[llvm] [Passes] Move LoopInterchange into optimization pipeline (PR #145503)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj updated https://github.com/llvm/llvm-project/pull/145503

>From cba73f82cb4cce790c9739ca228f8bc683b3e56a Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Tue, 24 Jun 2025 11:55:51 +0000
Subject: [PATCH 1/2] [Passes] Move LoopInterchange into optimization pipeline
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

As mentioned in https://github.com/llvm/llvm-project/pull/145071,
LoopInterchange should be part of the optimization pipeline rather than
the simplification pipeline. This patch moves LoopInterchange into the
optimization pipeline.

More contexts:

- By default, LoopInterchange attempts to improve data locality,
  however, it also takes vectorization opportunities into account. Given
  that, it is reasonable to run it as close to vectorization as
  possible.
- I looked into previous changes related to the placement of
  LoopInterchange, but couldn’t find any strong motivation suggesting
  that it benefits other simplifications.
- As far as I tried some tests (including llvm-test-suite), removing
  LoopInterchange from the simplification pipeline does not affect other
  simplifications. Therefore, there doesn't seem to be much value in
  keeping it there.
---
 llvm/lib/Passes/PassBuilderPipelines.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index c83d2dc1f1514..98821bb1408a7 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -690,9 +690,6 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   LPM2.addPass(LoopDeletionPass());
 
-  if (PTO.LoopInterchange)
-    LPM2.addPass(LoopInterchangePass());
-
   // Do not enable unrolling in PreLinkThinLTO phase during sample PGO
   // because it changes IR to makes profile annotation in back compile
   // inaccurate. The normal unroller doesn't pay attention to forced full unroll
@@ -1547,6 +1544,10 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   //        this may need to be revisited once we run GVN before loop deletion
   //        in the simplification pipeline.
   LPM.addPass(LoopDeletionPass());
+
+  if (PTO.LoopInterchange)
+    LPM.addPass(LoopInterchangePass());
+
   OptimizePM.addPass(createFunctionToLoopPassAdaptor(
       std::move(LPM), /*UseMemorySSA=*/false, /*UseBlockFrequencyInfo=*/false));
 

>From 11e2519e811efa07208d3d74236ccf34374b0221 Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Thu, 26 Jun 2025 12:55:56 +0000
Subject: [PATCH 2/2] Add test to check the position of LoopInterchange in the
 pipeline

---
 .../LoopInterchange/position-in-pipeline.ll   | 48 +++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 llvm/test/Transforms/LoopInterchange/position-in-pipeline.ll

diff --git a/llvm/test/Transforms/LoopInterchange/position-in-pipeline.ll b/llvm/test/Transforms/LoopInterchange/position-in-pipeline.ll
new file mode 100644
index 0000000000000..d0b6447d92fe7
--- /dev/null
+++ b/llvm/test/Transforms/LoopInterchange/position-in-pipeline.ll
@@ -0,0 +1,48 @@
+; RUN: opt -passes='default<O3>' -enable-loopinterchange -disable-output \
+; RUN:     -disable-verify -verify-analysis-invalidation=0 \
+; RUN:     -debug-pass-manager=quiet %s 2>&1 | FileCheck %s
+
+; Test the position of LoopInterchange in the pass pipeline.
+
+; CHECK-NOT:  Running pass: LoopInterchangePass
+; CHECK:      Running pass: ControlHeightReductionPass
+; CHECK-NEXT: Running pass: LoopSimplifyPass
+; CHECK-NEXT: Running pass: LCSSAPass
+; CHECK-NEXT: Running pass: LoopRotatePass
+; CHECK-NEXT: Running pass: LoopDeletionPass
+; CHECK-NEXT: Running pass: LoopRotatePass
+; CHECK-NEXT: Running pass: LoopDeletionPass
+; CHECK-NEXT: Running pass: LoopInterchangePass
+; CHECK-NEXT: Running pass: LoopDistributePass
+; CHECK-NEXT: Running pass: InjectTLIMappings
+; CHECK-NEXT: Running pass: LoopVectorizePass
+
+
+define void @foo(ptr %a, i32 %n) {
+entry:
+  br label %for.i.header
+
+for.i.header:
+  %i = phi i32 [ 0, %entry ], [ %i.next, %for.i.latch ]
+  br label %for.j
+
+for.j:
+  %j = phi i32 [ 0, %for.i.header ], [ %j.next, %for.j ]
+  %tmp = mul i32 %i, %n
+  %offset = add i32 %tmp, %j
+  %idx = getelementptr inbounds i32, ptr %a, i32 %offset
+  %load = load i32, ptr %idx, align 4
+  %inc = add i32 %load, 1
+  store i32 %inc, ptr %idx, align 4
+  %j.next = add i32 %j, 1
+  %j.exit = icmp eq i32 %j.next, %n
+  br i1 %j.exit, label %for.i.latch, label %for.j
+
+for.i.latch:
+  %i.next = add i32 %i, 1
+  %i.exit = icmp eq i32 %i.next, %n
+  br i1 %i.exit, label %for.i.header, label %exit
+
+exit:
+  ret void
+}