP.isProfitable(InnerLoopId, OuterLoopId, DependencyMatrix)) {
       DEBUG(dbgs() << "Interchanging loops not profitable\n");
+      ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                         "InterchangeNotProfitable",
+                                         InnerLoop->getStartLoc(),
+                                         InnerLoop->getHeader())
+                << "Interchanging loops is not profitable.");
+
+
       return false;
     }
 
+    ORE->emit(OptimizationRemark(DEBUG_TYPE, "Interchanged",
+                                 InnerLoop->getStartLoc(),
+                                 InnerLoop->getHeader())
+              << "Loop interchanged with enclosing loop.");
+
     LoopInterchangeTransform LIT(OuterLoop, InnerLoop, SE, LI, DT,
                                  LoopNestExit, LIL.hasInnerLoopReduction());
     LIT.transform();
@@ -760,6 +781,12 @@
   if (!findInductionAndReductions(InnerLoop, Inductions, Reductions)) {
     DEBUG(dbgs() << "Only inner loops with induction or reduction PHI nodes "
                  << "are supported currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Only inner loops with induction or reduction PHI nodes can be"
+                 " interchange currently.");
     return true;
   }
 
@@ -767,6 +794,12 @@
   if (Inductions.size() != 1) {
     DEBUG(dbgs() << "We currently only support loops with 1 induction variable."
                  << "Failed to interchange due to current limitation\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Only inner loops with 1 induction variable can be "
+                 "interchanged currently.");
     return true;
   }
   if (Reductions.size() > 0)
@@ -777,6 +810,12 @@
   if (!findInductionAndReductions(OuterLoop, Inductions, Reductions)) {
     DEBUG(dbgs() << "Only outer loops with induction or reduction PHI nodes "
                  << "are supported currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Only outer loops with induction or reduction PHI nodes can be"
+                 " interchanged currently.");
     return true;
   }
 
@@ -785,18 +824,36 @@
   if (!Reductions.empty()) {
     DEBUG(dbgs() << "Outer loops with reductions are not supported "
                  << "currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Outer loops with reductions cannot be interchange "
+                 "currently.");
+
     return true;
   }
   // TODO: Currently we handle only loops with 1 induction variable.
   if (Inductions.size() != 1) {
     DEBUG(dbgs() << "Loops with more than 1 induction variables are not "
                  << "supported currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       OuterLoop->getStartLoc(),
+                                       OuterLoop->getHeader())
+              << "Only outer loops with 1 induction variable can be "
+                 "interchanged currently.");
     return true;
   }
 
   // TODO: Triangular loops are not handled for now.
   if (!isLoopStructureUnderstood(InnerInductionVar)) {
     DEBUG(dbgs() << "Loop structure not understood by pass\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Loop structure not understood currently.");
     return true;
   }
 
@@ -805,12 +862,24 @@
       getLoopLatchExitBlock(OuterLoopLatch, OuterLoopHeader);
   if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, true)) {
     DEBUG(dbgs() << "Can only handle LCSSA PHIs in outer loops currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       OuterLoop->getStartLoc(),
+                                       OuterLoop->getHeader())
+              << "Only outer loops with LCSSA PHIs can be interchange "
+                 "currently.");
     return true;
   }
 
   LoopExitBlock = getLoopLatchExitBlock(InnerLoopLatch, InnerLoopHeader);
   if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, false)) {
     DEBUG(dbgs() << "Can only handle LCSSA PHIs in inner loops currently.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Only inner loops with LCSSA PHIs can be interchange "
+                 "currently.");
     return true;
   }
 
@@ -835,6 +904,11 @@
   if (!InnerIndexVarInc) {
     DEBUG(dbgs() << "Did not find an instruction to increment the induction "
                  << "variable.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "The inner loop does not increment the induction variable.");
     return true;
   }
 
@@ -852,6 +926,13 @@
     if (!I.isIdenticalTo(InnerIndexVarInc)) {
       DEBUG(dbgs() << "Found unsupported instructions between induction "
                    << "variable increment and branch.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Found unsupported instruction between induction variable "
+                 "increment and branch.");
+
       return true;
     }
 
@@ -862,6 +943,11 @@
   // current limitation.
   if (!FoundInduction) {
     DEBUG(dbgs() << "Did not find the induction variable.\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "CurrentLimitations",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Did not find the induction variable.");
     return true;
   }
   return false;
@@ -875,6 +961,11 @@
     DEBUG(dbgs() << "Failed interchange InnerLoopId = " << InnerLoopId
                  << " and OuterLoopId = " << OuterLoopId
                  << " due to dependence\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "Dependence",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Cannot interchange loops due to dependences.");
     return false;
   }
 
@@ -910,6 +1001,12 @@
   // Check if the loops are tightly nested.
   if (!tightlyNested(OuterLoop, InnerLoop)) {
     DEBUG(dbgs() << "Loops not tightly nested\n");
+    ORE->emit(OptimizationRemarkMissed(DEBUG_TYPE,
+                                       "TightlyNested",
+                                       InnerLoop->getStartLoc(),
+                                       InnerLoop->getHeader())
+              << "Cannot interchange loops because they are not tightly "
+                 "nested.");
     return false;
   }
 
Index: test/Transforms/LoopInterchange/loop-interchange-optimization-remarks.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopInterchange/loop-interchange-optimization-remarks.ll
@@ -0,0 +1,208 @@
+; Test optimization remarks generated by the LoopInterchange pass.
+;
+; RUN: opt < %s -basicaa -loop-interchange -pass-remarks-output=%t -pass-remarks-missed='loop-interchange' \
+; RUN:          -pass-remarks='loop-interchange' -S
+; RUN: cat %t |  FileCheck %s
+
+@A = common global [100 x [100 x i32]] zeroinitializer
+@B = common global [100 x [100 x i32]] zeroinitializer
+@C = common global [100 x i32] zeroinitializer
+
+;;---------------------------------------Test case 01---------------------------------
+;; Loops interchange is not profitable.
+;;   for(int i=1;i<N;i++)
+;;     for(int j=1;j<N;j++)
+;;       A[i-1][j-1] = A[i - 1][j-1] + B[i][j];
+
+define void @test01(i32 %N){
+entry:
+  %cmp31 = icmp sgt i32 %N, 1
+  br i1 %cmp31, label %for.cond1.preheader.lr.ph, label %for.end19
+
+for.cond1.preheader.lr.ph:
+  %0 = add i32 %N, -1
+  br label %for.body3.lr.ph
+
+for.body3.lr.ph:
+  %indvars.iv34 = phi i64 [ 1, %for.cond1.preheader.lr.ph ], [ %indvars.iv.next35, %for.inc17 ]
+  %1 = add nsw i64 %indvars.iv34, -1
+  br label %for.body3
+
+for.body3:
+  %indvars.iv = phi i64 [ 1, %for.body3.lr.ph ], [ %indvars.iv.next, %for.body3 ]
+  %2 = add nsw i64 %indvars.iv, -1
+  %arrayidx6 = getelementptr inbounds [100 x [100 x i32]], [100 x [100 x i32]]* @A, i64 0, i64 %1, i64 %2
+  %3 = load i32, i32* %arrayidx6
+  %arrayidx10 = getelementptr inbounds [100 x [100 x i32]], [100 x [100 x i32]]* @B, i64 0, i64 %indvars.iv34, i64 %indvars.iv
+  %4 = load i32, i32* %arrayidx10
+  %add = add nsw i32 %4, %3
+  store i32 %add, i32* %arrayidx6
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %0
+  br i1 %exitcond, label %for.inc17, label %for.body3
+
+for.inc17:
+  %indvars.iv.next35 = add nuw nsw i64 %indvars.iv34, 1
+  %lftr.wideiv37 = trunc i64 %indvars.iv34 to i32
+  %exitcond38 = icmp eq i32 %lftr.wideiv37, %0
+  br i1 %exitcond38, label %for.end19, label %for.body3.lr.ph
+
+for.end19:
+  ret void
+}
+
+; CHECK: !Missed
+; CHECK-NEXT: loop-interchange
+; CHECK-NEXT: InterchangeNotProfitable
+; CHECK-NEXT: test01
+; CHECK: Interchanging loops is not profitable.
+
+;;--------------------------------------Test case 02------------------------------------
+;; [FIXME] This loop though valid is currently not interchanged due to the
+;; limitation that we cannot split the inner loop latch due to multiple use of inner induction
+;; variable.(used to increment the loop counter and to access A[j+1][i+1]
+;;  for(int i=0;i<N-1;i++)
+;;    for(int j=1;j<N-1;j++)
+;;      A[j+1][i+1] = A[j+1][i+1] + k;
+
+define void @test02(i32 %k, i32 %N) {
+ entry:
+   %sub = add nsw i32 %N, -1
+   %cmp26 = icmp sgt i32 %N, 1
+   br i1 %cmp26, label %for.cond1.preheader.lr.ph, label %for.end17
+
+ for.cond1.preheader.lr.ph:
+   %cmp324 = icmp sgt i32 %sub, 1
+   %0 = add i32 %N, -2
+   %1 = sext i32 %sub to i64
+   br label %for.cond1.preheader
+
+ for.cond.loopexit:
+   %cmp = icmp slt i64 %indvars.iv.next29, %1
+   br i1 %cmp, label %for.cond1.preheader, label %for.end17
+
+ for.cond1.preheader:
+   %indvars.iv28 = phi i64 [ 0, %for.cond1.preheader.lr.ph ], [ %indvars.iv.next29, %for.cond.loopexit ]
+   %indvars.iv.next29 = add nuw nsw i64 %indvars.iv28, 1
+   br i1 %cmp324, label %for.body4, label %for.cond.loopexit
+
+ for.body4:
+   %indvars.iv = phi i64 [ %indvars.iv.next, %for.body4 ], [ 1, %for.cond1.preheader ]
+   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+   %arrayidx7 = getelementptr inbounds [100 x [100 x i32]], [100 x [100 x i32]]* @A, i64 0, i64 %indvars.iv.next, i64 %indvars.iv.next29
+   %2 = load i32, i32* %arrayidx7
+   %add8 = add nsw i32 %2, %k
+   store i32 %add8, i32* %arrayidx7
+   %lftr.wideiv = trunc i64 %indvars.iv to i32
+   %exitcond = icmp eq i32 %lftr.wideiv, %0
+   br i1 %exitcond, label %for.cond.loopexit, label %for.body4
+
+ for.end17:
+   ret void
+}
+
+; CHECK: !Missed
+; CHECK-NEXT: loop-interchange
+; CHECK-NEXT: CurrentLimitations
+; CHECK-NEXT: test02
+; CHECK: Found unsupported instruction between induction variable increment and branch.
+
+;;-----------------------------------Test case 03-------------------------------
+;; Test to make sure we can handle output dependencies.
+;;
+;;  for (int i = 0; i < 2; ++i)
+;;    for(int j = 0; j < 3; ++j) {
+;;      A[j][i] = i;
+;;      A[j][i+1] = j;
+;;    }
+
+@A10 = local_unnamed_addr global [3 x [3 x i32]] zeroinitializer, align 16
+
+define void @test03() {
+entry:
+  br label %for.cond1.preheader
+
+for.cond.loopexit:                                ; preds = %for.body4
+  %exitcond28 = icmp ne i64 %indvars.iv.next27, 2
+  br i1 %exitcond28, label %for.cond1.preheader, label %for.cond.cleanup
+
+for.cond1.preheader:                              ; preds = %for.cond.loopexit, %entry
+  %indvars.iv26 = phi i64 [ 0, %entry ], [ %indvars.iv.next27, %for.cond.loopexit ]
+  %indvars.iv.next27 = add nuw nsw i64 %indvars.iv26, 1
+  br label %for.body4
+
+for.cond.cleanup:                                 ; preds = %for.cond.loopexit
+  ret void
+
+for.body4:                                        ; preds = %for.body4, %for.cond1.preheader
+  %indvars.iv = phi i64 [ 0, %for.cond1.preheader ], [ %indvars.iv.next, %for.body4 ]
+  %arrayidx6 = getelementptr inbounds [3 x [3 x i32]], [3 x [3 x i32]]* @A10, i64 0, i64 %indvars.iv, i64 %indvars.iv26
+  %tmp = trunc i64 %indvars.iv26 to i32
+  store i32 %tmp, i32* %arrayidx6, align 4
+  %arrayidx10 = getelementptr inbounds [3 x [3 x i32]], [3 x [3 x i32]]* @A10, i64 0, i64 %indvars.iv, i64 %indvars.iv.next27
+  %tmp1 = trunc i64 %indvars.iv to i32
+  store i32 %tmp1, i32* %arrayidx10, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp ne i64 %indvars.iv.next, 3
+  br i1 %exitcond, label %for.body4, label %for.cond.loopexit
+}
+
+; CHECK: !Passed
+; CHECK-NEXT: loop-interchange
+; CHECK-NEXT: Interchange
+; CHECK-NEXT: test03
+; CHECK: Loop interchanged with enclosing loop.
+
+;;--------------------------------------Test case 04-------------------------------------
+;; Loops not tightly nested are not interchanged
+;;  for(int j=0;j<N;j++) {
+;;    B[j] = j+k;
+;;    for(int i=0;i<N;i++)
+;;      A[j][i] = A[j][i]+B[j];
+;;  }
+
+define void @test04(i32 %k, i32 %N){
+entry:
+  %cmp30 = icmp sgt i32 %N, 0
+  br i1 %cmp30, label %for.body.lr.ph, label %for.end17
+
+for.body.lr.ph:
+  %0 = add i32 %N, -1
+  %1 = zext i32 %k to i64
+  br label %for.body
+
+for.body:
+  %indvars.iv32 = phi i64 [ 0, %for.body.lr.ph ], [ %indvars.iv.next33, %for.inc15 ]
+  %2 = add nsw i64 %indvars.iv32, %1
+  %arrayidx = getelementptr inbounds [100 x i32], [100 x i32]* @C, i64 0, i64 %indvars.iv32
+  %3 = trunc i64 %2 to i32
+  store i32 %3, i32* %arrayidx
+  br label %for.body3
+
+for.body3:
+  %indvars.iv = phi i64 [ 0, %for.body ], [ %indvars.iv.next, %for.body3 ]
+  %arrayidx7 = getelementptr inbounds [100 x [100 x i32]], [100 x [100 x i32]]* @A, i64 0, i64 %indvars.iv32, i64 %indvars.iv
+  %4 = load i32, i32* %arrayidx7
+  %add10 = add nsw i32 %3, %4
+  store i32 %add10, i32* %arrayidx7
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %0
+  br i1 %exitcond, label %for.inc15, label %for.body3
+
+for.inc15:
+  %indvars.iv.next33 = add nuw nsw i64 %indvars.iv32, 1
+  %lftr.wideiv35 = trunc i64 %indvars.iv32 to i32
+  %exitcond36 = icmp eq i32 %lftr.wideiv35, %0
+  br i1 %exitcond36, label %for.end17, label %for.body
+
+for.end17:
+  ret void
+}
+
+; CHECK: !Missed
+; CHECK-NEXT: loop-interchange
+; CHECK-NEXT: TightlyNested
+; CHECK-NEXT: test04
+; CHECK: Cannot interchange loops because they are not tightly nested.