Revert "BlockFrequency: Saturate at 1 instead of 0 when multiplying a…

… frequency with a branch probability."

This reverts commit r184584. Breaks PPC selfhost.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184590 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/BlockFrequency.h

@@ -30,20 +30,12 @@ class BlockFrequency {
 public:
   BlockFrequency(uint64_t Freq = 0) : Frequency(Freq) { }
 
-  /// \brief Returns the frequency of the entry block of the function.
   static uint64_t getEntryFrequency() { return ENTRY_FREQ; }
-
-  /// \brief Returns the frequency as a fixpoint number scaled by the entry
-  /// frequency.
   uint64_t getFrequency() const { return Frequency; }
 
-  /// \brief Multiplies with a branch probability. The computation will never
-  /// overflow. If the result is equal to zero but the input wasn't this method
-  /// will return a frequency of one.
   BlockFrequency &operator*=(const BranchProbability &Prob);
   const BlockFrequency operator*(const BranchProbability &Prob) const;
 
-  /// \brief Adds another block frequency using saturating arithmetic.
   BlockFrequency &operator+=(const BlockFrequency &Freq);
   const BlockFrequency operator+(const BlockFrequency &Freq) const;
 

lib/Support/BlockFrequency.cpp

@@ -65,9 +65,6 @@ uint64_t div96bit(uint64_t W[2], uint32_t D) {
 
 
 BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
-  if (Frequency == 0)
-    return *this;
-
   uint32_t n = Prob.getNumerator();
   uint32_t d = Prob.getDenominator();
 
@@ -87,15 +84,10 @@ BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
     // 64-bit.
     mult96bit(Frequency, n, W);
     Frequency = div96bit(W, d);
-  } else {
-    // Fast case.
-    Frequency = mulRes / d;
+    return *this;
   }
 
-  // Limit the result to 1; 0 is a sentinel value. This keeps BlockFrequencyInfo
-  // from getting stuck at zero frequencies just because a value became too
-  // small to be represented as a BlockFrequency.
-  Frequency = (n == 0 || Frequency != 0) ? Frequency : 1;
+  Frequency = mulRes / d;
   return *this;
 }
 

unittests/Support/BlockFrequencyTest.cpp

@@ -8,22 +8,11 @@ using namespace llvm;
 
 namespace {
 
-TEST(BlockFrequencyTest, ZeroToZero) {
-  BlockFrequency Freq(0);
-  BranchProbability Prob(UINT32_MAX - 1, UINT32_MAX);
-  Freq *= Prob;
-  EXPECT_EQ(Freq.getFrequency(), 0u);
-
-  Freq = 1;
-  Freq *= BranchProbability::getZero();
-  EXPECT_EQ(Freq.getFrequency(), 0u);
-}
-
 TEST(BlockFrequencyTest, OneToZero) {
   BlockFrequency Freq(1);
   BranchProbability Prob(UINT32_MAX - 1, UINT32_MAX);
   Freq *= Prob;
-  EXPECT_EQ(Freq.getFrequency(), 1u);
+  EXPECT_EQ(Freq.getFrequency(), 0u);
 }
 
 TEST(BlockFrequencyTest, OneToOne) {