[LoopInterchange] Prevent interchange if one index is constant and the other has loop carried dependence

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -82,6 +82,34 @@ static void printDepMatrix(CharMatrix &DepMatrix) {
 }
 #endif
 
+static unsigned getAffectedLoopNum(const SCEV *Expr, ScalarEvolution &SE) {
+  const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(Expr);
+  if (!AddRec)
+    return 0;
+
+  const SCEV *Start = AddRec->getStart();
+  const SCEV *Step = AddRec->getStepRecurrence(SE);
+  return getAffectedLoopNum(Start, SE) + getAffectedLoopNum(Step, SE) + 1;
+}
+
+static bool hasConstantIndex(Instruction *Src, Instruction *Dst,
+                             ScalarEvolution *SE) {
+  Value *PtrSrc = getLoadStorePointerOperand(Src);
+  Value *PtrDst = getLoadStorePointerOperand(Dst);
+  const SCEV *SrcSCEV = SE->getSCEV(PtrSrc);
+  const SCEV *DstSCEV = SE->getSCEV(PtrDst);
+
+  unsigned SrcLoops = getAffectedLoopNum(SrcSCEV, *SE);
+  unsigned DstLoops = getAffectedLoopNum(DstSCEV, *SE);
+  // Loop interchange would need at least two loops. If the SCEV form
+  // only has one loop or the loop numbers are not equal. There will be
+  // at least one constant index.
+  if (SrcLoops == 1 || DstLoops == 1 || SrcLoops != DstLoops)
+    return true;
+
+  return false;
+}
+
 static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
                                      Loop *L, DependenceInfo *DI,
                                      ScalarEvolution *SE) {
@@ -150,6 +178,13 @@ static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
               Direction = '=';
             else
               Direction = '*';
+
+            if (hasConstantIndex(Src, Dst, SE) &&
+                (Direction == '>' || Direction == '<')) {
+              LLVM_DEBUG(dbgs() << "Has constant index with loop carried"
+                                << " dependencies inside loop\n");
+              return false;
+            }
             Dep.push_back(Direction);
           }
         }

llvm/test/Transforms/LoopInterchange/pr54176.ll

@@ -0,0 +1,52 @@
+; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 -verify-dom-info -verify-loop-info \
+; RUN:     -S -debug 2>&1 | FileCheck %s
+
+@bb = global [1024 x [128 x float]] zeroinitializer, align 4
+@aa = global [1024 x [128 x float]] zeroinitializer, align 4
+@cc = global [1024 x [128 x float]] zeroinitializer, align 4
+
+
+;;  for (int j = 1; j < M; j++)
+;;    for (int i = 1; i < N; i++) {
+;;      aa[1][j-1] += bb[i][j];
+;;      cc[i][j] = aa[1][j];
+;;    }
+
+; CHECK: Has constant index with loop carried dependencies inside loop
+; CHECK: Populating dependency matrix failed
+
+define void @pr54176() {
+entry:
+  br label %for.cond1.preheader
+
+; Loop:
+for.cond1.preheader:                              ; preds = %entry, %for.cond.cleanup3
+  %indvars.iv28 = phi i64 [ 1, %entry ], [ %indvars.iv.next29, %for.cond.cleanup3 ]
+  %0 = add nsw i64 %indvars.iv28, -1
+  %arrayidx8 = getelementptr inbounds [1024 x [128 x float]], ptr @aa, i64 0, i64 1, i64 %0
+  %arrayidx10 = getelementptr inbounds [1024 x [128 x float]], ptr @aa, i64 0, i64 1, i64 %indvars.iv28
+  br label %for.body4
+
+for.cond.cleanup3:                                ; preds = %for.body4
+  %indvars.iv.next29 = add nuw nsw i64 %indvars.iv28, 1
+  %exitcond31 = icmp ne i64 %indvars.iv.next29, 128
+  br i1 %exitcond31, label %for.cond1.preheader, label %for.cond.cleanup
+
+for.body4:                                        ; preds = %for.cond1.preheader, %for.body4
+  %indvars.iv = phi i64 [ 1, %for.cond1.preheader ], [ %indvars.iv.next, %for.body4 ]
+  %arrayidx6 = getelementptr inbounds [1024 x [128 x float]], ptr @bb, i64 0, i64 %indvars.iv, i64 %indvars.iv28
+  %1 = load float, ptr %arrayidx6, align 4
+  %2 = load float, ptr %arrayidx8, align 4
+  %add = fadd float %1, %2
+  store float %add, ptr %arrayidx8, align 4
+  %3 = load float, ptr %arrayidx10, align 4
+  %arrayidx14 = getelementptr inbounds [1024 x [128 x float]], ptr @cc, i64 0, i64 %indvars.iv, i64 %indvars.iv28
+  store float %3, ptr %arrayidx14, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp ne i64 %indvars.iv.next, 1024
+  br i1 %exitcond, label %for.body4, label %for.cond.cleanup3
+
+; Exit blocks
+for.cond.cleanup:                                 ; preds = %for.cond.cleanup3
+  ret void
+}