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

[madhur13490]

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.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Madhur Amilkanthwar (madhur13490)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/115127.diff

2 Files Affected:

• (modified) llvm/lib/Passes/PassBuilderPipelines.cpp (+1-1)
• (modified) llvm/lib/Transforms/Scalar/LoopInterchange.cpp (+18-4)

diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index 16fe9a74bb9c0d..c1650db0e7059d 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -199,7 +199,7 @@ static cl::opt<bool> RunNewGVN("enable-newgvn", cl::init(false), cl::Hidden,
                                cl::desc("Run the NewGVN pass"));
 
 static cl::opt<bool> EnableLoopInterchange(
-    "enable-loopinterchange", cl::init(false), cl::Hidden,
+    "enable-loopinterchange", cl::init(true), cl::Hidden,
     cl::desc("Enable the experimental LoopInterchange Pass"));
 
 static cl::opt<bool> EnableUnrollAndJam("enable-unroll-and-jam",
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index db63bda1e6b926..09bd7d8eda8547 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/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;
-    unsigned LoopNestDepth = LoopList.size();
-    if (LoopNestDepth < 2) {
-      LLVM_DEBUG(dbgs() << "Loop doesn't contain minimum nesting level.\n");
+
+    if (!hasMinimumLoopDepth(LoopList))
       return false;
-    }
+
+    unsigned LoopNestDepth = LoopList.size();
     if (LoopNestDepth > MaxLoopNestDepth) {
       LLVM_DEBUG(dbgs() << "Cannot handle loops of depth greater than "
                         << MaxLoopNestDepth << "\n");
@@ -1713,6 +1721,12 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
                                            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 =
       CacheCost::getCacheCost(LN.getOutermostLoop(), AR, DI);

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 895a8e66c6d1e42519909981ab1bb0ad41231029 ed275fbe4a23aef5daa26ff0c263fca558382cc5 --extensions cpp -- llvm/lib/Passes/PassBuilderPipelines.cpp llvm/lib/Transforms/Scalar/LoopInterchange.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 09bd7d8eda..80b6c0132d 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1722,11 +1722,11 @@ PreservedAnalyses LoopInterchangePass::run(LoopNest &LN,
   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 =
       CacheCost::getCacheCost(LN.getOutermostLoop(), AR, DI);