[llvm] [LoopInterchange] Detect non-LCSSA PHI in inner loop exit correctly (PR #194323)
Ryotaro Kasuga via llvm-commits
llvm-commits at lists.llvm.org
Mon Apr 27 03:17:52 PDT 2026
https://github.com/kasuga-fj updated https://github.com/llvm/llvm-project/pull/194323
>From d69c96125204f3f2f75bef38042c38588236812e Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Mon, 27 Apr 2026 08:32:21 +0000
Subject: [PATCH] [LoopInterchange] Detect non-LCSSA PHI in inner loop exit
correctly
---
.../lib/Transforms/Scalar/LoopInterchange.cpp | 46 ++++++++++------
.../LoopInterchange/lcssa-phi-outer-latch.ll | 38 ++++++-------
.../non-lcssa-phi-in-inner-exit.ll | 53 +++++++++++++++++++
3 files changed, 101 insertions(+), 36 deletions(-)
create mode 100644 llvm/test/Transforms/LoopInterchange/non-lcssa-phi-in-inner-exit.ll
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 49a9e77ef8deb..ce2511ba8add1 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1329,27 +1329,39 @@ bool LoopInterchangeLegality::findInductions(
return !Inductions.empty();
}
-// We currently only support LCSSA PHI nodes in the inner loop exit, if their
-// users are either reduction PHIs or PHIs outside the outer loop (which means
-// the we are only interested in the final value after the loop).
+/// We currently only support LCSSA PHI nodes in the inner loop exit if their
+/// users are either of the following:
+///
+/// - Reduction PHIs
+/// - PHIs outside the outer loop
+/// - PHIs in the outer loop that have exactly one incoming value
+///
+/// These conditions mean that we are only interested in the final value after
+/// the inner loop.
static bool
-areInnerLoopExitPHIsSupported(Loop *InnerL, Loop *OuterL,
+areInnerLoopExitPHIsSupported(Loop *OuterL, Loop *InnerL,
SmallPtrSetImpl<PHINode *> &Reductions,
PHINode *LcssaReduction) {
- BasicBlock *InnerExit = OuterL->getUniqueExitBlock();
- for (PHINode &PHI : InnerExit->phis()) {
- // The reduction LCSSA PHI will have only one incoming block, which comes
- // from the loop latch.
- if (PHI.getNumIncomingValues() > 1)
- return false;
- if (&PHI == LcssaReduction)
- return true;
- if (any_of(PHI.users(), [&Reductions, OuterL](User *U) {
- PHINode *PN = dyn_cast<PHINode>(U);
- return !PN ||
- (!Reductions.count(PN) && OuterL->contains(PN->getParent()));
- })) {
+ BasicBlock *InnerExit = InnerL->getUniqueExitBlock();
+ SmallVector<PHINode *, 4> PHIs;
+ for (PHINode &PHI : InnerExit->phis())
+ PHIs.push_back(&PHI);
+
+ while (!PHIs.empty()) {
+ PHINode *PHI = PHIs.pop_back_val();
+ if (PHI->getNumIncomingValues() > 1)
return false;
+ if (PHI == LcssaReduction)
+ continue;
+ for (User *U : PHI->users()) {
+ PHINode *PN = dyn_cast<PHINode>(U);
+ if (!PN)
+ return false;
+ if (OuterL->contains(PN->getParent())) {
+ if (Reductions.count(PN))
+ continue;
+ PHIs.push_back(PN);
+ }
}
}
return true;
diff --git a/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll b/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll
index 92ce3288b4529..58ec391b23057 100644
--- a/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll
+++ b/llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll
@@ -12,38 +12,38 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
define i16 @main(ptr %a) {
; CHECK-LABEL: define i16 @main(
; CHECK-SAME: ptr [[A:%.*]]) {
-; CHECK-NEXT: [[ENTRY:.*:]]
-; CHECK-NEXT: br label %[[INNER_HEADER_PREHEADER:.*]]
-; CHECK: [[OUTER_HEADER_PREHEADER:.*]]:
-; CHECK-NEXT: br label %[[OUTER_HEADER:.*]]
-; CHECK: [[OUTER_HEADER]]:
-; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ], [ 1, %[[OUTER_HEADER_PREHEADER]] ]
-; CHECK-NEXT: br label %[[INNER_HEADER_SPLIT:.*]]
-; CHECK: [[INNER_HEADER_PREHEADER]]:
+; CHECK-NEXT: [[INNER_HEADER_PREHEADER:.*:]]
; CHECK-NEXT: br label %[[INNER_HEADER:.*]]
+; CHECK: [[OUTER_HEADER_PREHEADER1:.*]]:
+; CHECK-NEXT: br label %[[OUTER_HEADER_PREHEADER:.*]]
+; CHECK: [[OUTER_HEADER_PREHEADER]]:
+; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ], [ 1, %[[OUTER_HEADER_PREHEADER1]] ]
+; CHECK-NEXT: br label %[[INNER_HEADER_SPLIT:.*]]
; CHECK: [[INNER_HEADER]]:
-; CHECK-NEXT: [[J:%.*]] = phi i16 [ [[TMP1:%.*]], %[[INNER_LATCH_SPLIT:.*]] ], [ 0, %[[INNER_HEADER_PREHEADER]] ]
-; CHECK-NEXT: br label %[[OUTER_HEADER_PREHEADER]]
+; CHECK-NEXT: br label %[[INNER_HEADER1:.*]]
+; CHECK: [[INNER_HEADER1]]:
+; CHECK-NEXT: [[J:%.*]] = phi i16 [ [[TMP3:%.*]], %[[INNER_LATCH_SPLIT:.*]] ], [ 0, %[[INNER_HEADER]] ]
+; CHECK-NEXT: br label %[[OUTER_HEADER_PREHEADER1]]
; CHECK: [[INNER_HEADER_SPLIT]]:
; CHECK-NEXT: [[ARRAYIDX_US_US:%.*]] = getelementptr i16, ptr [[A]], 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 %[[INNER_LATCH:.*]]
-; CHECK: [[INNER_LATCH]]:
-; CHECK-NEXT: [[J_NEXT:%.*]] = add i16 [[J]], 1
-; CHECK-NEXT: [[CMP:%.*]] = icmp slt i16 [[J]], 42
; CHECK-NEXT: br label %[[OUTER_BODY:.*]]
+; CHECK: [[OUTER_BODY]]:
+; CHECK-NEXT: [[TMP1:%.*]] = add i16 [[J]], 1
+; CHECK-NEXT: [[TMP2:%.*]] = icmp slt i16 [[J]], 42
+; CHECK-NEXT: br label %[[OUTER_BODY1:.*]]
; CHECK: [[INNER_LATCH_SPLIT]]:
; CHECK-NEXT: [[NEW_COND_LCSSA:%.*]] = phi i16 [ [[COND]], %[[OUTER_LATCH]] ]
-; CHECK-NEXT: [[TMP1]] = add i16 [[J]], 1
-; CHECK-NEXT: [[TMP2:%.*]] = icmp slt i16 [[J]], 42
-; CHECK-NEXT: br i1 [[TMP2]], label %[[INNER_HEADER]], label %[[EXIT:.*]]
-; CHECK: [[OUTER_BODY]]:
+; CHECK-NEXT: [[TMP3]] = add i16 [[J]], 1
+; CHECK-NEXT: [[TMP4:%.*]] = icmp slt i16 [[J]], 42
+; CHECK-NEXT: br i1 [[TMP4]], label %[[INNER_HEADER1]], label %[[EXIT:.*]]
+; CHECK: [[OUTER_BODY1]]:
; CHECK-NEXT: br label %[[OUTER_LATCH]]
; CHECK: [[OUTER_LATCH]]:
; CHECK-NEXT: [[I_NEXT]] = add i64 [[I]], 1
; CHECK-NEXT: [[CMP286_US:%.*]] = icmp ugt i64 [[I]], 0
-; CHECK-NEXT: br i1 [[CMP286_US]], label %[[OUTER_HEADER]], label %[[INNER_LATCH_SPLIT]]
+; CHECK-NEXT: br i1 [[CMP286_US]], label %[[OUTER_HEADER_PREHEADER]], label %[[INNER_LATCH_SPLIT]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: [[OLD_COND_LCSSA_LCSSA:%.*]] = phi i16 [ [[NEW_COND_LCSSA]], %[[INNER_LATCH_SPLIT]] ]
; CHECK-NEXT: ret i16 [[OLD_COND_LCSSA_LCSSA]]
diff --git a/llvm/test/Transforms/LoopInterchange/non-lcssa-phi-in-inner-exit.ll b/llvm/test/Transforms/LoopInterchange/non-lcssa-phi-in-inner-exit.ll
new file mode 100644
index 0000000000000..575c7983452d9
--- /dev/null
+++ b/llvm/test/Transforms/LoopInterchange/non-lcssa-phi-in-inner-exit.ll
@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=loop-interchange -verify-dom-info -verify-loop-info -verify-scev -verify-loop-lcssa -S | FileCheck %s
+
+; Non-lcssa phi nodes in the inner loop exit are not supported.
+
+define void @f(i1 %cond) {
+; CHECK-LABEL: define void @f(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: br label %[[OUTER_HEADER:.*]]
+; CHECK: [[OUTER_HEADER]]:
+; CHECK-NEXT: [[I:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[I_INC:%.*]], %[[OUTER_LATCH:.*]] ]
+; CHECK-NEXT: br i1 [[COND]], label %[[OUTER_LATCH]], label %[[INNER_PREHEADER:.*]]
+; CHECK: [[INNER_PREHEADER]]:
+; CHECK-NEXT: br label %[[INNER:.*]]
+; CHECK: [[INNER]]:
+; CHECK-NEXT: [[J:%.*]] = phi i64 [ [[J_INC:%.*]], %[[INNER]] ], [ 0, %[[INNER_PREHEADER]] ]
+; CHECK-NEXT: [[J_INC]] = add i64 [[J]], 1
+; CHECK-NEXT: [[EC_J:%.*]] = icmp eq i64 [[J_INC]], 10
+; CHECK-NEXT: br i1 [[EC_J]], label %[[OUTER_LATCH_LOOPEXIT:.*]], label %[[INNER]]
+; CHECK: [[OUTER_LATCH_LOOPEXIT]]:
+; CHECK-NEXT: [[J_LCSSA:%.*]] = phi i64 [ [[J]], %[[INNER]] ]
+; CHECK-NEXT: br label %[[OUTER_LATCH]]
+; CHECK: [[OUTER_LATCH]]:
+; CHECK-NEXT: [[P:%.*]] = phi i64 [ [[I]], %[[OUTER_HEADER]] ], [ [[J_LCSSA]], %[[OUTER_LATCH_LOOPEXIT]] ]
+; CHECK-NEXT: [[I_INC]] = add i64 [[I]], 1
+; CHECK-NEXT: [[EC_I:%.*]] = icmp eq i64 [[I_INC]], 10
+; CHECK-NEXT: br i1 [[EC_I]], label %[[EXIT:.*]], label %[[OUTER_HEADER]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %outer.header
+
+outer.header:
+ %i = phi i64 [ 0, %entry ], [ %i.inc, %outer.latch ]
+ br i1 %cond, label %outer.latch, label %inner
+
+inner:
+ %j = phi i64 [ 0, %outer.header ], [ %j.inc, %inner ]
+ %j.inc = add i64 %j, 1
+ %ec.j = icmp eq i64 %j.inc, 10
+ br i1 %ec.j, label %outer.latch, label %inner
+
+outer.latch:
+ %p = phi i64 [ %i, %outer.header ], [ %j, %inner ]
+ %i.inc = add i64 %i, 1
+ %ec.i = icmp eq i64 %i.inc, 10
+ br i1 %ec.i, label %exit, label %outer.header
+
+exit:
+ ret void
+}
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