[PATCH] D15548: Remove the restriction that known and unknown probabilities cannot coexist when being normalized.

[Cong Hou via llvm-commits]

congh created this revision.
congh added a reviewer: davidxl.
congh added a subscriber: llvm-commits.

The current BranchProbability::normalizeProbabilities() forbids known and unknown probabilities to coexist in the list. This was once used to help capture probability exceptions but has caused some reported build failures (https://llvm.org/bugs/show_bug.cgi?id=25838).

This patch removes this restriction by evenly distributing the complement of the sum of all known probabilities to unknown ones. We could still treat this as an abnormal behavior, but it is better to emit warnings in our future profile validator. 

http://reviews.llvm.org/D15548

Files:
  include/llvm/Support/BranchProbability.h
  unittests/Support/BranchProbabilityTest.cpp

Index: unittests/Support/BranchProbabilityTest.cpp
===================================================================
--- unittests/Support/BranchProbabilityTest.cpp
+++ unittests/Support/BranchProbabilityTest.cpp
@@ -288,6 +288,7 @@
 }
 
 TEST(BranchProbabilityTest, NormalizeProbabilities) {
+  const auto UnknownProb = BranchProbability::getUnknown();
   {
     SmallVector<BranchProbability, 2> Probs{{0, 1}, {0, 1}};
     BranchProbability::normalizeProbabilities(Probs.begin(), Probs.end());
@@ -322,6 +323,36 @@
     EXPECT_EQ(BranchProbability::getDenominator() / 3 + 1,
               Probs[2].getNumerator());
   }
+  {
+    SmallVector<BranchProbability, 2> Probs{{0, 1}, UnknownProb};
+    BranchProbability::normalizeProbabilities(Probs.begin(), Probs.end());
+    EXPECT_EQ(BranchProbability::getDenominator() / 2, Probs[0].getNumerator());
+    EXPECT_EQ(BranchProbability::getDenominator() / 2, Probs[1].getNumerator());
+  }
+  {
+    SmallVector<BranchProbability, 2> Probs{{1, 1}, UnknownProb};
+    BranchProbability::normalizeProbabilities(Probs.begin(), Probs.end());
+    EXPECT_EQ(BranchProbability::getDenominator(), Probs[0].getNumerator());
+    EXPECT_EQ(0, Probs[1].getNumerator());
+  }
+  {
+    SmallVector<BranchProbability, 2> Probs{{1, 2}, UnknownProb};
+    BranchProbability::normalizeProbabilities(Probs.begin(), Probs.end());
+    EXPECT_EQ(BranchProbability::getDenominator() / 2, Probs[0].getNumerator());
+    EXPECT_EQ(BranchProbability::getDenominator() / 2, Probs[1].getNumerator());
+  }
+  {
+    SmallVector<BranchProbability, 4> Probs{
+        {1, 2}, {1, 2}, {1, 2}, UnknownProb};
+    BranchProbability::normalizeProbabilities(Probs.begin(), Probs.end());
+    EXPECT_EQ(BranchProbability::getDenominator() / 3 + 1,
+              Probs[0].getNumerator());
+    EXPECT_EQ(BranchProbability::getDenominator() / 3 + 1,
+              Probs[1].getNumerator());
+    EXPECT_EQ(BranchProbability::getDenominator() / 3 + 1,
+              Probs[2].getNumerator());
+    EXPECT_EQ(0, Probs[3].getNumerator());
+  }
 }
 
 }
Index: include/llvm/Support/BranchProbability.h
===================================================================
--- include/llvm/Support/BranchProbability.h
+++ include/llvm/Support/BranchProbability.h
@@ -183,16 +183,31 @@
   if (Begin == End)
     return;
 
-  auto UnknownProbCount =
-      std::count(Begin, End, BranchProbability::getUnknown());
-  assert((UnknownProbCount == 0 ||
-          UnknownProbCount == std::distance(Begin, End)) &&
-         "Cannot normalize probabilities with known and unknown ones.");
-  (void)UnknownProbCount;
-
-  uint64_t Sum = std::accumulate(
-      Begin, End, uint64_t(0),
-      [](uint64_t S, const BranchProbability &BP) { return S + BP.N; });
+  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 && Sum > 0) {
+    BranchProbability UnknownProb = 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())
+      UnknownProb = BranchProbability::getRaw(
+          (BranchProbability::getDenominator() - Sum) / UnknownProbCount);
+
+    std::replace_if(Begin, End,
+                    [](const BranchProbability &BP) { return BP.isUnknown(); },
+                    UnknownProb);
+
+    if (Sum <= BranchProbability::getDenominator())
+      return;
+  }
 
   if (Sum == 0) {
     BranchProbability BP(1, std::distance(Begin, End));


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