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[Passes] Move LoopInterchange into optimization pipeline #145503

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Merged
merged 2 commits into from
Jul 4, 2025
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[Passes] Move LoopInterchange into optimization pipeline #145503

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cba73f8
[Passes] Move LoopInterchange into optimization pipeline
kasuga-fj Jun 24, 2025
11e2519
Add test to check the position of LoopInterchange in the pipeline
kasuga-fj Jun 26, 2025
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7 changes: 4 additions & 3 deletions llvm/lib/Passes/PassBuilderPipelines.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -690,9 +690,6 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,

LPM2.addPass(LoopDeletionPass());

if (PTO.LoopInterchange)
LPM2.addPass(LoopInterchangePass());
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It might be reasonable to keep this, at least for now?


// 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
Expand Down Expand Up @@ -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());
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I'm not sure where LoopInterchange should be placed within the optimization pipeline...

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Hmm, would be interesting to get some more data about the impact on different positions, but current palcement seems reasonable to me at least

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Yes, collecting diverse data would be ideal, but since the current LoopInterchange is rarely triggered in practice, I haven’t been able to gather enough data to have a meaningful discussion about the optimal placement...

TSVC s235 might be an interesting example. If LoopDistribute were extended to support non-innermost loops and it ran before LoopInterchange, the following could become possible:

Original:

for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++) {
    a[i] += b[i] * c[i];
    for (int j = 1; j < LEN2; j++) {
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];
    }
  }

Distributed:

for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    a[i] += b[i] * c[i];

for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    for (int j = 1; j < LEN2; j++)
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];

Interchanged:

for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    a[i] += b[i] * c[i];

// The i-loop and j-loop are interchanged.
for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int j = 1; j < LEN2; j++)
    for (int i = 0; i < LEN2; i++)
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];

IIRC, when I ran the last code in the past, it was about 8x faster than the original. But achieving this would require a lot of work on LoopDistribute...


OptimizePM.addPass(createFunctionToLoopPassAdaptor(
std::move(LPM), /*UseMemorySSA=*/false, /*UseBlockFrequencyInfo=*/false));

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48 changes: 48 additions & 0 deletions llvm/test/Transforms/LoopInterchange/position-in-pipeline.ll
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@@ -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
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@kasuga-fj kasuga-fj Jun 26, 2025 •
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Please let me know if there is a better way...



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
}