--- a/include/llvm/Support/BranchProbability.h	Tue Oct 13 17:49:56 2015 +0900
+++ b/include/llvm/Support/BranchProbability.h	Tue Jan 26 22:53:40 2016 +0900
@@ -15,7 +15,10 @@
 #define LLVM_SUPPORT_BRANCHPROBABILITY_H
 
 #include "llvm/Support/DataTypes.h"
+#include <algorithm>
 #include <cassert>
+#include <climits>
+#include <numeric>
 
 namespace llvm {
 
@@ -31,27 +34,34 @@
 
   // Denominator, which is a constant value.
   static const uint32_t D = 1u << 31;
+  static const uint32_t UnknownN = UINT32_MAX;
 
   // Construct a BranchProbability with only numerator assuming the denominator
   // is 1<<31. For internal use only.
   explicit BranchProbability(uint32_t n) : N(n) {}
 
 public:
-  BranchProbability() : N(0) {}
+  BranchProbability() : N(UnknownN) {}
   BranchProbability(uint32_t Numerator, uint32_t Denominator);
 
   bool isZero() const { return N == 0; }
+  bool isUnknown() const { return N == UnknownN; }
 
   static BranchProbability getZero() { return BranchProbability(0); }
   static BranchProbability getOne() { return BranchProbability(D); }
+  static BranchProbability getUnknown() { return BranchProbability(UnknownN); }
   // Create a BranchProbability object with the given numerator and 1<<31
   // as denominator.
   static BranchProbability getRaw(uint32_t N) { return BranchProbability(N); }
+  // Create a BranchProbability object from 64-bit integers.
+  static BranchProbability getBranchProbability(uint64_t Numerator,
+                                                uint64_t Denominator);
 
   // Normalize given probabilties so that the sum of them becomes approximate
   // one.
-  template <class ProbabilityList>
-  static void normalizeProbabilities(ProbabilityList &Probs);
+  template <class ProbabilityIter>
+  static void normalizeProbabilities(ProbabilityIter Begin,
+                                     ProbabilityIter End);
 
   uint32_t getNumerator() const { return N; }
   static uint32_t getDenominator() { return D; }
@@ -80,24 +90,36 @@
   uint64_t scaleByInverse(uint64_t Num) const;
 
   BranchProbability &operator+=(BranchProbability RHS) {
-    assert(N <= D - RHS.N &&
-           "The sum of branch probabilities should not exceed one!");
-    N += RHS.N;
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in arithmetics.");
+    // Saturate the result in case of overflow.
+    N = (uint64_t(N) + RHS.N > D) ? D : N + RHS.N;
     return *this;
   }
 
   BranchProbability &operator-=(BranchProbability RHS) {
-    assert(N >= RHS.N &&
-           "Can only subtract a smaller probability from a larger one!");
-    N -= RHS.N;
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in arithmetics.");
+    // Saturate the result in case of underflow.
+    N = N < RHS.N ? 0 : N - RHS.N;
     return *this;
   }
 
   BranchProbability &operator*=(BranchProbability RHS) {
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in arithmetics.");
     N = (static_cast<uint64_t>(N) * RHS.N + D / 2) / D;
     return *this;
   }
 
+  BranchProbability &operator/=(uint32_t RHS) {
+    assert(N != UnknownN &&
+           "Unknown probability cannot participate in arithmetics.");
+    assert(RHS > 0 && "The divider cannot be zero.");
+    N /= RHS;
+    return *this;
+  }
+
   BranchProbability operator+(BranchProbability RHS) const {
     BranchProbability Prob(*this);
     return Prob += RHS;
@@ -113,26 +135,83 @@
     return Prob *= RHS;
   }
 
+  BranchProbability operator/(uint32_t RHS) const {
+    BranchProbability Prob(*this);
+    return Prob /= RHS;
+  }
+
   bool operator==(BranchProbability RHS) const { return N == RHS.N; }
   bool operator!=(BranchProbability RHS) const { return !(*this == RHS); }
-  bool operator<(BranchProbability RHS) const { return N < RHS.N; }
-  bool operator>(BranchProbability RHS) const { return RHS < *this; }
-  bool operator<=(BranchProbability RHS) const { return !(RHS < *this); }
-  bool operator>=(BranchProbability RHS) const { return !(*this < RHS); }
+
+  bool operator<(BranchProbability RHS) const {
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in comparisons.");
+    return N < RHS.N;
+  }
+
+  bool operator>(BranchProbability RHS) const {
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in comparisons.");
+    return RHS < *this;
+  }
+
+  bool operator<=(BranchProbability RHS) const {
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in comparisons.");
+    return !(RHS < *this);
+  }
+
+  bool operator>=(BranchProbability RHS) const {
+    assert(N != UnknownN && RHS.N != UnknownN &&
+           "Unknown probability cannot participate in comparisons.");
+    return !(*this < RHS);
+  }
 };
 
 inline raw_ostream &operator<<(raw_ostream &OS, BranchProbability Prob) {
   return Prob.print(OS);
 }
 
-template <class ProbabilityList>
-void BranchProbability::normalizeProbabilities(ProbabilityList &Probs) {
-  uint64_t Sum = 0;
-  for (auto Prob : Probs)
-    Sum += Prob.N;
-  assert(Sum > 0);
-  for (auto &Prob : Probs)
-    Prob.N = (Prob.N * uint64_t(D) + Sum / 2) / Sum;
+template <class ProbabilityIter>
+void BranchProbability::normalizeProbabilities(ProbabilityIter Begin,
+                                               ProbabilityIter End) {
+  if (Begin == End)
+    return;
+
+  unsigned UnknownProbCount = 0;
+  uint64_t Sum = std::accumulate(Begin, End, uint64_t(0),
+                                 [&](uint64_t S, const BranchProbability &BP) {
+                                   if (!BP.isUnknown())
+                                     return S + BP.N;
+                                   UnknownProbCount++;
+                                   return S;
+                                 });
+
+  if (UnknownProbCount > 0) {
+    BranchProbability ProbForUnknown = BranchProbability::getZero();
+    // If the sum of all known probabilities is less than one, evenly distribute
+    // the complement of sum to unknown probabilities. Otherwise, set unknown
+    // probabilities to zeros and continue to normalize known probabilities.
+    if (Sum < BranchProbability::getDenominator())
+      ProbForUnknown = BranchProbability::getRaw(
+          (BranchProbability::getDenominator() - Sum) / UnknownProbCount);
+
+    std::replace_if(Begin, End,
+                    [](const BranchProbability &BP) { return BP.isUnknown(); },
+                    ProbForUnknown);
+
+    if (Sum <= BranchProbability::getDenominator())
+      return;
+  }
+ 
+  if (Sum == 0) {
+    BranchProbability BP(1, std::distance(Begin, End));
+    std::fill(Begin, End, BP);
+    return;
+  }
+
+  for (auto I = Begin; I != End; ++I)
+    I->N = (I->N * uint64_t(D) + Sum / 2) / Sum;
 }
 
 }