Index: llvm/include/llvm/Analysis/BranchProbabilityInfo.h
===================================================================
--- llvm/include/llvm/Analysis/BranchProbabilityInfo.h
+++ llvm/include/llvm/Analysis/BranchProbabilityInfo.h
@@ -131,6 +131,12 @@
   void setEdgeProbability(const BasicBlock *Src,
                           const SmallVectorImpl<BranchProbability> &Probs);
 
+  /// Copy outgoing edge probabilities from \p Src to \p Dst.
+  ///
+  /// This allows to keep probabilities unset for the destination if they were
+  /// unset for source.
+  void copyEdgeProbabilities(BasicBlock *Src, BasicBlock *Dst);
+
   static BranchProbability getBranchProbStackProtector(bool IsLikely) {
     static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
     return IsLikely ? LikelyProb : LikelyProb.getCompl();
Index: llvm/lib/Analysis/BranchProbabilityInfo.cpp
===================================================================
--- llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -1157,6 +1157,24 @@
   assert(TotalNumerator >= BranchProbability::getDenominator() - Probs.size());
 }
 
+void BranchProbabilityInfo::copyEdgeProbabilities(BasicBlock *Src, BasicBlock *Dst) {
+  eraseBlock(Dst); // Erase stale data if any.
+  unsigned NumSuccessors = Src->getTerminator()->getNumSuccessors();
+  assert(NumSuccessors == Dst->getTerminator()->getNumSuccessors());
+  if (NumSuccessors == 0)
+    return; // Nothing to set.
+  if (this->Probs.find(std::make_pair(Src, 0)) == this->Probs.end())
+    return; // No probability is set for edges from Src. Keep the same for Dst.
+
+  Handles.insert(BasicBlockCallbackVH(Dst, this));
+  for (unsigned SuccIdx = 0; SuccIdx < NumSuccessors; ++SuccIdx) {
+    auto Prob = this->Probs[std::make_pair(Src, SuccIdx)];
+    this->Probs[std::make_pair(Dst, SuccIdx)] = Prob;
+    LLVM_DEBUG(dbgs() << "set edge " << Dst->getName() << " -> " << SuccIdx
+                      << " successor probability to " << Prob << "\n");
+  }
+}
+
 raw_ostream &
 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
                                             const BasicBlock *Src,