Update the branch weight metadata in JumpThreading pass.

Cong Hou · Cong Hou · commit b74d3b3b86c6 · 2015-10-14T23:14:17.000Z
Currently in JumpThreading pass, the branch weight metadata is not updated after CFG modification. Consider the jump threading on PredBB, BB, and SuccBB. After jump threading, the weight on BB->SuccBB should be adjusted as some of it is contributed by the edge PredBB->BB, which doesn't exist anymore. This patch tries to update the edge weight in metadata on BB->SuccBB by scaling it by 1 - Freq(PredBB->BB) / Freq(BB->SuccBB). This is the third attempt to submit this patch, while the first two led to failures in some FDO tests. After investigation, it is the edge weight normalization that caused those failures. In this patch the edge weight normalization is fixed so that there is no zero weight in the output and the sum of all weights can fit in 32-bit integer. Several unit tests are added. Differential revision: http://reviews.llvm.org/D10979 llvm-svn: 250345
diff --git a/llvm/include/llvm/Analysis/BlockFrequencyInfo.h b/llvm/include/llvm/Analysis/BlockFrequencyInfo.h
@@ -45,6 +45,9 @@ class BlockFrequencyInfo {
   /// floating points.
   BlockFrequency getBlockFreq(const BasicBlock *BB) const;
 
+  // Set the frequency of the given basic block.
+  void setBlockFreq(const BasicBlock *BB, uint64_t Freq);
+
   /// calculate - compute block frequency info for the given function.
   void calculate(const Function &F, const BranchProbabilityInfo &BPI,
                  const LoopInfo &LI);
diff --git a/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h b/llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h
@@ -477,6 +477,8 @@ class BlockFrequencyInfoImplBase {
 
   BlockFrequency getBlockFreq(const BlockNode &Node) const;
 
+  void setBlockFreq(const BlockNode &Node, uint64_t Freq);
+
   raw_ostream &printBlockFreq(raw_ostream &OS, const BlockNode &Node) const;
   raw_ostream &printBlockFreq(raw_ostream &OS,
                               const BlockFrequency &Freq) const;
@@ -913,6 +915,7 @@ template <class BT> class BlockFrequencyInfoImpl : BlockFrequencyInfoImplBase {
   BlockFrequency getBlockFreq(const BlockT *BB) const {
     return BlockFrequencyInfoImplBase::getBlockFreq(getNode(BB));
   }
+  void setBlockFreq(const BlockT *BB, uint64_t Freq);
   Scaled64 getFloatingBlockFreq(const BlockT *BB) const {
     return BlockFrequencyInfoImplBase::getFloatingBlockFreq(getNode(BB));
   }
@@ -965,6 +968,21 @@ void BlockFrequencyInfoImpl<BT>::calculate(const FunctionT &F,
   finalizeMetrics();
 }
 
+template <class BT>
+void BlockFrequencyInfoImpl<BT>::setBlockFreq(const BlockT *BB, uint64_t Freq) {
+  if (Nodes.count(BB))
+    BlockFrequencyInfoImplBase::setBlockFreq(getNode(BB), Freq);
+  else {
+    // If BB is a newly added block after BFI is done, we need to create a new
+    // BlockNode for it assigned with a new index. The index can be determined
+    // by the size of Freqs.
+    BlockNode NewNode(Freqs.size());
+    Nodes[BB] = NewNode;
+    Freqs.emplace_back();
+    BlockFrequencyInfoImplBase::setBlockFreq(NewNode, Freq);
+  }
+}
+
 template <class BT> void BlockFrequencyInfoImpl<BT>::initializeRPOT() {
   const BlockT *Entry = &F->front();
   RPOT.reserve(F->size());
diff --git a/llvm/include/llvm/Support/BranchProbability.h b/llvm/include/llvm/Support/BranchProbability.h
@@ -15,7 +15,10 @@
 #define LLVM_SUPPORT_BRANCHPROBABILITY_H
 
 #include "llvm/Support/DataTypes.h"
+#include <algorithm>
 #include <cassert>
+#include <climits>
+#include <numeric>
 
 namespace llvm {
 
@@ -53,6 +56,11 @@ class BranchProbability {
   template <class ProbabilityList>
   static void normalizeProbabilities(ProbabilityList &Probs);
 
+  // Normalize a list of weights by scaling them down so that the sum of them
+  // doesn't exceed UINT32_MAX.
+  template <class WeightListIter>
+  static void normalizeEdgeWeights(WeightListIter Begin, WeightListIter End);
+
   uint32_t getNumerator() const { return N; }
   static uint32_t getDenominator() { return D; }
 
@@ -135,6 +143,49 @@ void BranchProbability::normalizeProbabilities(ProbabilityList &Probs) {
     Prob.N = (Prob.N * uint64_t(D) + Sum / 2) / Sum;
 }
 
+template <class WeightListIter>
+void BranchProbability::normalizeEdgeWeights(WeightListIter Begin,
+                                             WeightListIter End) {
+  // First we compute the sum with 64-bits of precision.
+  uint64_t Sum = std::accumulate(Begin, End, uint64_t(0));
+
+  if (Sum > UINT32_MAX) {
+    // Compute the scale necessary to cause the weights to fit, and re-sum with
+    // that scale applied.
+    assert(Sum / UINT32_MAX < UINT32_MAX &&
+           "The sum of weights exceeds UINT32_MAX^2!");
+    uint32_t Scale = Sum / UINT32_MAX + 1;
+    for (auto I = Begin; I != End; ++I)
+      *I /= Scale;
+    Sum = std::accumulate(Begin, End, uint64_t(0));
+  }
+
+  // Eliminate zero weights.
+  auto ZeroWeightNum = std::count(Begin, End, 0u);
+  if (ZeroWeightNum > 0) {
+    // If all weights are zeros, replace them by 1.
+    if (Sum == 0)
+      std::fill(Begin, End, 1u);
+    else {
+      // We are converting zeros into ones, and here we need to make sure that
+      // after this the sum won't exceed UINT32_MAX.
+      if (Sum + ZeroWeightNum > UINT32_MAX) {
+        for (auto I = Begin; I != End; ++I)
+          *I /= 2;
+        ZeroWeightNum = std::count(Begin, End, 0u);
+        Sum = std::accumulate(Begin, End, uint64_t(0));
+      }
+      // Scale up non-zero weights and turn zero weights into ones.
+      uint64_t ScalingFactor = (UINT32_MAX - ZeroWeightNum) / Sum;
+      assert(ScalingFactor >= 1);
+      if (ScalingFactor > 1)
+        for (auto I = Begin; I != End; ++I)
+          *I *= ScalingFactor;
+      std::replace(Begin, End, 0u, 1u);
+    }
+  }
+}
+
 }
 
 #endif
diff --git a/llvm/lib/Analysis/BlockFrequencyInfo.cpp b/llvm/lib/Analysis/BlockFrequencyInfo.cpp
@@ -129,6 +129,12 @@ BlockFrequency BlockFrequencyInfo::getBlockFreq(const BasicBlock *BB) const {
   return BFI ? BFI->getBlockFreq(BB) : 0;
 }
 
+void BlockFrequencyInfo::setBlockFreq(const BasicBlock *BB,
+                                      uint64_t Freq) {
+  assert(BFI && "Expected analysis to be available");
+  BFI->setBlockFreq(BB, Freq);
+}
+
 /// Pop up a ghostview window with the current block frequency propagation
 /// rendered using dot.
 void BlockFrequencyInfo::view() const {
diff --git a/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp b/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
@@ -530,6 +530,13 @@ BlockFrequencyInfoImplBase::getFloatingBlockFreq(const BlockNode &Node) const {
   return Freqs[Node.Index].Scaled;
 }
 
+void BlockFrequencyInfoImplBase::setBlockFreq(const BlockNode &Node,
+                                              uint64_t Freq) {
+  assert(Node.isValid() && "Expected valid node");
+  assert(Node.Index < Freqs.size() && "Expected legal index");
+  Freqs[Node.Index].Integer = Freq;
+}
+
 std::string
 BlockFrequencyInfoImplBase::getBlockName(const BlockNode &Node) const {
   return std::string();
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -20,14 +20,19 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/BlockFrequencyInfoImpl.h"
+#include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LazyValueInfo.h"
 #include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Pass.h"
@@ -37,6 +42,8 @@
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/SSAUpdater.h"
+#include <algorithm>
+#include <memory>
 using namespace llvm;
 
 #define DEBUG_TYPE "jump-threading"
@@ -81,6 +88,9 @@ namespace {
   class JumpThreading : public FunctionPass {
     TargetLibraryInfo *TLI;
     LazyValueInfo *LVI;
+    std::unique_ptr<BlockFrequencyInfo> BFI;
+    std::unique_ptr<BranchProbabilityInfo> BPI;
+    bool HasProfileData;
 #ifdef NDEBUG
     SmallPtrSet<BasicBlock*, 16> LoopHeaders;
 #else
@@ -119,6 +129,11 @@ namespace {
       AU.addRequired<TargetLibraryInfoWrapperPass>();
     }
 
+    void releaseMemory() override {
+      BFI.reset();
+      BPI.reset();
+    }
+
     void FindLoopHeaders(Function &F);
     bool ProcessBlock(BasicBlock *BB);
     bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
@@ -139,6 +154,12 @@ namespace {
 
     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
     bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
+
+  private:
+    BasicBlock *SplitBlockPreds(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
+                                const char *Suffix);
+    void UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB, BasicBlock *BB,
+                                      BasicBlock *NewBB, BasicBlock *SuccBB);
   };
 }
 
@@ -162,6 +183,16 @@ bool JumpThreading::runOnFunction(Function &F) {
   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
   TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
   LVI = &getAnalysis<LazyValueInfo>();
+  BFI.reset();
+  BPI.reset();
+  // When profile data is available, we need to update edge weights after
+  // successful jump threading, which requires both BPI and BFI being available.
+  HasProfileData = F.getEntryCount().hasValue();
+  if (HasProfileData) {
+    LoopInfo LI{DominatorTree(F)};
+    BPI.reset(new BranchProbabilityInfo(F, LI));
+    BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
+  }
 
   // Remove unreachable blocks from function as they may result in infinite
   // loop. We do threading if we found something profitable. Jump threading a
@@ -977,8 +1008,7 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
     }
 
     // Split them out to their own block.
-    UnavailablePred =
-      SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split");
+    UnavailablePred = SplitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
   }
 
   // If the value isn't available in all predecessors, then there will be
@@ -1403,7 +1433,7 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB,
   else {
     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
           << " common predecessors.\n");
-    PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
+    PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
   }
 
   // And finally, do it!
@@ -1424,6 +1454,13 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB,
                                          BB->getParent(), BB);
   NewBB->moveAfter(PredBB);
 
+  // Set the block frequency of NewBB.
+  if (HasProfileData) {
+    auto NewBBFreq =
+        BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
+    BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
+  }
+
   BasicBlock::iterator BI = BB->begin();
   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
@@ -1447,7 +1484,7 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB,
 
   // We didn't copy the terminator from BB over to NewBB, because there is now
   // an unconditional jump to SuccBB.  Insert the unconditional jump.
-  BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
+  BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
   NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
 
   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
@@ -1508,11 +1545,85 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB,
   // frequently happens because of phi translation.
   SimplifyInstructionsInBlock(NewBB, TLI);
 
+  // Update the edge weight from BB to SuccBB, which should be less than before.
+  UpdateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
+
   // Threaded an edge!
   ++NumThreads;
   return true;
 }
 
+/// Create a new basic block that will be the predecessor of BB and successor of
+/// all blocks in Preds. When profile data is availble, update the frequency of
+/// this new block.
+BasicBlock *JumpThreading::SplitBlockPreds(BasicBlock *BB,
+                                           ArrayRef<BasicBlock *> Preds,
+                                           const char *Suffix) {
+  // Collect the frequencies of all predecessors of BB, which will be used to
+  // update the edge weight on BB->SuccBB.
+  BlockFrequency PredBBFreq(0);
+  if (HasProfileData)
+    for (auto Pred : Preds)
+      PredBBFreq += BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB);
+
+  BasicBlock *PredBB = SplitBlockPredecessors(BB, Preds, Suffix);
+
+  // Set the block frequency of the newly created PredBB, which is the sum of
+  // frequencies of Preds.
+  if (HasProfileData)
+    BFI->setBlockFreq(PredBB, PredBBFreq.getFrequency());
+  return PredBB;
+}
+
+/// Update the block frequency of BB and branch weight and the metadata on the
+/// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
+/// Freq(PredBB->BB) / Freq(BB->SuccBB).
+void JumpThreading::UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
+                                                 BasicBlock *BB,
+                                                 BasicBlock *NewBB,
+                                                 BasicBlock *SuccBB) {
+  if (!HasProfileData)
+    return;
+
+  assert(BFI && BPI && "BFI & BPI should have been created here");
+
+  // As the edge from PredBB to BB is deleted, we have to update the block
+  // frequency of BB.
+  auto BBOrigFreq = BFI->getBlockFreq(BB);
+  auto NewBBFreq = BFI->getBlockFreq(NewBB);
+  auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
+  auto BBNewFreq = BBOrigFreq - NewBBFreq;
+  BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
+
+  // Collect updated outgoing edges' frequencies from BB and use them to update
+  // edge weights.
+  SmallVector<uint64_t, 4> BBSuccFreq;
+  for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
+    auto SuccFreq = (*I == SuccBB)
+                        ? BB2SuccBBFreq - NewBBFreq
+                        : BBOrigFreq * BPI->getEdgeProbability(BB, *I);
+    BBSuccFreq.push_back(SuccFreq.getFrequency());
+  }
+
+  // Normalize edge weights in Weights64 so that the sum of them can fit in
+  BranchProbability::normalizeEdgeWeights(BBSuccFreq.begin(), BBSuccFreq.end());
+
+  SmallVector<uint32_t, 4> Weights;
+  for (auto Freq : BBSuccFreq)
+    Weights.push_back(static_cast<uint32_t>(Freq));
+
+  // Update edge weights in BPI.
+  for (int I = 0, E = Weights.size(); I < E; I++)
+    BPI->setEdgeWeight(BB, I, Weights[I]);
+
+  if (Weights.size() >= 2) {
+    auto TI = BB->getTerminator();
+    TI->setMetadata(
+        LLVMContext::MD_prof,
+        MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights));
+  }
+}
+
 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
 /// If we can duplicate the contents of BB up into PredBB do so now, this
@@ -1546,7 +1657,7 @@ bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
   else {
     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
           << " common predecessors.\n");
-    PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
+    PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
   }
 
   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
diff --git a/llvm/test/Transforms/JumpThreading/update-edge-weight.ll b/llvm/test/Transforms/JumpThreading/update-edge-weight.ll
diff --git a/llvm/unittests/Support/BranchProbabilityTest.cpp b/llvm/unittests/Support/BranchProbabilityTest.cpp
