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

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3602,6 +3602,9 @@ 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 +1,30 @@
+; RUN: opt -passes='simplifycfg' -S < %s | FileCheck %s
+
+; CHECK: br i1 %4, label %3, label %1, 
+; CHECK-SAME: llvm.loop
+
+define void @test(i32 %1 ) {
+.critedge:
+  br label %107
+
+107:                                              ; preds = %147, .critedge 
+  %111 = icmp eq i32 %1, 0
+  br i1 %111, label %112, label %156
+
+112:                                              ; preds = %107
+  br label %147
+
+147:                                              ; preds = %149, %112
+  %148 = phi i1 [ false, %149 ], [ true, %112 ]
+  br i1 %148, label %149, label %107, !llvm.loop !32
+
+149:                                              ; preds = %147
+  br label %147
+
+156:                                              ; preds = %107
+   ret void
+} 
+
+!32 = distinct !{!32, !33, !34}
+!33 = !{!"llvm.loop.unroll.enable"}
+!34 = !{!"llvm.loop.unroll.full"}