[LoopInterchange] Bail out early if minimum loop nest is not met

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -234,6 +234,14 @@ static void populateWorklist(Loop &L, LoopVector &LoopList) {
   LoopList.push_back(CurrentLoop);
 }
 
+static bool hasMinimumLoopDepth(SmallVectorImpl<Loop *> &LoopList) {
+  unsigned LoopNestDepth = LoopList.size();
+  if (LoopNestDepth < 2) {
+    LLVM_DEBUG(dbgs() << "Loop doesn't contain minimum nesting level.\n");
+    return false;
+  }
+  return true;
+}
 namespace {
 
 /// LoopInterchangeLegality checks if it is legal to interchange the loop.
@@ -416,11 +424,11 @@ struct LoopInterchange {
 
   bool processLoopList(SmallVectorImpl<Loop *> &LoopList) {
     bool Changed = false;
+
+    // Ensure minimum loop nest depth.
+    assert(hasMinimumLoopDepth(LoopList) && "Loop nest does not meet minimum depth.");
+
     unsigned LoopNestDepth = LoopList.size();
-    if (LoopNestDepth < 2) {
-      LLVM_DEBUG(dbgs() << "Loop doesn't contain minimum nesting level.\n");
-      return false;
-    }
     if (LoopNestDepth > MaxLoopNestDepth) {
       LLVM_DEBUG(dbgs() << "Cannot handle loops of depth greater than "
                         << MaxLoopNestDepth << "\n");
@@ -1712,6 +1720,10 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
                                            LoopStandardAnalysisResults &AR,
                                            LPMUpdater &U) {
   Function &F = *LN.getParent();
+  SmallVector<Loop *, 8> LoopList(LN.getLoops());
+  // Ensure minimum depth of the loop nest to do the interchange.
+  if (!hasMinimumLoopDepth(LoopList))
+    return PreservedAnalyses::all();
 
   DependenceInfo DI(&F, &AR.AA, &AR.SE, &AR.LI);
   std::unique_ptr<CacheCost> CC =

llvm/test/Transforms/LoopInterchange/bail-out-one-loop.ll

@@ -0,0 +1,65 @@
+; REQUIRES: asserts
+
+; RUN: opt < %s -passes=loop-interchange -debug -disable-output 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128-Fn32"
+
+@N = dso_local global i32 0, align 4
+@a = dso_local global ptr null, align 8
+@b = dso_local global ptr null, align 8
+@c = dso_local global ptr null, align 8
+
+; Loop interchange should not run delinearization
+; for one loop case and should bail out early.
+
+; CHECK-NOT: Delinearizing
+; CHECK-NOT: Strides:
+; CHECK-NOT: Terms:
+; CHECK: Loop doesn't contain minimum nesting level.
+
+define void @foo() {
+entry:
+  %retval = alloca i32, align 4
+  %i = alloca i32, align 4
+  store i32 0, ptr %retval, align 4
+  store i32 0, ptr %i, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i32, ptr %i, align 4
+  %1 = load i32, ptr @N, align 4
+  %cmp = icmp ult i32 %0, %1
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  br label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %2 = load ptr, ptr @b, align 8
+  %3 = load i32, ptr %i, align 4
+  %idxprom = zext i32 %3 to i64
+  %arrayidx = getelementptr inbounds nuw i32, ptr %2, i64 %idxprom
+  %4 = load i32, ptr %arrayidx, align 4
+  %5 = load ptr, ptr @c, align 8
+  %6 = load i32, ptr %i, align 4
+  %idxprom1 = zext i32 %6 to i64
+  %arrayidx2 = getelementptr inbounds nuw i32, ptr %5, i64 %idxprom1
+  %7 = load i32, ptr %arrayidx2, align 4
+  %add = add nsw i32 %4, %7
+  %8 = load ptr, ptr @a, align 8
+  %9 = load i32, ptr %i, align 4
+  %idxprom3 = zext i32 %9 to i64
+  %arrayidx4 = getelementptr inbounds nuw i32, ptr %8, i64 %idxprom3
+  store i32 %add, ptr %arrayidx4, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %10 = load i32, ptr %i, align 4
+  %inc = add i32 %10, 1
+  store i32 %inc, ptr %i, align 4
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond.cleanup
+  ret void
+}
+