PGO: preserve branch-weight metadata when simplifying two branches with a common

destination in SimplifyCondBranchToCondBranch.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164054 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 091d63d67d..5a8d2e4689 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2150,6 +2150,33 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
   PBI->setSuccessor(0, CommonDest);
   PBI->setSuccessor(1, OtherDest);
 
+  // Update branch weight for PBI.
+  uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
+  bool PredHasWeights = ExtractBranchMetadata(PBI, PredTrueWeight,
+                                              PredFalseWeight);
+  bool SuccHasWeights = ExtractBranchMetadata(BI, SuccTrueWeight,
+                                              SuccFalseWeight);
+  if (PredHasWeights && SuccHasWeights) {
+    uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
+    uint64_t PredOther = PBIOp ?PredTrueWeight : PredFalseWeight;
+    uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
+    uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
+    // The weight to CommonDest should be PredCommon * SuccTotal +
+    //                                    PredOther * SuccCommon.
+    // The weight to OtherDest should be PredOther * SuccOther.
+    SmallVector<uint64_t, 2> NewWeights;
+    NewWeights.push_back(PredCommon * (SuccCommon + SuccOther) +
+                         PredOther * SuccCommon);
+    NewWeights.push_back(PredOther * SuccOther);
+    // Halve the weights if any of them cannot fit in an uint32_t
+    FitWeights(NewWeights);
+
+    SmallVector<uint32_t, 2> MDWeights(NewWeights.begin(),NewWeights.end());
+    PBI->setMetadata(LLVMContext::MD_prof,
+                     MDBuilder(BI->getContext()).
+                     createBranchWeights(MDWeights));
+  }
+
   // OtherDest may have phi nodes.  If so, add an entry from PBI's
   // block that are identical to the entries for BI's block.
   AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);