[LoopUnroll] Fix block frequencies for epilogue

llvm/include/llvm/Support/BranchProbability.h

@@ -97,6 +97,9 @@ class BranchProbability {
   /// \return \c Num divided by \c this.
   LLVM_ABI uint64_t scaleByInverse(uint64_t Num) const;
 
+  /// Compute pow(Probability, N).
+  BranchProbability pow(unsigned N) const;
+
   BranchProbability &operator+=(BranchProbability RHS) {
     assert(N != UnknownN && RHS.N != UnknownN &&
            "Unknown probability cannot participate in arithmetics.");

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -365,6 +365,40 @@ LLVM_ABI bool setLoopEstimatedTripCount(
     Loop *L, unsigned EstimatedTripCount,
     std::optional<unsigned> EstimatedLoopInvocationWeight = std::nullopt);
 
+/// Based on branch weight metadata, return either:
+/// - An unknown probability if the implementation is unable to handle the loop
+///   form of \p L (e.g., \p L must have a latch block that controls the loop
+///   exit).
+/// - The probability \c P that, at the end of any iteration, the latch of \p L
+///   will start another iteration such that `1 - P` is the probability of
+///   exiting the loop.
+BranchProbability getLoopProbability(Loop *L);
+
+/// Set branch weight metadata for the latch of \p L to indicate that, at the
+/// end of any iteration, \p P and `1 - P` are the probabilities of starting
+/// another iteration and exiting the loop, respectively.  Return false if the
+/// implementation is unable to handle the loop form of \p L (e.g., \p L must
+/// have a latch block that controls the loop exit).  Otherwise, return true.
+bool setLoopProbability(Loop *L, BranchProbability P);
+
+/// Based on branch weight metadata, return either:
+/// - An unknown probability if the implementation cannot extract the
+///   probability (e.g., \p B must have exactly two target labels, so it must be
+///   a conditional branch).
+/// - The probability \c P that control flows from \p B to its first target
+///   label such that `1 - P` is the probability of control flowing to its
+///   second target label, or vice-versa if \p ForFirstTarget is false.
+BranchProbability getBranchProbability(BranchInst *B, bool ForFirstTarget);
+
+/// Set branch weight metadata for \p B to indicate that \p P and `1 - P` are
+/// the probabilities of control flowing to its first and second target labels,
+/// respectively, or vice-versa if \p ForFirstTarget is false.  Return false if
+/// the implementation cannot set the probability (e.g., \p B must have exactly
+/// two target labels, so it must be a conditional branch).  Otherwise, return
+/// true.
+bool setBranchProbability(BranchInst *B, BranchProbability P,
+                          bool ForFirstTarget);
+
 /// Check inner loop (L) backedge count is known to be invariant on all
 /// iterations of its outer loop. If the loop has no parent, this is trivially
 /// true.

llvm/include/llvm/Transforms/Utils/UnrollLoop.h

@@ -97,7 +97,9 @@ LLVM_ABI bool UnrollRuntimeLoopRemainder(
     LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
     const TargetTransformInfo *TTI, bool PreserveLCSSA,
     unsigned SCEVExpansionBudget, bool RuntimeUnrollMultiExit,
-    Loop **ResultLoop = nullptr);
+    Loop **ResultLoop = nullptr,
+    std::optional<unsigned> OriginalTripCount = std::nullopt,
+    BranchProbability OriginalLoopProb = BranchProbability::getUnknown());
 
 LLVM_ABI LoopUnrollResult UnrollAndJamLoop(
     Loop *L, unsigned Count, unsigned TripCount, unsigned TripMultiple,

llvm/lib/Support/BranchProbability.cpp

@@ -111,3 +111,10 @@ uint64_t BranchProbability::scale(uint64_t Num) const {
 uint64_t BranchProbability::scaleByInverse(uint64_t Num) const {
   return ::scale<0>(Num, D, N);
 }
+
+BranchProbability BranchProbability::pow(unsigned N) const {
+  BranchProbability Res = BranchProbability::getOne();
+  for (unsigned I = 0; I < N; ++I)
+    Res *= *this;
+  return Res;
+}

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -499,9 +499,9 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
 
   const unsigned MaxTripCount = SE->getSmallConstantMaxTripCount(L);
   const bool MaxOrZero = SE->isBackedgeTakenCountMaxOrZero(L);
-  unsigned EstimatedLoopInvocationWeight = 0;
   std::optional<unsigned> OriginalTripCount =
-      llvm::getLoopEstimatedTripCount(L, &EstimatedLoopInvocationWeight);
+      llvm::getLoopEstimatedTripCount(L);
+  BranchProbability OriginalLoopProb = llvm::getLoopProbability(L);
 
   // Effectively "DCE" unrolled iterations that are beyond the max tripcount
   // and will never be executed.
@@ -592,11 +592,11 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
                                               : isEpilogProfitable(L);
 
   if (ULO.Runtime &&
-      !UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount,
-                                  EpilogProfitability, ULO.UnrollRemainder,
-                                  ULO.ForgetAllSCEV, LI, SE, DT, AC, TTI,
-                                  PreserveLCSSA, ULO.SCEVExpansionBudget,
-                                  ULO.RuntimeUnrollMultiExit, RemainderLoop)) {
+      !UnrollRuntimeLoopRemainder(
+          L, ULO.Count, ULO.AllowExpensiveTripCount, EpilogProfitability,
+          ULO.UnrollRemainder, ULO.ForgetAllSCEV, LI, SE, DT, AC, TTI,
+          PreserveLCSSA, ULO.SCEVExpansionBudget, ULO.RuntimeUnrollMultiExit,
+          RemainderLoop, OriginalTripCount, OriginalLoopProb)) {
     if (ULO.Force)
       ULO.Runtime = false;
     else {
@@ -1131,11 +1131,46 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     LI->erase(L);
     // We shouldn't try to use `L` anymore.
     L = nullptr;
-  } else if (OriginalTripCount) {
-    // Update the trip count. Note that the remainder has already logic
-    // computing it in `UnrollRuntimeLoopRemainder`.
-    setLoopEstimatedTripCount(L, *OriginalTripCount / ULO.Count,
-                              EstimatedLoopInvocationWeight);
+  } else {
+    // Update metadata for the loop's branch weights and estimated trip count:
+    // - If ULO.Runtime, UnrollRuntimeLoopRemainder sets the guard branch
+    //   weights, latch branch weights, and estimated trip count of the
+    //   remainder loop it creates.  It also sets the branch weights for the
+    //   unrolled loop guard it creates.  The branch weights for the unrolled
+    //   loop latch are adjusted below.  FIXME: Handle prologue loops.
+    // - Otherwise, if unrolled loop iteration latches become unconditional,
+    //   branch weights are adjusted above.  FIXME: Actually handle such
+    //   unconditional latches.
+    // - Otherwise, the original loop's branch weights are correct for the
+    //   unrolled loop, so do not adjust them.
+    // - In all cases, the unrolled loop's estimated trip count is set below.
+    //
+    // As an example of the last case, consider what happens if the unroll count
+    // is 4 for a loop with an estimated trip count of 10 when we do not create
+    // a remainder loop and all iterations' latches remain conditional.  Each
+    // unrolled iteration's latch still has the same probability of exiting the
+    // loop as it did when in the original loop, and thus it should still have
+    // the same branch weights.  Each unrolled iteration's non-zero probability
+    // of exiting already appropriately reduces the probability of reaching the
+    // remaining iterations just as it did in the original loop.  Trying to also
+    // adjust the branch weights of the final unrolled iteration's latch (i.e.,
+    // the backedge for the unrolled loop as a whole) to reflect its new trip
+    // count of 3 will erroneously further reduce its block frequencies.
+    // However, in case an analysis later needs to estimate the trip count of
+    // the unrolled loop as a whole without considering the branch weights for
+    // each unrolled iteration's latch within it, we store the new trip count as
+    // separate metadata.
+    if (!OriginalLoopProb.isUnknown() && ULO.Runtime && EpilogProfitability) {
+      // Where p is always the probability of executing at least 1 more
+      // iteration, the probability for at least n more iterations is p^n.
+      setLoopProbability(L, OriginalLoopProb.pow(ULO.Count));
+    }
+    if (OriginalTripCount) {
+      unsigned NewTripCount = *OriginalTripCount / ULO.Count;
+      if (!ULO.Runtime && *OriginalTripCount % ULO.Count)
+        NewTripCount += 1;
+      setLoopEstimatedTripCount(L, NewTripCount);
+    }
   }
 
   // LoopInfo should not be valid, confirm that.

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -40,6 +40,7 @@
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
 #include "llvm/Transforms/Utils/UnrollLoop.h"
+#include <cmath>
 
 using namespace llvm;
 
@@ -195,6 +196,21 @@ static void ConnectProlog(Loop *L, Value *BECount, unsigned Count,
   }
 }
 
+/// Assume, due to our position in the remainder loop or its guard, anywhere
+/// from 0 to \p N more iterations can possibly execute.  Among such cases in
+/// the original loop (with loop probability \p OriginalLoopProb), what is the
+/// probability of executing at least one more iteration?
+static BranchProbability
+probOfNextInRemainder(BranchProbability OriginalLoopProb, unsigned N) {
+  // Each of these variables holds the original loop's probability that the
+  // number of iterations it will execute is some m in the specified range.
+  BranchProbability ProbOne = OriginalLoopProb;                // 1 <= m
+  BranchProbability ProbTooMany = ProbOne.pow(N + 1);          // N + 1 <= m
+  BranchProbability ProbNotTooMany = ProbTooMany.getCompl();   // 0 <= m <= N
+  BranchProbability ProbOneNotTooMany = ProbOne - ProbTooMany; // 1 <= m <= N
+  return ProbOneNotTooMany / ProbNotTooMany;
+}
+
 /// Connect the unrolling epilog code to the original loop.
 /// The unrolling epilog code contains code to execute the
 /// 'extra' iterations if the run-time trip count modulo the
@@ -221,7 +237,8 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
                           BasicBlock *EpilogPreHeader, BasicBlock *NewPreHeader,
                           ValueToValueMapTy &VMap, DominatorTree *DT,
                           LoopInfo *LI, bool PreserveLCSSA, ScalarEvolution &SE,
-                          unsigned Count, AssumptionCache &AC) {
+                          unsigned Count, AssumptionCache &AC,
+                          BranchProbability OriginalLoopProb) {
   BasicBlock *Latch = L->getLoopLatch();
   assert(Latch && "Loop must have a latch");
   BasicBlock *EpilogLatch = cast<BasicBlock>(VMap[Latch]);
@@ -332,12 +349,19 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
                          PreserveLCSSA);
   // Add the branch to the exit block (around the epilog loop)
   MDNode *BranchWeights = nullptr;
-  if (hasBranchWeightMD(*Latch->getTerminator())) {
+  if (OriginalLoopProb.isUnknown() &&
+      hasBranchWeightMD(*Latch->getTerminator())) {
     // Assume equal distribution in interval [0, Count).
     MDBuilder MDB(B.getContext());
     BranchWeights = MDB.createBranchWeights(1, Count - 1);
   }
-  B.CreateCondBr(BrLoopExit, EpilogPreHeader, Exit, BranchWeights);
+  BranchInst *RemainderLoopGuard =
+      B.CreateCondBr(BrLoopExit, EpilogPreHeader, Exit, BranchWeights);
+  if (!OriginalLoopProb.isUnknown()) {
+    setBranchProbability(RemainderLoopGuard,
+                         probOfNextInRemainder(OriginalLoopProb, Count - 1),
+                         /*ForFirstTarget=*/true);
+  }
   InsertPt->eraseFromParent();
   if (DT) {
     auto *NewDom = DT->findNearestCommonDominator(Exit, NewExit);
@@ -357,14 +381,15 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
 /// The cloned blocks should be inserted between InsertTop and InsertBot.
 /// InsertTop should be new preheader, InsertBot new loop exit.
 /// Returns the new cloned loop that is created.
-static Loop *
-CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
-                const bool UnrollRemainder,
-                BasicBlock *InsertTop,
-                BasicBlock *InsertBot, BasicBlock *Preheader,
+static Loop *CloneLoopBlocks(Loop *L, Value *NewIter,
+                             const bool UseEpilogRemainder,
+                             const bool UnrollRemainder, BasicBlock *InsertTop,
+                             BasicBlock *InsertBot, BasicBlock *Preheader,
                              std::vector<BasicBlock *> &NewBlocks,
                              LoopBlocksDFS &LoopBlocks, ValueToValueMapTy &VMap,
-                             DominatorTree *DT, LoopInfo *LI, unsigned Count) {
+                             DominatorTree *DT, LoopInfo *LI, unsigned Count,
+                             std::optional<unsigned> OriginalTripCount,
+                             BranchProbability OriginalLoopProb) {
   StringRef suffix = UseEpilogRemainder ? "epil" : "prol";
   BasicBlock *Header = L->getHeader();
   BasicBlock *Latch = L->getLoopLatch();
@@ -419,7 +444,8 @@ CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
           Builder.CreateAdd(NewIdx, One, NewIdx->getName() + ".next");
       Value *IdxCmp = Builder.CreateICmpNE(IdxNext, NewIter, NewIdx->getName() + ".cmp");
       MDNode *BranchWeights = nullptr;
-      if (hasBranchWeightMD(*LatchBR)) {
+      if ((OriginalLoopProb.isUnknown() || !UseEpilogRemainder) &&
+          hasBranchWeightMD(*LatchBR)) {
         uint32_t ExitWeight;
         uint32_t BackEdgeWeight;
         if (Count >= 3) {
@@ -437,7 +463,29 @@ CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
         MDBuilder MDB(Builder.getContext());
         BranchWeights = MDB.createBranchWeights(BackEdgeWeight, ExitWeight);
       }
-      Builder.CreateCondBr(IdxCmp, FirstLoopBB, InsertBot, BranchWeights);
+      BranchInst *RemainderLoopLatch =
+          Builder.CreateCondBr(IdxCmp, FirstLoopBB, InsertBot, BranchWeights);
+      if (!OriginalLoopProb.isUnknown() && UseEpilogRemainder) {
+        // Compute the total frequency of the original loop body from the
+        // remainder iterations.  Once we've reached them, the first of them
+        // always executes, so its frequency and probability are 1.
+        double FreqRemIters = 1;
+        if (Count > 2) {
+          BranchProbability ProbReaching = BranchProbability::getOne();
+          for (unsigned N = Count - 2; N >= 1; --N) {
+            ProbReaching *= probOfNextInRemainder(OriginalLoopProb, N);
+            FreqRemIters += double(ProbReaching.getNumerator()) /
+                            ProbReaching.getDenominator();
+          }
+        }
+        // Solve for the loop probability that would produce that frequency.
+        // Sum(i=0..inf)(Prob^i) = 1/(1-Prob) = FreqRemIters.
+        double ProbDouble = 1 - 1 / FreqRemIters;
+        BranchProbability Prob = BranchProbability::getBranchProbability(
+            std::round(ProbDouble * BranchProbability::getDenominator()),
+            BranchProbability::getDenominator());
+        setBranchProbability(RemainderLoopLatch, Prob, /*ForFirstTarget=*/true);
+      }
       NewIdx->addIncoming(Zero, InsertTop);
       NewIdx->addIncoming(IdxNext, NewBB);
       LatchBR->eraseFromParent();
@@ -469,6 +517,8 @@ CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
 
   std::optional<MDNode *> NewLoopID = makeFollowupLoopID(
       LoopID, {LLVMLoopUnrollFollowupAll, LLVMLoopUnrollFollowupRemainder});
+  if (OriginalTripCount && UseEpilogRemainder)
+    setLoopEstimatedTripCount(NewLoop, *OriginalTripCount % Count);
   if (NewLoopID) {
     NewLoop->setLoopID(*NewLoopID);
 
@@ -603,7 +653,8 @@ bool llvm::UnrollRuntimeLoopRemainder(
     LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
     const TargetTransformInfo *TTI, bool PreserveLCSSA,
     unsigned SCEVExpansionBudget, bool RuntimeUnrollMultiExit,
-    Loop **ResultLoop) {
+    Loop **ResultLoop, std::optional<unsigned> OriginalTripCount,
+    BranchProbability OriginalLoopProb) {
   LLVM_DEBUG(dbgs() << "Trying runtime unrolling on Loop: \n");
   LLVM_DEBUG(L->dump());
   LLVM_DEBUG(UseEpilogRemainder ? dbgs() << "Using epilog remainder.\n"
@@ -823,12 +874,23 @@ bool llvm::UnrollRuntimeLoopRemainder(
   BasicBlock *UnrollingLoop = UseEpilogRemainder ? NewPreHeader : PrologExit;
   // Branch to either remainder (extra iterations) loop or unrolling loop.
   MDNode *BranchWeights = nullptr;
-  if (hasBranchWeightMD(*Latch->getTerminator())) {
+  if ((OriginalLoopProb.isUnknown() || !UseEpilogRemainder) &&
+      hasBranchWeightMD(*Latch->getTerminator())) {
     // Assume loop is nearly always entered.
     MDBuilder MDB(B.getContext());
     BranchWeights = MDB.createBranchWeights(EpilogHeaderWeights);
   }
-  B.CreateCondBr(BranchVal, RemainderLoop, UnrollingLoop, BranchWeights);
+  BranchInst *UnrollingLoopGuard =
+      B.CreateCondBr(BranchVal, RemainderLoop, UnrollingLoop, BranchWeights);
+  if (!OriginalLoopProb.isUnknown() && UseEpilogRemainder) {
+    // The original loop's first iteration always happens.  Compute the
+    // probability of the original loop executing Count-1 iterations after that
+    // to complete the first iteration of the unrolled loop.
+    BranchProbability ProbOne = OriginalLoopProb;
+    BranchProbability ProbRest = ProbOne.pow(Count - 1);
+    setBranchProbability(UnrollingLoopGuard, ProbRest,
+                         /*ForFirstTarget=*/false);
+  }
   PreHeaderBR->eraseFromParent();
   if (DT) {
     if (UseEpilogRemainder)
@@ -855,9 +917,10 @@ bool llvm::UnrollRuntimeLoopRemainder(
   // iterations. This function adds the appropriate CFG connections.
   BasicBlock *InsertBot = UseEpilogRemainder ? LatchExit : PrologExit;
   BasicBlock *InsertTop = UseEpilogRemainder ? EpilogPreHeader : PrologPreHeader;
-  Loop *remainderLoop = CloneLoopBlocks(
-      L, ModVal, UseEpilogRemainder, UnrollRemainder, InsertTop, InsertBot,
-      NewPreHeader, NewBlocks, LoopBlocks, VMap, DT, LI, Count);
+  Loop *remainderLoop =
+      CloneLoopBlocks(L, ModVal, UseEpilogRemainder, UnrollRemainder, InsertTop,
+                      InsertBot, NewPreHeader, NewBlocks, LoopBlocks, VMap, DT,
+                      LI, Count, OriginalTripCount, OriginalLoopProb);
 
   // Insert the cloned blocks into the function.
   F->splice(InsertBot->getIterator(), F, NewBlocks[0]->getIterator(), F->end());
@@ -956,7 +1019,8 @@ bool llvm::UnrollRuntimeLoopRemainder(
     // Connect the epilog code to the original loop and update the
     // PHI functions.
     ConnectEpilog(L, ModVal, NewExit, LatchExit, PreHeader, EpilogPreHeader,
-                  NewPreHeader, VMap, DT, LI, PreserveLCSSA, *SE, Count, *AC);
+                  NewPreHeader, VMap, DT, LI, PreserveLCSSA, *SE, Count, *AC,
+                  OriginalLoopProb);
 
     // Update counter in loop for unrolling.
     // Use an incrementing IV.  Pre-incr/post-incr is backedge/trip count.