[LoopInterchange] Bail out early if minimum loop nest is not met (llvm#115128)

This patch bails out early if minimum depth
is not met. As it stands today, the pass computes
CacheCost before it attempts to do the transform.
This is not needed if minimum depth is not met.
This handles basic cases where depth is typically 1.

As the patch avoids unnecessary computation, it is aimed to improve
compile-time.

Author: madhur13490

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
+}
+