[llvm] [LoopInterchange] Fix crash when followLCSSA returns constant (PR #203515)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj updated https://github.com/llvm/llvm-project/pull/203515

>From 229e5478e6f0385034b77b263237494febfa29b9 Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Fri, 12 Jun 2026 12:30:13 +0000
Subject: [PATCH 1/2] [LoopInterchange] Fix crash when followLCSSA returns
 constant

---
 .../lib/Transforms/Scalar/LoopInterchange.cpp | 12 ++--
 .../lcssa-incoming-value-is-not-instr.ll      | 70 +++++++++++++++++++
 2 files changed, 77 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 3da4e99860e45..becac190dc01c 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -2271,11 +2271,13 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
 
     // In case of multi-level nested loops, follow LCSSA to find the incoming
     // value defined from the innermost loop.
-    auto IncIInnerMost = cast<Instruction>(followLCSSA(IncI));
-    // Skip phis with incoming values from the inner loop body, excluding the
-    // header and latch.
-    if (IncIInnerMost->getParent() != InnerLatch &&
-        IncIInnerMost->getParent() != InnerHeader)
+    auto *IncIInnerMost = dyn_cast<Instruction>(followLCSSA(IncI));
+    // Skip phis when:
+    // - Constant incoming values.
+    // - Incomming values from the inner loop body, excluding the header and
+    //   latch.
+    if (!IncIInnerMost || (IncIInnerMost->getParent() != InnerLatch &&
+                           IncIInnerMost->getParent() != InnerHeader))
       continue;
 
     assert(all_of(P.users(),
diff --git a/llvm/test/Transforms/LoopInterchange/lcssa-incoming-value-is-not-instr.ll b/llvm/test/Transforms/LoopInterchange/lcssa-incoming-value-is-not-instr.ll
index 9624ed7d38ed7..b0abe98b5f5d7 100644
--- a/llvm/test/Transforms/LoopInterchange/lcssa-incoming-value-is-not-instr.ll
+++ b/llvm/test/Transforms/LoopInterchange/lcssa-incoming-value-is-not-instr.ll
@@ -62,3 +62,73 @@ exit:
   %cst.lcssa = phi i32 [ %cst, %outer.latch ]
   ret i32 %cst.lcssa
 }
+
+
+; Ensure that we can handle LCSSA PHI even if the root of the use-def chain of
+; the incoming value is not an instruction.
+
+define i32 @g() {
+; CHECK-LABEL: define i32 @g() {
+; 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:.*]] ], [ 0, %[[OUTER_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[INNER_LATCH:.*]]
+; CHECK:       [[INNER_HEADER_PREHEADER]]:
+; CHECK-NEXT:    br label %[[INNER_HEADER:.*]]
+; CHECK:       [[INNER_HEADER]]:
+; CHECK-NEXT:    [[J:%.*]] = phi i64 [ [[TMP1:%.*]], %[[INNER_LATCH_SPLIT:.*]] ], [ 0, %[[INNER_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[OUTER_HEADER_PREHEADER]]
+; CHECK:       [[INNER_LATCH]]:
+; CHECK-NEXT:    [[X:%.*]] = phi i32 [ 42, %[[OUTER_HEADER]] ]
+; CHECK-NEXT:    [[J_NEXT:%.*]] = add i64 [[J]], 1
+; CHECK-NEXT:    [[EC_J:%.*]] = icmp eq i64 [[J_NEXT]], 2
+; CHECK-NEXT:    br label %[[INNER_EXIT:.*]]
+; CHECK:       [[INNER_LATCH_SPLIT]]:
+; CHECK-NEXT:    [[X_LCSSA:%.*]] = phi i32 [ [[X]], %[[OUTER_LATCH]] ]
+; CHECK-NEXT:    [[TMP1]] = add i64 [[J]], 1
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i64 [[TMP1]], 2
+; CHECK-NEXT:    br i1 [[TMP2]], label %[[EXIT:.*]], label %[[INNER_HEADER]]
+; CHECK:       [[INNER_EXIT]]:
+; CHECK-NEXT:    br label %[[OUTER_LATCH]]
+; CHECK:       [[OUTER_LATCH]]:
+; CHECK-NEXT:    [[I_NEXT]] = add i64 [[I]], 1
+; CHECK-NEXT:    [[EC_I:%.*]] = icmp eq i64 [[I_NEXT]], 2
+; CHECK-NEXT:    br i1 [[EC_I]], label %[[INNER_LATCH_SPLIT]], label %[[OUTER_HEADER]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[X_FINAL:%.*]] = phi i32 [ [[X_LCSSA]], %[[INNER_LATCH_SPLIT]] ]
+; CHECK-NEXT:    ret i32 [[X_FINAL]]
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  %i = phi i64 [ 0, %entry ], [ %i.next, %outer.latch ]
+  br label %inner.header
+
+inner.header:
+  %j = phi i64 [ 0, %outer.header ], [ %j.next, %inner.latch ]
+  br label %inner.latch
+
+inner.latch:
+  %x = phi i32 [ 42, %inner.header ]
+  %j.next = add i64 %j, 1
+  %ec.j = icmp eq i64 %j.next, 2
+  br i1 %ec.j, label %inner.exit, label %inner.header
+
+inner.exit:
+  %x.lcssa = phi i32 [ %x, %inner.latch ]
+  br label %outer.latch
+
+outer.latch:
+  %x.lcssa2 = phi i32 [ %x.lcssa, %inner.exit ]
+  %i.next = add i64 %i, 1
+  %ec.i = icmp eq i64 %i.next, 2
+  br i1 %ec.i, label %exit, label %outer.header
+
+exit:
+  %x.final = phi i32 [ %x.lcssa2, %outer.latch ]
+  ret i32 %x.final
+}

>From f248fec86aa114c89c0c85087bda1bc4f96f34be Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Sat, 13 Jun 2026 05:05:43 +0900
Subject: [PATCH 2/2] address review

---
 llvm/lib/Transforms/Scalar/LoopInterchange.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index becac190dc01c..ca24d6165913e 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -2273,7 +2273,7 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
     // value defined from the innermost loop.
     auto *IncIInnerMost = dyn_cast<Instruction>(followLCSSA(IncI));
     // Skip phis when:
-    // - Constant incoming values.
+    // - they are not an instruction, e.g. incoming values are constants.
     // - Incomming values from the inner loop body, excluding the header and
     //   latch.
     if (!IncIInnerMost || (IncIInnerMost->getParent() != InnerLatch &&