[llvm] [LoopInterchange] Also look at lcssa phis in outer loop latch block (PR #160889)
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Fri Sep 26 07:09:47 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Sjoerd Meijer (sjoerdmeijer)
<details>
<summary>Changes</summary>
This deals with a corner case of LCSSA phi nodes in the outer loop latch block: the loop was in LCSSA form, some transformations can come along (e.g. unswitch) and create an empty block:
BB4:
br label %BB5
BB5:
%old.cond.lcssa = phi i16 [ %cond, %BB4 ]
br outer.header
Interchange then brings it in LCSSA form again and we get:
BB4:
%new.cond.lcssa = phi i16 [ %cond, %BB3 ]
br label %BB5
BB5:
%old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
Which means that we have a chain of LCSSA phi nodes from %new.cond.lcssa to %old.cond.lcssa. The problem is that interchange can reoder blocks BB4 and BB5 placing the use before the def if we don't check this. The observation is that %old.cond.lcssa is unused, so instead of moving and renaming these phi nodes, just delete it if it's trivially dead. If it isn't trivially dead, it is handled elsewhere. The loop should still be in LCSSA form, and if it isn't, formLCSSARecursively is called after the interchange rewrite.
Fixes #<!-- -->160068
---
Full diff: https://github.com/llvm/llvm-project/pull/160889.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/Scalar/LoopInterchange.cpp (+33)
- (added) llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll (+75)
``````````diff
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 28ae4f0a0aad9..e42d82d1533e1 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -44,6 +44,7 @@
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
#include <cassert>
#include <utility>
#include <vector>
@@ -1837,6 +1838,38 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
for (PHINode *P : LcssaInnerLatch)
P->moveBefore(InnerExit->getFirstNonPHIIt());
+ // This deals with a corner case of LCSSA phi nodes in the outer loop latch
+ // block: the loop was in LCSSA form, some transformations can come along
+ // (e.g. unswitch) and create an empty block:
+ //
+ // BB4:
+ // br label %BB5
+ // BB5:
+ // %old.cond.lcssa = phi i16 [ %cond, %BB4 ]
+ // br outer.header
+ //
+ // Interchange then brings it in LCSSA form again and we get:
+ //
+ // BB4:
+ // %new.cond.lcssa = phi i16 [ %cond, %BB3 ]
+ // br label %BB5
+ // BB5:
+ // %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
+ //
+ // Which means that we have a chain of LCSSA phi nodes from %new.cond.lcssa
+ // to %old.cond.lcssa. The problem is that interchange can reoder blocks BB4
+ // and BB5 placing the use before the def if we don't check this. The
+ // observation is that %old.cond.lcssa is unused, so instead of moving and
+ // renaming these phi nodes, just delete it if it's trivially dead. If it
+ // isn't trivially dead, it is handled above. The loop should still be in
+ // LCSSA form, and if it isn't, formLCSSARecursively is called after the
+ // interchange rewrite.
+ SmallVector<PHINode *, 8> LcssaOuterLatch(
+ llvm::make_pointer_range(OuterLatch->phis()));
+ for (PHINode *P : LcssaOuterLatch)
+ if (isInstructionTriviallyDead(P))
+ P->eraseFromParent();
+
// Deal with LCSSA PHI nodes in the loop nest exit block. For PHIs that have
// incoming values defined in the outer loop, we have to add a new PHI
// in the inner loop latch, which became the exit block of the outer loop,
diff --git a/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll b/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll
new file mode 100644
index 0000000000000..482db85fe33e8
--- /dev/null
+++ b/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll
@@ -0,0 +1,75 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --prefix-filecheck-ir-name SJM --version 6
+; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 -verify-dom-info -verify-loop-info -verify-scev -verify-loop-lcssa -S | FileCheck %s
+
+; This test is checking that blocks BB4 and BB5, where BB4 is the exit
+; block of the inner loop and BB5 the latch of the outer loop, correctly
+; deal with the phi-node use-def chain %new.cond.lcssa -> %old.cond.lcssa.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define i16 @main() {
+; CHECK-LABEL: define i16 @main() {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: br label %[[BB2_PREHEADER:.*]]
+; CHECK: [[BB1_PREHEADER:.*]]:
+; CHECK-NEXT: br label %[[SJMBB1:.*]]
+; CHECK: [[SJMBB1]]:
+; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[BB5:.*]] ], [ 1, %[[BB1_PREHEADER]] ]
+; CHECK-NEXT: br label %[[BB2_SPLIT:.*]]
+; CHECK: [[BB2_PREHEADER]]:
+; CHECK-NEXT: br label %[[SJMBB2:.*]]
+; CHECK: [[SJMBB2]]:
+; CHECK-NEXT: [[J:%.*]] = phi i16 [ [[TMP1:%.*]], %[[BB3_SPLIT:.*]] ], [ 0, %[[BB2_PREHEADER]] ]
+; CHECK-NEXT: br label %[[BB1_PREHEADER]]
+; CHECK: [[BB2_SPLIT]]:
+; CHECK-NEXT: [[ARRAYIDX_US_US:%.*]] = getelementptr i16, ptr null, i16 [[J]]
+; CHECK-NEXT: [[TMP0:%.*]] = load i16, ptr [[ARRAYIDX_US_US]], align 1
+; CHECK-NEXT: [[COND:%.*]] = select i1 false, i16 0, i16 0
+; CHECK-NEXT: br label %[[SJMBB3:.*]]
+; CHECK: [[SJMBB3]]:
+; CHECK-NEXT: [[J_NEXT:%.*]] = add i16 [[J]], 1
+; CHECK-NEXT: br label %[[SJMBB4:.*]]
+; CHECK: [[BB3_SPLIT]]:
+; CHECK-NEXT: [[NEW_COND_LCSSA:%.*]] = phi i16 [ [[COND]], %[[BB5]] ]
+; CHECK-NEXT: [[TMP1]] = add i16 [[J]], 1
+; CHECK-NEXT: br i1 true, label %[[EXIT:.*]], label %[[SJMBB2]]
+; CHECK: [[SJMBB4]]:
+; CHECK-NEXT: br label %[[BB5]]
+; CHECK: [[BB5]]:
+; CHECK-NEXT: [[I_NEXT]] = add i64 [[I]], 1
+; CHECK-NEXT: [[CMP286_US:%.*]] = icmp ugt i64 [[I]], 0
+; CHECK-NEXT: br i1 [[CMP286_US]], label %[[SJMBB1]], label %[[BB3_SPLIT]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret i16 0
+;
+entry:
+ br label %BB1
+
+BB1:
+ %i = phi i64 [ 1, %entry ], [ %i.next, %BB5 ]
+ br label %BB2
+
+BB2:
+ %j = phi i16 [ 0, %BB1 ], [ %j.next, %BB3 ]
+ %arrayidx.us.us = getelementptr i16, ptr null, i16 %j
+ %0 = load i16, ptr %arrayidx.us.us, align 1
+ %cond = select i1 false, i16 0, i16 0
+ br label %BB3
+
+BB3:
+ %j.next = add i16 %j, 1
+ br i1 true, label %BB4, label %BB2
+
+BB4:
+ %new.cond.lcssa = phi i16 [ %cond, %BB3 ]
+ br label %BB5
+
+BB5:
+ %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
+ %i.next = add i64 %i, 1
+ %cmp286.us = icmp ugt i64 %i, 0
+ br i1 %cmp286.us, label %BB1, label %exit
+
+exit:
+ ret i16 0
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/160889
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