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;
 }
 

test/Analysis/BlockFrequencyInfo/singularity.ll

@@ -1,65 +0,0 @@
-; RUN: opt < %s -analyze -block-freq | FileCheck %s
-; PR16402
-
-define void @test1(i32 %n) nounwind {
-entry:
-  %call = tail call i32* @cond() nounwind
-  %tobool = icmp eq i32* %call, null
-  br i1 %tobool, label %land.lhs.true, label %if.end
-
-land.lhs.true:                                    ; preds = %entry
-  %call1 = tail call i32* @cond() nounwind
-  %tobool2 = icmp eq i32* %call1, null
-  br i1 %tobool2, label %land.lhs.true3, label %if.end
-
-land.lhs.true3:                                   ; preds = %land.lhs.true
-  %call4 = tail call i32* @cond() nounwind
-  %tobool5 = icmp eq i32* %call4, null
-  br i1 %tobool5, label %land.lhs.true6, label %if.end
-
-land.lhs.true6:                                   ; preds = %land.lhs.true3
-  %call7 = tail call i32* @cond() nounwind
-  %tobool8 = icmp eq i32* %call7, null
-  br i1 %tobool8, label %land.lhs.true9, label %if.end
-
-land.lhs.true9:                                   ; preds = %land.lhs.true6
-  %call10 = tail call i32* @cond() nounwind
-  %tobool11 = icmp eq i32* %call10, null
-  br i1 %tobool11, label %land.lhs.true12, label %if.end
-
-land.lhs.true12:                                  ; preds = %land.lhs.true9
-  %call13 = tail call i32* @cond() nounwind
-  %tobool14 = icmp eq i32* %call13, null
-  br i1 %tobool14, label %land.lhs.true15, label %if.end
-
-land.lhs.true15:                                  ; preds = %land.lhs.true12
-  %call16 = tail call i32* @cond() nounwind
-  %tobool17 = icmp eq i32* %call16, null
-  br i1 %tobool17, label %for.cond.preheader, label %if.end
-
-for.cond.preheader:                               ; preds = %land.lhs.true15
-  %cmp21 = icmp eq i32 %n, 0
-  br i1 %cmp21, label %for.end, label %for.body
-
-for.body:                                         ; preds = %for.cond.preheader, %for.body
-  %i.022 = phi i32 [ %inc, %for.body ], [ 0, %for.cond.preheader ]
-  %call18 = tail call i32 @call() nounwind
-  %inc = add nsw i32 %i.022, 1
-  %cmp = icmp eq i32 %inc, %n
-  br i1 %cmp, label %for.end, label %for.body
-
-for.end:                                          ; preds = %for.body, %for.cond.preheader
-  %call19 = tail call i32* @cond() nounwind
-  br label %if.end
-
-if.end:                                           ; preds = %land.lhs.true15, %land.lhs.true12, %land.lhs.true9, %land.lhs.true6, %land.lhs.true3, %land.lhs.true, %entry, %for.end
-  ret void
-
-; CHECK: entry = 1024
-; CHECK-NOT: for.body = 0
-; CHECK-NOT: for.end = 0
-}
-
-declare i32* @cond() nounwind
-
-declare i32 @call() nounwind

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) {