[BasicBlockUtils] Do not move loop metadata if outer loop header.

Fixes a bug preventing moving the loop's metadata to an outer loop's header,
which happens if the loop's exit is also the header of an outer loop.

Adjusts test for above.

Fixes #55416.

Differential Revision: https://reviews.llvm.org/D125574

Author: Hendrik Greving

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -1162,7 +1162,11 @@ SplitBlockPredecessorsImpl(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
     if (NewLatch != OldLatch) {
       MDNode *MD = OldLatch->getTerminator()->getMetadata("llvm.loop");
       NewLatch->getTerminator()->setMetadata("llvm.loop", MD);
-      OldLatch->getTerminator()->setMetadata("llvm.loop", nullptr);
+      // It's still possible that OldLatch is the latch of another inner loop,
+      // in which case we do not remove the metadata.
+      Loop *IL = LI->getLoopFor(OldLatch);
+      if (IL && IL->getLoopLatch() != OldLatch)
+        OldLatch->getTerminator()->setMetadata("llvm.loop", nullptr);
     }
   }
 

llvm/test/Transforms/LoopSimplify/update_latch_md2.ll

@@ -1,13 +1,11 @@
 ; Tests loop-simplify does not move the loop metadata, because
 ; the loopexit block is not the latch of the loop _bb6.
 
-; FIXME(#55416): The metadata should not move.
-
 ; RUN: opt < %s -passes=loop-simplify -S | FileCheck %s
 ; CHECK-LABEL: loop.header.loopexit:
 ; CHECK: br label %loop.header, !llvm.loop !0
 ; CHECK-LABEL: loop.latch:
-; CHECK-NOT: br i1 %p, label %loop.latch, label %loop.header.loopexit, !llvm.loop !0
+; CHECK: br i1 %p, label %loop.latch, label %loop.header.loopexit, !llvm.loop !0
 
 define void @func(i1 %p) {
 entry: