[BPI] Adjust the probability for floating point unordered comparison

Since NaN is very rare in normal programs, so the probability for floating point unordered comparison should be extremely small. Current probability is 3/8, it is too large, this patch changes it to a tiny number.

Differential Revision: https://reviews.llvm.org/D65303

llvm-svn: 371541

1 files changed, 14 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/BranchProbabilityInfo.cpp b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
index 65c9cf2f06d..a06ee096d54 100644
--- a/llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -118,6 +118,13 @@ static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
 static const uint32_t FPH_TAKEN_WEIGHT = 20;
 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
 
+/// This is the probability for an ordered floating point comparison.
+static const uint32_t FPH_ORD_WEIGHT = 1024 * 1024 - 1;
+/// This is the probability for an unordered floating point comparison, it means
+/// one or two of the operands are NaN. Usually it is used to test for an
+/// exceptional case, so the result is unlikely.
+static const uint32_t FPH_UNO_WEIGHT = 1;
+
 /// Invoke-terminating normal branch taken weight
 ///
 /// This is the weight for branching to the normal destination of an invoke
@@ -778,6 +785,8 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   if (!FCmp)
     return false;
 
+  uint32_t TakenWeight = FPH_TAKEN_WEIGHT;
+  uint32_t NontakenWeight = FPH_NONTAKEN_WEIGHT;
   bool isProb;
   if (FCmp->isEquality()) {
     // f1 == f2 -> Unlikely
@@ -786,9 +795,13 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
     // !isnan -> Likely
     isProb = true;
+    TakenWeight = FPH_ORD_WEIGHT;
+    NontakenWeight = FPH_UNO_WEIGHT;
   } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
     // isnan -> Unlikely
     isProb = false;
+    TakenWeight = FPH_ORD_WEIGHT;
+    NontakenWeight = FPH_UNO_WEIGHT;
   } else {
     return false;
   }
@@ -798,8 +811,7 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   if (!isProb)
     std::swap(TakenIdx, NonTakenIdx);
 
-  BranchProbability TakenProb(FPH_TAKEN_WEIGHT,
-                              FPH_TAKEN_WEIGHT + FPH_NONTAKEN_WEIGHT);
+  BranchProbability TakenProb(TakenWeight, TakenWeight + NontakenWeight);
   setEdgeProbability(BB, TakenIdx, TakenProb);
   setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
   return true;