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, 12 insertions, 1 deletions

diff --git a/llvm/unittests/Support/BlockFrequencyTest.cpp b/llvm/unittests/Support/BlockFrequencyTest.cpp
index ff66bc4e45a..bcb88c88950 100644
--- a/llvm/unittests/Support/BlockFrequencyTest.cpp
+++ b/llvm/unittests/Support/BlockFrequencyTest.cpp
@@ -8,11 +8,22 @@ 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(), 0u);
+  EXPECT_EQ(Freq.getFrequency(), 1u);
 }
 
 TEST(BlockFrequencyTest, OneToOne) {