[SimplifyCFG] Add loop metadata to the new branch when doing jump-threading

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3605,6 +3605,9 @@ foldCondBranchOnValueKnownInPredecessorImpl(BranchInst *BI, DomTreeUpdater *DTU,
     EdgeBB->setName(RealDest->getName() + ".critedge");
     EdgeBB->moveBefore(RealDest);
 
+    if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
+      EdgeBB->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopMD);
+
     // Update PHI nodes.
     addPredecessorToBlock(RealDest, EdgeBB, BB);
 

llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata-2.ll

@@ -0,0 +1,49 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes='simplifycfg' -S < %s | FileCheck %s
+
+; CHECK: br i1 %2, label %loop2, label %loop1
+; CHECK-SAME: llvm.loop
+
+define void @test(i32 %1 ) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: i32 [[TMP0:%.*]]) {
+; CHECK-NEXT:  [[_CRITEDGE:.*:]]
+; CHECK-NEXT:    br label %[[LOOP1:.*]]
+; CHECK:       [[LOOP1]]:
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[TMP0]], 0
+; CHECK-NEXT:    br i1 [[TMP1]], label %[[LOOP2:.*]], label %[[BB3:.*]]
+; CHECK:       [[LOOP2]]:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi i1 [ true, %[[LOOP1]] ], [ false, %[[LOOP2]] ]
+; CHECK-NEXT:    br i1 [[TMP2]], label %[[LOOP2]], label %[[LOOP1]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       [[BB3]]:
+; CHECK-NEXT:    ret void
+;
+.critedge:
+  br label %loop1
+
+loop1:                                              ; preds = %loop2, .critedge
+  %111 = icmp eq i32 %1, 0
+  br i1 %111, label %112, label %156
+
+112:                                              ; preds = %loop1
+  br label %loop2
+
+loop2:                                              ; preds = %149, %112
+  %148 = phi i1 [ false, %149 ], [ true, %112 ]
+  br i1 %148, label %149, label %loop1, !llvm.loop !32
+
+149:                                              ; preds = %loop2
+  br label %loop2
+
+156:                                              ; preds = loop1
+  ret void
+}
+
+!32 = distinct !{!32, !33, !34}
+!33 = !{!"llvm.loop.unroll.enable"}
+!34 = !{!"llvm.loop.unroll.full"}
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.unroll.enable"}
+; CHECK: [[META2]] = !{!"llvm.loop.unroll.full"}
+;.