[llvm] [LoopInterchange] Detect unsupported LCSSA-related PHIs correctly (PR #194323)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj updated https://github.com/llvm/llvm-project/pull/194323

>From 1ea2f20eceed685fb329887a80d4abb83d0ad0f3 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 ++++++++++------
 .../non-lcssa-phi-in-inner-exit.ll            | 53 +++++++++++++++++++
 2 files changed, 82 insertions(+), 17 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/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
+}