[NFC] Refactor LoopInterchange into a loop-nest pass

This is the preliminary patch of converting `LoopInterchange` pass to a loop-nest pass and has no intended functional change.
Changes that are not loop-nest related are split to D96650.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D96644

include/llvm/Analysis/LoopNestAnalysis.h

@@ -139,6 +139,11 @@ public:
     return all_of(Loops, [](const Loop *L) { return L->isRotatedForm(); });
   }
 
+  /// Return the function to which the loop-nest belongs.
+  Function *getParent() const {
+    return Loops.front()->getHeader()->getParent();
+  }
+
   StringRef getName() const { return Loops.front()->getName(); }
 
 protected:

include/llvm/Transforms/Scalar/LoopInterchange.h

@@ -15,7 +15,7 @@
 namespace llvm {
 
 struct LoopInterchangePass : public PassInfoMixin<LoopInterchangePass> {
-  PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
+  PreservedAnalyses run(LoopNest &L, LoopAnalysisManager &AM,
                         LoopStandardAnalysisResults &AR, LPMUpdater &U);
 };
 

lib/Transforms/Scalar/LoopInterchange.cpp

@@ -449,7 +449,15 @@ struct LoopInterchange {
     return processLoopList(populateWorklist(*L));
   }
 
-  bool isComputableLoopNest(LoopVector LoopList) {
+  bool run(LoopNest &LN) {
+    const auto &LoopList = LN.getLoops();
+    for (unsigned I = 1; I < LoopList.size(); ++I)
+      if (LoopList[I]->getParentLoop() != LoopList[I - 1])
+        return false;
+    return processLoopList(LoopList);
+  }
+
+  bool isComputableLoopNest(ArrayRef<Loop *> LoopList) {
     for (Loop *L : LoopList) {
       const SCEV *ExitCountOuter = SE->getBackedgeTakenCount(L);
       if (isa<SCEVCouldNotCompute>(ExitCountOuter)) {
@@ -468,13 +476,13 @@ struct LoopInterchange {
     return true;
   }
 
-  unsigned selectLoopForInterchange(const LoopVector &LoopList) {
+  unsigned selectLoopForInterchange(ArrayRef<Loop *> LoopList) {
     // TODO: Add a better heuristic to select the loop to be interchanged based
     // on the dependence matrix. Currently we select the innermost loop.
     return LoopList.size() - 1;
   }
 
-  bool processLoopList(LoopVector LoopList) {
+  bool processLoopList(ArrayRef<Loop *> LoopList) {
     bool Changed = false;
     unsigned LoopNestDepth = LoopList.size();
     if (LoopNestDepth < 2) {
@@ -515,14 +523,12 @@ struct LoopInterchange {
 
     unsigned SelecLoopId = selectLoopForInterchange(LoopList);
     // Move the selected loop outwards to the best possible position.
+    Loop *LoopToBeInterchanged = LoopList[SelecLoopId];
     for (unsigned i = SelecLoopId; i > 0; i--) {
-      bool Interchanged = processLoop(LoopList[i], LoopList[i - 1], i, i - 1,
-                                      LoopNestExit, DependencyMatrix);
+      bool Interchanged = processLoop(LoopToBeInterchanged, LoopList[i - 1], i,
+                                      i - 1, LoopNestExit, DependencyMatrix);
       if (!Interchanged)
         return Changed;
-      // Loops interchanged reflect the same in LoopList
-      std::swap(LoopList[i - 1], LoopList[i]);
-
       // Update the DependencyMatrix
       interChangeDependencies(DependencyMatrix, i, i - 1);
 #ifdef DUMP_DEP_MATRICIES
@@ -539,7 +545,6 @@ struct LoopInterchange {
                    std::vector<std::vector<char>> &DependencyMatrix) {
     LLVM_DEBUG(dbgs() << "Processing InnerLoopId = " << InnerLoopId
                       << " and OuterLoopId = " << OuterLoopId << "\n");
-
     LoopInterchangeLegality LIL(OuterLoop, InnerLoop, SE, ORE);
     if (!LIL.canInterchangeLoops(InnerLoopId, OuterLoopId, DependencyMatrix)) {
       LLVM_DEBUG(dbgs() << "Not interchanging loops. Cannot prove legality.\n");
@@ -1680,14 +1685,15 @@ Pass *llvm::createLoopInterchangePass() {
   return new LoopInterchangeLegacyPass();
 }
 
-PreservedAnalyses LoopInterchangePass::run(Loop &L, LoopAnalysisManager &AM,
+PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
+                                           LoopAnalysisManager &AM,
                                            LoopStandardAnalysisResults &AR,
                                            LPMUpdater &U) {
-  Function &F = *L.getHeader()->getParent();
+  Function &F = *LN.getParent();
 
   DependenceInfo DI(&F, &AR.AA, &AR.SE, &AR.LI);
   OptimizationRemarkEmitter ORE(&F);
-  if (!LoopInterchange(&AR.SE, &AR.LI, &DI, &AR.DT, &ORE).run(&L))
+  if (!LoopInterchange(&AR.SE, &AR.LI, &DI, &AR.DT, &ORE).run(LN))
     return PreservedAnalyses::all();
   return getLoopPassPreservedAnalyses();
 }

test/Transforms/LoopInterchange/interchanged-loop-nest-3.ll

@@ -0,0 +1,56 @@
+; REQUIRES: asserts
+; RUN: opt < %s -basic-aa -loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:     -S -debug 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@D = common global [100 x [100 x [100 x i32]]] zeroinitializer
+
+;; Test for interchange in loop nest greater than 2.
+;;  for(int i=0;i<100;i++)
+;;    for(int j=0;j<100;j++)
+;;      for(int k=0;k<100;k++)
+;;        D[k][j][i] = D[k][j][i]+t;
+
+; CHECK: Processing InnerLoopId = 2 and OuterLoopId = 1
+; CHECK: Loops interchanged.
+
+; CHECK: Processing InnerLoopId = 1 and OuterLoopId = 0
+; CHECK: Loops interchanged.
+
+define void @interchange_08(i32 %t){
+entry:
+  br label %for.cond1.preheader
+
+for.cond1.preheader:                              ; preds = %for.inc15, %entry
+  %i.028 = phi i32 [ 0, %entry ], [ %inc16, %for.inc15 ]
+  br label %for.cond4.preheader
+
+for.cond4.preheader:                              ; preds = %for.inc12, %for.cond1.preheader
+  %j.027 = phi i32 [ 0, %for.cond1.preheader ], [ %inc13, %for.inc12 ]
+  br label %for.body6
+
+for.body6:                                        ; preds = %for.body6, %for.cond4.preheader
+  %k.026 = phi i32 [ 0, %for.cond4.preheader ], [ %inc, %for.body6 ]
+  %arrayidx8 = getelementptr inbounds [100 x [100 x [100 x i32]]], [100 x [100 x [100 x i32]]]* @D, i32 0, i32 %k.026, i32 %j.027, i32 %i.028
+  %0 = load i32, i32* %arrayidx8
+  %add = add nsw i32 %0, %t
+  store i32 %add, i32* %arrayidx8
+  %inc = add nuw nsw i32 %k.026, 1
+  %exitcond = icmp eq i32 %inc, 100
+  br i1 %exitcond, label %for.inc12, label %for.body6
+
+for.inc12:                                        ; preds = %for.body6
+  %inc13 = add nuw nsw i32 %j.027, 1
+  %exitcond29 = icmp eq i32 %inc13, 100
+  br i1 %exitcond29, label %for.inc15, label %for.cond4.preheader
+
+for.inc15:                                        ; preds = %for.inc12
+  %inc16 = add nuw nsw i32 %i.028, 1
+  %exitcond30 = icmp eq i32 %inc16, 100
+  br i1 %exitcond30, label %for.end17, label %for.cond1.preheader
+
+for.end17:                                        ; preds = %for.inc15
+  ret void
+}