[LoopInterchange] Don't consider loops with BTC=0

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -46,6 +46,7 @@
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include <cassert>
+#include <map>
 #include <utility>
 #include <vector>
 
@@ -421,9 +422,11 @@ static bool hasSupportedLoopDepth(ArrayRef<Loop *> LoopList,
 }
 
 static bool isComputableLoopNest(ScalarEvolution *SE,
-                                 ArrayRef<Loop *> LoopList) {
+                                 ArrayRef<Loop *> LoopList,
+                                 std::map<const Loop *, const SCEV *> &LoopBTC) {
   for (Loop *L : LoopList) {
     const SCEV *ExitCountOuter = SE->getBackedgeTakenCount(L);
+    LoopBTC[L] = ExitCountOuter;
     if (isa<SCEVCouldNotCompute>(ExitCountOuter)) {
       LLVM_DEBUG(dbgs() << "Couldn't compute backedge count\n");
       return false;
@@ -545,7 +548,8 @@ class LoopInterchangeProfitability {
   /// Check if the loop interchange is profitable.
   bool isProfitable(const Loop *InnerLoop, const Loop *OuterLoop,
                     unsigned InnerLoopId, unsigned OuterLoopId,
-                    CharMatrix &DepMatrix, CacheCostManager &CCM);
+                    CharMatrix &DepMatrix, CacheCostManager &CCM,
+                    std::map<const Loop *, const SCEV *> &LoopBTC);
 
 private:
   int getInstrOrderCost();
@@ -602,14 +606,17 @@ struct LoopInterchange {
   DependenceInfo *DI = nullptr;
   DominatorTree *DT = nullptr;
   LoopStandardAnalysisResults *AR = nullptr;
-
   /// Interface to emit optimization remarks.
   OptimizationRemarkEmitter *ORE;
+  // A cache to avoid recalculating the backedge-taken count for a loop.
+  std::map<const Loop *, const SCEV *> LoopBTC;
 
   LoopInterchange(ScalarEvolution *SE, LoopInfo *LI, DependenceInfo *DI,
                   DominatorTree *DT, LoopStandardAnalysisResults *AR,
-                  OptimizationRemarkEmitter *ORE)
-      : SE(SE), LI(LI), DI(DI), DT(DT), AR(AR), ORE(ORE) {}
+                  OptimizationRemarkEmitter *ORE,
+                  std::map<const Loop *, const SCEV *> &&LoopBTC)
+      : SE(SE), LI(LI), DI(DI), DT(DT), AR(AR), ORE(ORE),
+        LoopBTC(std::move(LoopBTC)) {}
 
   bool run(Loop *L) {
     if (L->getParentLoop())
@@ -701,7 +708,7 @@ struct LoopInterchange {
     LLVM_DEBUG(dbgs() << "Loops are legal to interchange\n");
     LoopInterchangeProfitability LIP(OuterLoop, InnerLoop, SE, ORE);
     if (!LIP.isProfitable(InnerLoop, OuterLoop, InnerLoopId, OuterLoopId,
-                          DependencyMatrix, CCM)) {
+                          DependencyMatrix, CCM, LoopBTC)) {
       LLVM_DEBUG(dbgs() << "Interchanging loops not profitable.\n");
       return false;
     }
@@ -1461,7 +1468,30 @@ std::optional<bool> LoopInterchangeProfitability::isProfitableForVectorization(
 
 bool LoopInterchangeProfitability::isProfitable(
     const Loop *InnerLoop, const Loop *OuterLoop, unsigned InnerLoopId,
-    unsigned OuterLoopId, CharMatrix &DepMatrix, CacheCostManager &CCM) {
+    unsigned OuterLoopId, CharMatrix &DepMatrix, CacheCostManager &CCM,
+    std::map<const Loop *, const SCEV *> &LoopBTC) {
+
+  auto *InnerBTC = LoopBTC[InnerLoop];
+  auto *OuterBTC = LoopBTC[OuterLoop];
+  assert(InnerBTC && OuterBTC &&
+         "Loop BTC should exist in cache but not found");
+  // A loop with a backedge that isn't taken, e.g. an unconditional branch
+  // true, isn't really a loop and we don't want to consider it as a
+  // candidate.
+  // TODO: when interchange is forced, we should probably also allow
+  // interchange for these loops, and thus this logic should be moved just
+  // below the cost-model ignore check below. But this check is done first
+  // to avoid the issue in #163954.
+  if (InnerBTC && InnerBTC->isZero()) {
+    LLVM_DEBUG(dbgs() << "Inner loop back-edge isn't taken, rejecting "
+                         "single iteration loop\n");
+    return false;
+  }
+  if (OuterBTC && OuterBTC->isZero()) {
+    LLVM_DEBUG(dbgs() << "Outer loop back-edge isn't taken, rejecting "
+                         "single iteration loop\n");
+    return false;
+  }
 
   // Return true if interchange is forced and the cost-model ignored.
   if (Profitabilities.size() == 1 && Profitabilities[0] == RuleTy::Ignore)
@@ -2098,6 +2128,7 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
                                            LPMUpdater &U) {
   Function &F = *LN.getParent();
   SmallVector<Loop *, 8> LoopList(LN.getLoops());
+  std::map<const Loop *, const SCEV *> LoopBTC;
 
   if (MaxMemInstrCount < 1) {
     LLVM_DEBUG(dbgs() << "MaxMemInstrCount should be at least 1");
@@ -2109,7 +2140,7 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
   if (!hasSupportedLoopDepth(LoopList, ORE))
     return PreservedAnalyses::all();
   // Ensure computable loop nest.
-  if (!isComputableLoopNest(&AR.SE, LoopList)) {
+  if (!isComputableLoopNest(&AR.SE, LoopList, LoopBTC)) {
     LLVM_DEBUG(dbgs() << "Not valid loop candidate for interchange\n");
     return PreservedAnalyses::all();
   }
@@ -2122,7 +2153,9 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
   });
 
   DependenceInfo DI(&F, &AR.AA, &AR.SE, &AR.LI);
-  if (!LoopInterchange(&AR.SE, &AR.LI, &DI, &AR.DT, &AR, &ORE).run(LN))
+  if (!LoopInterchange(&AR.SE, &AR.LI, &DI, &AR.DT, &AR, &ORE,
+                       std::move(LoopBTC))
+           .run(LN))
     return PreservedAnalyses::all();
   U.markLoopNestChanged(true);
   return getLoopPassPreservedAnalyses();

llvm/test/Transforms/LoopInterchange/loopnest-with-outer-btc0.ll

@@ -0,0 +1,73 @@
+; RUN: opt < %s -passes=loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:       -pass-remarks-output=%t -pass-remarks='loop-interchange' -S
+; RUN: cat %t |  FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+@D = common global [100 x [100 x [100 x i32]]] zeroinitializer
+
+; Test for interchange in
+;
+;  for(int i=0;i<1;i++)
+;    for(int j=0;j<100;j++)
+;      for(int k=0;k<100;k++)
+;        D[i][k][j] = D[i][k][j]+t;
+;
+
+; CHECK:        --- !Analysis
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            Dependence
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Computed dependence info, invoking the transform.
+; CHECK-NEXT:   ...
+; CHECK-NEXT:   --- !Passed
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            Interchanged
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Loop interchanged with enclosing loop.
+; CHECK-NEXT:   ...
+; CHECK-NEXT:   --- !Missed
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            InterchangeNotProfitable
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Insufficient information to calculate the cost of loop for interchange.
+; CHECK-NEXT:   ...
+
+define void @interchange_i_and_j(i32 %t){
+entry:
+  br label %outer.header
+
+outer.header:
+  %i = phi i64 [ 0, %entry ], [ %inc16, %for.inc15 ]
+  br label %inner1.header
+
+inner1.header:
+  %j = phi i64 [ 0, %outer.header ], [ %inc13, %for.inc12 ]
+  br label %inner2.body
+
+inner2.body:
+  %k = phi i64 [ 0, %inner1.header ], [ %inc, %inner2.body ]
+  %arrayidx8 = getelementptr inbounds [100 x [100 x [100 x i32]]], ptr @D, i64 0, i64 %i, i64 %k, i64 %j
+  %0 = load i32, ptr %arrayidx8
+  %add = add nsw i32 %0, %t
+  store i32 %add, ptr %arrayidx8
+  %inc = add nuw nsw i64 %k, 1
+  %exitcond = icmp eq i64 %inc, 100
+  br i1 %exitcond, label %for.inc12, label %inner2.body
+
+for.inc12:
+  %inc13 = add nuw nsw i64 %j, 1
+  %exitcond29 = icmp eq i64 %inc13, 100
+  br i1 %exitcond29, label %for.inc15, label %inner1.header
+
+for.inc15:
+  %inc16 = add nuw nsw i64 %i, 1
+  %exitcond30 = icmp eq i64 %inc16, 1
+  br i1 %exitcond30, label %for.end17, label %outer.header
+
+for.end17:
+  ret void
+}

llvm/test/Transforms/LoopInterchange/pr43326.ll

@@ -64,7 +64,7 @@ for.end:                                          ; preds = %for.inc
 
 for.inc10:                                        ; preds = %for.end
   %j.next = add i8 %j, -1
-  %cmp = icmp sgt i8 %j.next, -1
+  %cmp = icmp sgt i8 %j.next, -10
   br i1 %cmp, label %inner1.header, label %for.end11
 
 for.end11:                                        ; preds = %for.inc10
@@ -75,8 +75,8 @@ for.end11:                                        ; preds = %for.inc10
 
 for.inc12:                                        ; preds = %for.end11
   %inc13 = add nsw i32 %inc1312, 1
-  %tobool.not = icmp eq i32 %inc13, 0
-  br i1 %tobool.not, label %for.cond.for.end14_crit_edge, label %outer.header
+  %tobool.not = icmp slt i32 %inc13, 42
+  br i1 %tobool.not, label %outer.header, label %for.cond.for.end14_crit_edge
 
 for.cond.for.end14_crit_edge:                     ; preds = %for.inc12
   %inc13.lcssa = phi i32 [ %inc13, %for.inc12 ]