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.

llvm-svn: 184584

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Support/BlockFrequency.cpp b/llvm/lib/Support/BlockFrequency.cpp
index 84a993e3e5b..53bbd8ad12d 100644
--- a/llvm/lib/Support/BlockFrequency.cpp
+++ b/llvm/lib/Support/BlockFrequency.cpp
@@ -65,6 +65,9 @@ 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();
 
@@ -84,10 +87,15 @@ BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
     // 64-bit.
     mult96bit(Frequency, n, W);
     Frequency = div96bit(W, d);
-    return *this;
+  } else {
+    // Fast case.
+    Frequency = mulRes / d;
   }
 
-  Frequency = mulRes / d;
+  // 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;
   return *this;
 }