BlockFrequency: Saturate at 1 instead of 0 when multiplying a frequency with a branch probability.

Zero is used by BlockFrequencyInfo as a special "don't know" value. It also
causes a sink for frequencies as you can't ever get off a zero frequency with
more multiplies.

This recovers a 10% regression on MultiSource/Benchmarks/7zip. A zero frequency
was propagated into an inner loop causing excessive spilling.

PR16402.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184584 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 0 deletions

diff --git a/include/llvm/Support/BlockFrequency.h b/include/llvm/Support/BlockFrequency.h
index 839cf93712..257888f8d0 100644
--- a/include/llvm/Support/BlockFrequency.h
+++ b/include/llvm/Support/BlockFrequency.h
@@ -30,12 +30,20 @@ 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;