llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp - toolchain/llvm-project - Gitiles

 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by
 // inserting a dummy basic block.  This pass may be "required" by passes that
 // cannot deal with critical edges.  For this usage, the structure type is
 // forward declared.  This pass obviously invalidates the CFG, but can update
 // dominator trees.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/Utils/BreakCriticalEdges.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/BlockFrequencyInfo.h"
 #include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemorySSAUpdater.h"
 #include "llvm/Analysis/PostDominators.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Type.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Transforms/Utils.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/ValueMapper.h"
 using namespace llvm;

 #define DEBUG_TYPE "break-crit-edges"

 STATISTIC(NumBroken, "Number of blocks inserted");

 namespace {
   struct BreakCriticalEdges : public FunctionPass {
     static char ID; // Pass identification, replacement for typeid
     BreakCriticalEdges() : FunctionPass(ID) {
       initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
     }

     bool runOnFunction(Function &F) override {
       auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
       auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;

       auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
       auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;

       auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
       auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
       unsigned N =
           SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI, nullptr, PDT));
       NumBroken += N;
       return N > 0;
     }

     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.addPreserved<DominatorTreeWrapperPass>();
       AU.addPreserved<LoopInfoWrapperPass>();

       // No loop canonicalization guarantees are broken by this pass.
       AU.addPreservedID(LoopSimplifyID);
     }
   };
 }

 char BreakCriticalEdges::ID = 0;
 INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
                 "Break critical edges in CFG", false, false)

 // Publicly exposed interface to pass...
 char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
 FunctionPass *llvm::createBreakCriticalEdgesPass() {
   return new BreakCriticalEdges();
 }

 PreservedAnalyses BreakCriticalEdgesPass::run(Function &F,
                                               FunctionAnalysisManager &AM) {
   auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
   auto *LI = AM.getCachedResult<LoopAnalysis>(F);
   unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
   NumBroken += N;
   if (N == 0)
     return PreservedAnalyses::all();
   PreservedAnalyses PA;
   PA.preserve<DominatorTreeAnalysis>();
   PA.preserve<LoopAnalysis>();
   return PA;
 }

 //===----------------------------------------------------------------------===//
 //    Implementation of the external critical edge manipulation functions
 //===----------------------------------------------------------------------===//

 /// When a loop exit edge is split, LCSSA form may require new PHIs in the new
 /// exit block. This function inserts the new PHIs, as needed. Preds is a list
 /// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is
 /// the old loop exit, now the successor of SplitBB.
 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
                                        BasicBlock *SplitBB,
                                        BasicBlock *DestBB) {
   // SplitBB shouldn't have anything non-trivial in it yet.
   assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
           SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");

   // For each PHI in the destination block.
   for (PHINode &PN : DestBB->phis()) {
     unsigned Idx = PN.getBasicBlockIndex(SplitBB);
     Value *V = PN.getIncomingValue(Idx);

     // If the input is a PHI which already satisfies LCSSA, don't create
     // a new one.
     if (const PHINode *VP = dyn_cast<PHINode>(V))
       if (VP->getParent() == SplitBB)
         continue;

     // Otherwise a new PHI is needed. Create one and populate it.
     PHINode *NewPN = PHINode::Create(
         PN.getType(), Preds.size(), "split",
         SplitBB->isLandingPad() ? &SplitBB->front() : SplitBB->getTerminator());
     for (unsigned i = 0, e = Preds.size(); i != e; ++i)
       NewPN->addIncoming(V, Preds[i]);

     // Update the original PHI.
     PN.setIncomingValue(Idx, NewPN);
   }
 }

 BasicBlock *
 llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
                         const CriticalEdgeSplittingOptions &Options) {
   if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
     return nullptr;

   assert(!isa<IndirectBrInst>(TI) &&
          "Cannot split critical edge from IndirectBrInst");

   BasicBlock *TIBB = TI->getParent();
   BasicBlock *DestBB = TI->getSuccessor(SuccNum);

   // Splitting the critical edge to a pad block is non-trivial. Don't do
   // it in this generic function.
   if (DestBB->isEHPad()) return nullptr;

   // Don't split the non-fallthrough edge from a callbr.
   if (isa<CallBrInst>(TI) && SuccNum > 0)
     return nullptr;

   if (Options.IgnoreUnreachableDests &&
       isa<UnreachableInst>(DestBB->getFirstNonPHIOrDbgOrLifetime()))
     return nullptr;

   // Create a new basic block, linking it into the CFG.
   BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
                       TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
   // Create our unconditional branch.
   BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
   NewBI->setDebugLoc(TI->getDebugLoc());

   // Branch to the new block, breaking the edge.
   TI->setSuccessor(SuccNum, NewBB);

   // Insert the block into the function... right after the block TI lives in.
   Function &F = *TIBB->getParent();
   Function::iterator FBBI = TIBB->getIterator();
   F.getBasicBlockList().insert(++FBBI, NewBB);

   // If there are any PHI nodes in DestBB, we need to update them so that they
   // merge incoming values from NewBB instead of from TIBB.
   {
     unsigned BBIdx = 0;
     for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
       // We no longer enter through TIBB, now we come in through NewBB.
       // Revector exactly one entry in the PHI node that used to come from
       // TIBB to come from NewBB.
       PHINode *PN = cast<PHINode>(I);

       // Reuse the previous value of BBIdx if it lines up.  In cases where we
       // have multiple phi nodes with *lots* of predecessors, this is a speed
       // win because we don't have to scan the PHI looking for TIBB.  This
       // happens because the BB list of PHI nodes are usually in the same
       // order.
       if (PN->getIncomingBlock(BBIdx) != TIBB)
         BBIdx = PN->getBasicBlockIndex(TIBB);
       PN->setIncomingBlock(BBIdx, NewBB);
     }
   }

   // If there are any other edges from TIBB to DestBB, update those to go
   // through the split block, making those edges non-critical as well (and
   // reducing the number of phi entries in the DestBB if relevant).
   if (Options.MergeIdenticalEdges) {
     for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
       if (TI->getSuccessor(i) != DestBB) continue;

       // Remove an entry for TIBB from DestBB phi nodes.
       DestBB->removePredecessor(TIBB, Options.KeepOneInputPHIs);

       // We found another edge to DestBB, go to NewBB instead.
       TI->setSuccessor(i, NewBB);
     }
   }

   // If we have nothing to update, just return.
   auto *DT = Options.DT;
   auto *PDT = Options.PDT;
   auto *LI = Options.LI;
   auto *MSSAU = Options.MSSAU;
   if (MSSAU)
     MSSAU->wireOldPredecessorsToNewImmediatePredecessor(
         DestBB, NewBB, {TIBB}, Options.MergeIdenticalEdges);

   if (!DT && !PDT && !LI)
     return NewBB;

   if (DT || PDT) {
     // Update the DominatorTree.
     //       ---> NewBB -----\
     //      /                 V
     //  TIBB -------\\------> DestBB
     //
     // First, inform the DT about the new path from TIBB to DestBB via NewBB,
     // then delete the old edge from TIBB to DestBB. By doing this in that order
     // DestBB stays reachable in the DT the whole time and its subtree doesn't
     // get disconnected.
     SmallVector<DominatorTree::UpdateType, 3> Updates;
     Updates.push_back({DominatorTree::Insert, TIBB, NewBB});
     Updates.push_back({DominatorTree::Insert, NewBB, DestBB});
     if (llvm::find(successors(TIBB), DestBB) == succ_end(TIBB))
       Updates.push_back({DominatorTree::Delete, TIBB, DestBB});

     if (DT)
       DT->applyUpdates(Updates);
     if (PDT)
       PDT->applyUpdates(Updates);
   }

   // Update LoopInfo if it is around.
   if (LI) {
     if (Loop *TIL = LI->getLoopFor(TIBB)) {
       // If one or the other blocks were not in a loop, the new block is not
       // either, and thus LI doesn't need to be updated.
       if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
         if (TIL == DestLoop) {
           // Both in the same loop, the NewBB joins loop.
           DestLoop->addBasicBlockToLoop(NewBB, *LI);
         } else if (TIL->contains(DestLoop)) {
           // Edge from an outer loop to an inner loop.  Add to the outer loop.
           TIL->addBasicBlockToLoop(NewBB, *LI);
         } else if (DestLoop->contains(TIL)) {
           // Edge from an inner loop to an outer loop.  Add to the outer loop.
           DestLoop->addBasicBlockToLoop(NewBB, *LI);
         } else {
           // Edge from two loops with no containment relation.  Because these
           // are natural loops, we know that the destination block must be the
           // header of its loop (adding a branch into a loop elsewhere would
           // create an irreducible loop).
           assert(DestLoop->getHeader() == DestBB &&
                  "Should not create irreducible loops!");
           if (Loop *P = DestLoop->getParentLoop())
             P->addBasicBlockToLoop(NewBB, *LI);
         }
       }

       // If TIBB is in a loop and DestBB is outside of that loop, we may need
       // to update LoopSimplify form and LCSSA form.
       if (!TIL->contains(DestBB)) {
         assert(!TIL->contains(NewBB) &&
                "Split point for loop exit is contained in loop!");

         // Update LCSSA form in the newly created exit block.
         if (Options.PreserveLCSSA) {
           createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
         }

         // The only that we can break LoopSimplify form by splitting a critical
         // edge is if after the split there exists some edge from TIL to DestBB
         // *and* the only edge into DestBB from outside of TIL is that of
         // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
         // is the new exit block and it has no non-loop predecessors. If the
         // second isn't true, then DestBB was not in LoopSimplify form prior to
         // the split as it had a non-loop predecessor. In both of these cases,
         // the predecessor must be directly in TIL, not in a subloop, or again
         // LoopSimplify doesn't hold.
         SmallVector<BasicBlock *, 4> LoopPreds;
         for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
              ++I) {
           BasicBlock *P = *I;
           if (P == NewBB)
             continue; // The new block is known.
           if (LI->getLoopFor(P) != TIL) {
             // No need to re-simplify, it wasn't to start with.
             LoopPreds.clear();
             break;
           }
           LoopPreds.push_back(P);
         }
         if (!LoopPreds.empty()) {
           assert(!DestBB->isEHPad() && "We don't split edges to EH pads!");
           BasicBlock *NewExitBB = SplitBlockPredecessors(
               DestBB, LoopPreds, "split", DT, LI, MSSAU, Options.PreserveLCSSA);
           if (Options.PreserveLCSSA)
             createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
         }
       }
     }
   }

   return NewBB;
 }

 // Return the unique indirectbr predecessor of a block. This may return null
 // even if such a predecessor exists, if it's not useful for splitting.
 // If a predecessor is found, OtherPreds will contain all other (non-indirectbr)
 // predecessors of BB.
 static BasicBlock *
 findIBRPredecessor(BasicBlock *BB, SmallVectorImpl<BasicBlock *> &OtherPreds) {
   // If the block doesn't have any PHIs, we don't care about it, since there's
   // no point in splitting it.
   PHINode *PN = dyn_cast<PHINode>(BB->begin());
   if (!PN)
     return nullptr;

   // Verify we have exactly one IBR predecessor.
   // Conservatively bail out if one of the other predecessors is not a "regular"
   // terminator (that is, not a switch or a br).
   BasicBlock *IBB = nullptr;
   for (unsigned Pred = 0, E = PN->getNumIncomingValues(); Pred != E; ++Pred) {
     BasicBlock *PredBB = PN->getIncomingBlock(Pred);
     Instruction *PredTerm = PredBB->getTerminator();
     switch (PredTerm->getOpcode()) {
     case Instruction::IndirectBr:
       if (IBB)
         return nullptr;
       IBB = PredBB;
       break;
     case Instruction::Br:
     case Instruction::Switch:
       OtherPreds.push_back(PredBB);
       continue;
     default:
       return nullptr;
     }
   }

   return IBB;
 }

 bool llvm::SplitIndirectBrCriticalEdges(Function &F,
                                         BranchProbabilityInfo *BPI,
                                         BlockFrequencyInfo *BFI) {
   // Check whether the function has any indirectbrs, and collect which blocks
   // they may jump to. Since most functions don't have indirect branches,
   // this lowers the common case's overhead to O(Blocks) instead of O(Edges).
   SmallSetVector<BasicBlock *, 16> Targets;
   for (auto &BB : F) {
     auto *IBI = dyn_cast<IndirectBrInst>(BB.getTerminator());
     if (!IBI)
       continue;

     for (unsigned Succ = 0, E = IBI->getNumSuccessors(); Succ != E; ++Succ)
       Targets.insert(IBI->getSuccessor(Succ));
   }

   if (Targets.empty())
     return false;

   bool ShouldUpdateAnalysis = BPI && BFI;
   bool Changed = false;
   for (BasicBlock *Target : Targets) {
     SmallVector<BasicBlock *, 16> OtherPreds;
     BasicBlock *IBRPred = findIBRPredecessor(Target, OtherPreds);
     // If we did not found an indirectbr, or the indirectbr is the only
     // incoming edge, this isn't the kind of edge we're looking for.
     if (!IBRPred || OtherPreds.empty())
       continue;

     // Don't even think about ehpads/landingpads.
     Instruction *FirstNonPHI = Target->getFirstNonPHI();
     if (FirstNonPHI->isEHPad() || Target->isLandingPad())
       continue;

     BasicBlock *BodyBlock = Target->splitBasicBlock(FirstNonPHI, ".split");
     if (ShouldUpdateAnalysis) {
       // Copy the BFI/BPI from Target to BodyBlock.
       for (unsigned I = 0, E = BodyBlock->getTerminator()->getNumSuccessors();
            I < E; ++I)
         BPI->setEdgeProbability(BodyBlock, I,
                                 BPI->getEdgeProbability(Target, I));
       BFI->setBlockFreq(BodyBlock, BFI->getBlockFreq(Target).getFrequency());
     }
     // It's possible Target was its own successor through an indirectbr.
     // In this case, the indirectbr now comes from BodyBlock.
     if (IBRPred == Target)
       IBRPred = BodyBlock;

     // At this point Target only has PHIs, and BodyBlock has the rest of the
     // block's body. Create a copy of Target that will be used by the "direct"
     // preds.
     ValueToValueMapTy VMap;
     BasicBlock *DirectSucc = CloneBasicBlock(Target, VMap, ".clone", &F);

     BlockFrequency BlockFreqForDirectSucc;
     for (BasicBlock *Pred : OtherPreds) {
       // If the target is a loop to itself, then the terminator of the split
       // block (BodyBlock) needs to be updated.
       BasicBlock *Src = Pred != Target ? Pred : BodyBlock;
       Src->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
       if (ShouldUpdateAnalysis)
         BlockFreqForDirectSucc += BFI->getBlockFreq(Src) *
             BPI->getEdgeProbability(Src, DirectSucc);
     }
     if (ShouldUpdateAnalysis) {
       BFI->setBlockFreq(DirectSucc, BlockFreqForDirectSucc.getFrequency());
       BlockFrequency NewBlockFreqForTarget =
           BFI->getBlockFreq(Target) - BlockFreqForDirectSucc;
       BFI->setBlockFreq(Target, NewBlockFreqForTarget.getFrequency());
       BPI->eraseBlock(Target);
     }

     // Ok, now fix up the PHIs. We know the two blocks only have PHIs, and that
     // they are clones, so the number of PHIs are the same.
     // (a) Remove the edge coming from IBRPred from the "Direct" PHI
     // (b) Leave that as the only edge in the "Indirect" PHI.
     // (c) Merge the two in the body block.
     BasicBlock::iterator Indirect = Target->begin(),
                          End = Target->getFirstNonPHI()->getIterator();
     BasicBlock::iterator Direct = DirectSucc->begin();
     BasicBlock::iterator MergeInsert = BodyBlock->getFirstInsertionPt();

     assert(&*End == Target->getTerminator() &&
            "Block was expected to only contain PHIs");

     while (Indirect != End) {
       PHINode *DirPHI = cast<PHINode>(Direct);
       PHINode *IndPHI = cast<PHINode>(Indirect);

       // Now, clean up - the direct block shouldn't get the indirect value,
       // and vice versa.
       DirPHI->removeIncomingValue(IBRPred);
       Direct++;

       // Advance the pointer here, to avoid invalidation issues when the old
       // PHI is erased.
       Indirect++;

       PHINode *NewIndPHI = PHINode::Create(IndPHI->getType(), 1, "ind", IndPHI);
       NewIndPHI->addIncoming(IndPHI->getIncomingValueForBlock(IBRPred),
                              IBRPred);

       // Create a PHI in the body block, to merge the direct and indirect
       // predecessors.
       PHINode *MergePHI =
           PHINode::Create(IndPHI->getType(), 2, "merge", &*MergeInsert);
       MergePHI->addIncoming(NewIndPHI, Target);
       MergePHI->addIncoming(DirPHI, DirectSucc);

       IndPHI->replaceAllUsesWith(MergePHI);
       IndPHI->eraseFromParent();
     }

     Changed = true;
   }

   return Changed;
 }
	//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//
	//
	// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
	// inserting a dummy basic block. This pass may be "required" by passes that
	// cannot deal with critical edges. For this usage, the structure type is
	// forward declared. This pass obviously invalidates the CFG, but can update
	// dominator trees.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/Utils/BreakCriticalEdges.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/BlockFrequencyInfo.h"
	#include "llvm/Analysis/BranchProbabilityInfo.h"
	#include "llvm/Analysis/CFG.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/Analysis/MemorySSAUpdater.h"
	#include "llvm/Analysis/PostDominators.h"
	#include "llvm/IR/CFG.h"
	#include "llvm/IR/Dominators.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/IR/Type.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Transforms/Utils.h"
	#include "llvm/Transforms/Utils/BasicBlockUtils.h"
	#include "llvm/Transforms/Utils/Cloning.h"
	#include "llvm/Transforms/Utils/ValueMapper.h"
	using namespace llvm;

	#define DEBUG_TYPE "break-crit-edges"

	STATISTIC(NumBroken, "Number of blocks inserted");

	namespace {
	struct BreakCriticalEdges : public FunctionPass {
	static char ID; // Pass identification, replacement for typeid
	BreakCriticalEdges() : FunctionPass(ID) {
	initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
	}

	bool runOnFunction(Function &F) override {
	auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
	auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;

	auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
	auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;

	auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
	auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
	unsigned N =
	SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI, nullptr, PDT));
	NumBroken += N;
	return N > 0;
	}

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.addPreserved<DominatorTreeWrapperPass>();
	AU.addPreserved<LoopInfoWrapperPass>();

	// No loop canonicalization guarantees are broken by this pass.
	AU.addPreservedID(LoopSimplifyID);
	}
	};
	}

	char BreakCriticalEdges::ID = 0;
	INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
	"Break critical edges in CFG", false, false)

	// Publicly exposed interface to pass...
	char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
	FunctionPass *llvm::createBreakCriticalEdgesPass() {
	return new BreakCriticalEdges();
	}

	PreservedAnalyses BreakCriticalEdgesPass::run(Function &F,
	FunctionAnalysisManager &AM) {
	auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
	auto *LI = AM.getCachedResult<LoopAnalysis>(F);
	unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
	NumBroken += N;
	if (N == 0)
	return PreservedAnalyses::all();
	PreservedAnalyses PA;
	PA.preserve<DominatorTreeAnalysis>();
	PA.preserve<LoopAnalysis>();
	return PA;
	}

	//===----------------------------------------------------------------------===//
	// Implementation of the external critical edge manipulation functions
	//===----------------------------------------------------------------------===//

	/// When a loop exit edge is split, LCSSA form may require new PHIs in the new
	/// exit block. This function inserts the new PHIs, as needed. Preds is a list
	/// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is
	/// the old loop exit, now the successor of SplitBB.
	static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
	BasicBlock *SplitBB,
	BasicBlock *DestBB) {
	// SplitBB shouldn't have anything non-trivial in it yet.
	assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() \|\|
	SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");

	// For each PHI in the destination block.
	for (PHINode &PN : DestBB->phis()) {
	unsigned Idx = PN.getBasicBlockIndex(SplitBB);
	Value *V = PN.getIncomingValue(Idx);

	// If the input is a PHI which already satisfies LCSSA, don't create
	// a new one.
	if (const PHINode *VP = dyn_cast<PHINode>(V))
	if (VP->getParent() == SplitBB)
	continue;

	// Otherwise a new PHI is needed. Create one and populate it.
	PHINode *NewPN = PHINode::Create(
	PN.getType(), Preds.size(), "split",
	SplitBB->isLandingPad() ? &SplitBB->front() : SplitBB->getTerminator());
	for (unsigned i = 0, e = Preds.size(); i != e; ++i)
	NewPN->addIncoming(V, Preds[i]);

	// Update the original PHI.
	PN.setIncomingValue(Idx, NewPN);
	}
	}

	BasicBlock *
	llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
	const CriticalEdgeSplittingOptions &Options) {
	if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
	return nullptr;

	assert(!isa<IndirectBrInst>(TI) &&
	"Cannot split critical edge from IndirectBrInst");

	BasicBlock *TIBB = TI->getParent();
	BasicBlock *DestBB = TI->getSuccessor(SuccNum);

	// Splitting the critical edge to a pad block is non-trivial. Don't do
	// it in this generic function.
	if (DestBB->isEHPad()) return nullptr;

	// Don't split the non-fallthrough edge from a callbr.
	if (isa<CallBrInst>(TI) && SuccNum > 0)
	return nullptr;

	if (Options.IgnoreUnreachableDests &&
	isa<UnreachableInst>(DestBB->getFirstNonPHIOrDbgOrLifetime()))
	return nullptr;

	// Create a new basic block, linking it into the CFG.
	BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
	TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
	// Create our unconditional branch.
	BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
	NewBI->setDebugLoc(TI->getDebugLoc());

	// Branch to the new block, breaking the edge.
	TI->setSuccessor(SuccNum, NewBB);

	// Insert the block into the function... right after the block TI lives in.
	Function &F = *TIBB->getParent();
	Function::iterator FBBI = TIBB->getIterator();
	F.getBasicBlockList().insert(++FBBI, NewBB);

	// If there are any PHI nodes in DestBB, we need to update them so that they
	// merge incoming values from NewBB instead of from TIBB.
	{
	unsigned BBIdx = 0;
	for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
	// We no longer enter through TIBB, now we come in through NewBB.
	// Revector exactly one entry in the PHI node that used to come from
	// TIBB to come from NewBB.
	PHINode *PN = cast<PHINode>(I);

	// Reuse the previous value of BBIdx if it lines up. In cases where we
	// have multiple phi nodes with lots of predecessors, this is a speed
	// win because we don't have to scan the PHI looking for TIBB. This
	// happens because the BB list of PHI nodes are usually in the same
	// order.
	if (PN->getIncomingBlock(BBIdx) != TIBB)
	BBIdx = PN->getBasicBlockIndex(TIBB);
	PN->setIncomingBlock(BBIdx, NewBB);
	}
	}

	// If there are any other edges from TIBB to DestBB, update those to go
	// through the split block, making those edges non-critical as well (and
	// reducing the number of phi entries in the DestBB if relevant).
	if (Options.MergeIdenticalEdges) {
	for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
	if (TI->getSuccessor(i) != DestBB) continue;

	// Remove an entry for TIBB from DestBB phi nodes.
	DestBB->removePredecessor(TIBB, Options.KeepOneInputPHIs);

	// We found another edge to DestBB, go to NewBB instead.
	TI->setSuccessor(i, NewBB);
	}
	}

	// If we have nothing to update, just return.
	auto *DT = Options.DT;
	auto *PDT = Options.PDT;
	auto *LI = Options.LI;
	auto *MSSAU = Options.MSSAU;
	if (MSSAU)
	MSSAU->wireOldPredecessorsToNewImmediatePredecessor(
	DestBB, NewBB, {TIBB}, Options.MergeIdenticalEdges);

	if (!DT && !PDT && !LI)
	return NewBB;

	if (DT \|\| PDT) {
	// Update the DominatorTree.
	// ---> NewBB -----\
	// / V
	// TIBB -------\\------> DestBB
	//
	// First, inform the DT about the new path from TIBB to DestBB via NewBB,
	// then delete the old edge from TIBB to DestBB. By doing this in that order
	// DestBB stays reachable in the DT the whole time and its subtree doesn't
	// get disconnected.
	SmallVector<DominatorTree::UpdateType, 3> Updates;
	Updates.push_back({DominatorTree::Insert, TIBB, NewBB});
	Updates.push_back({DominatorTree::Insert, NewBB, DestBB});
	if (llvm::find(successors(TIBB), DestBB) == succ_end(TIBB))
	Updates.push_back({DominatorTree::Delete, TIBB, DestBB});

	if (DT)
	DT->applyUpdates(Updates);
	if (PDT)
	PDT->applyUpdates(Updates);
	}

	// Update LoopInfo if it is around.
	if (LI) {
	if (Loop *TIL = LI->getLoopFor(TIBB)) {
	// If one or the other blocks were not in a loop, the new block is not
	// either, and thus LI doesn't need to be updated.
	if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
	if (TIL == DestLoop) {
	// Both in the same loop, the NewBB joins loop.
	DestLoop->addBasicBlockToLoop(NewBB, *LI);
	} else if (TIL->contains(DestLoop)) {
	// Edge from an outer loop to an inner loop. Add to the outer loop.
	TIL->addBasicBlockToLoop(NewBB, *LI);
	} else if (DestLoop->contains(TIL)) {
	// Edge from an inner loop to an outer loop. Add to the outer loop.
	DestLoop->addBasicBlockToLoop(NewBB, *LI);
	} else {
	// Edge from two loops with no containment relation. Because these
	// are natural loops, we know that the destination block must be the
	// header of its loop (adding a branch into a loop elsewhere would
	// create an irreducible loop).
	assert(DestLoop->getHeader() == DestBB &&
	"Should not create irreducible loops!");
	if (Loop *P = DestLoop->getParentLoop())
	P->addBasicBlockToLoop(NewBB, *LI);
	}
	}

	// If TIBB is in a loop and DestBB is outside of that loop, we may need
	// to update LoopSimplify form and LCSSA form.
	if (!TIL->contains(DestBB)) {
	assert(!TIL->contains(NewBB) &&
	"Split point for loop exit is contained in loop!");

	// Update LCSSA form in the newly created exit block.
	if (Options.PreserveLCSSA) {
	createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
	}

	// The only that we can break LoopSimplify form by splitting a critical
	// edge is if after the split there exists some edge from TIL to DestBB
	// and the only edge into DestBB from outside of TIL is that of
	// NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
	// is the new exit block and it has no non-loop predecessors. If the
	// second isn't true, then DestBB was not in LoopSimplify form prior to
	// the split as it had a non-loop predecessor. In both of these cases,
	// the predecessor must be directly in TIL, not in a subloop, or again
	// LoopSimplify doesn't hold.
	SmallVector<BasicBlock *, 4> LoopPreds;
	for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
	++I) {
	BasicBlock P = I;
	if (P == NewBB)
	continue; // The new block is known.
	if (LI->getLoopFor(P) != TIL) {
	// No need to re-simplify, it wasn't to start with.
	LoopPreds.clear();
	break;
	}
	LoopPreds.push_back(P);
	}
	if (!LoopPreds.empty()) {
	assert(!DestBB->isEHPad() && "We don't split edges to EH pads!");
	BasicBlock *NewExitBB = SplitBlockPredecessors(
	DestBB, LoopPreds, "split", DT, LI, MSSAU, Options.PreserveLCSSA);
	if (Options.PreserveLCSSA)
	createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
	}
	}
	}
	}

	return NewBB;
	}

	// Return the unique indirectbr predecessor of a block. This may return null
	// even if such a predecessor exists, if it's not useful for splitting.
	// If a predecessor is found, OtherPreds will contain all other (non-indirectbr)
	// predecessors of BB.
	static BasicBlock *
	findIBRPredecessor(BasicBlock BB, SmallVectorImpl<BasicBlock > &OtherPreds) {
	// If the block doesn't have any PHIs, we don't care about it, since there's
	// no point in splitting it.
	PHINode *PN = dyn_cast<PHINode>(BB->begin());
	if (!PN)
	return nullptr;

	// Verify we have exactly one IBR predecessor.
	// Conservatively bail out if one of the other predecessors is not a "regular"
	// terminator (that is, not a switch or a br).
	BasicBlock *IBB = nullptr;
	for (unsigned Pred = 0, E = PN->getNumIncomingValues(); Pred != E; ++Pred) {
	BasicBlock *PredBB = PN->getIncomingBlock(Pred);
	Instruction *PredTerm = PredBB->getTerminator();
	switch (PredTerm->getOpcode()) {
	case Instruction::IndirectBr:
	if (IBB)
	return nullptr;
	IBB = PredBB;
	break;
	case Instruction::Br:
	case Instruction::Switch:
	OtherPreds.push_back(PredBB);
	continue;
	default:
	return nullptr;
	}
	}

	return IBB;
	}

	bool llvm::SplitIndirectBrCriticalEdges(Function &F,
	BranchProbabilityInfo *BPI,
	BlockFrequencyInfo *BFI) {
	// Check whether the function has any indirectbrs, and collect which blocks
	// they may jump to. Since most functions don't have indirect branches,
	// this lowers the common case's overhead to O(Blocks) instead of O(Edges).
	SmallSetVector<BasicBlock *, 16> Targets;
	for (auto &BB : F) {
	auto *IBI = dyn_cast<IndirectBrInst>(BB.getTerminator());
	if (!IBI)
	continue;

	for (unsigned Succ = 0, E = IBI->getNumSuccessors(); Succ != E; ++Succ)
	Targets.insert(IBI->getSuccessor(Succ));
	}

	if (Targets.empty())
	return false;

	bool ShouldUpdateAnalysis = BPI && BFI;
	bool Changed = false;
	for (BasicBlock *Target : Targets) {
	SmallVector<BasicBlock *, 16> OtherPreds;
	BasicBlock *IBRPred = findIBRPredecessor(Target, OtherPreds);
	// If we did not found an indirectbr, or the indirectbr is the only
	// incoming edge, this isn't the kind of edge we're looking for.
	if (!IBRPred \|\| OtherPreds.empty())
	continue;

	// Don't even think about ehpads/landingpads.
	Instruction *FirstNonPHI = Target->getFirstNonPHI();
	if (FirstNonPHI->isEHPad() \|\| Target->isLandingPad())
	continue;

	BasicBlock *BodyBlock = Target->splitBasicBlock(FirstNonPHI, ".split");
	if (ShouldUpdateAnalysis) {
	// Copy the BFI/BPI from Target to BodyBlock.
	for (unsigned I = 0, E = BodyBlock->getTerminator()->getNumSuccessors();
	I < E; ++I)
	BPI->setEdgeProbability(BodyBlock, I,
	BPI->getEdgeProbability(Target, I));
	BFI->setBlockFreq(BodyBlock, BFI->getBlockFreq(Target).getFrequency());
	}
	// It's possible Target was its own successor through an indirectbr.
	// In this case, the indirectbr now comes from BodyBlock.
	if (IBRPred == Target)
	IBRPred = BodyBlock;

	// At this point Target only has PHIs, and BodyBlock has the rest of the
	// block's body. Create a copy of Target that will be used by the "direct"
	// preds.
	ValueToValueMapTy VMap;
	BasicBlock *DirectSucc = CloneBasicBlock(Target, VMap, ".clone", &F);

	BlockFrequency BlockFreqForDirectSucc;
	for (BasicBlock *Pred : OtherPreds) {
	// If the target is a loop to itself, then the terminator of the split
	// block (BodyBlock) needs to be updated.
	BasicBlock *Src = Pred != Target ? Pred : BodyBlock;
	Src->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
	if (ShouldUpdateAnalysis)
	BlockFreqForDirectSucc += BFI->getBlockFreq(Src) *
	BPI->getEdgeProbability(Src, DirectSucc);
	}
	if (ShouldUpdateAnalysis) {
	BFI->setBlockFreq(DirectSucc, BlockFreqForDirectSucc.getFrequency());
	BlockFrequency NewBlockFreqForTarget =
	BFI->getBlockFreq(Target) - BlockFreqForDirectSucc;
	BFI->setBlockFreq(Target, NewBlockFreqForTarget.getFrequency());
	BPI->eraseBlock(Target);
	}

	// Ok, now fix up the PHIs. We know the two blocks only have PHIs, and that
	// they are clones, so the number of PHIs are the same.
	// (a) Remove the edge coming from IBRPred from the "Direct" PHI
	// (b) Leave that as the only edge in the "Indirect" PHI.
	// (c) Merge the two in the body block.
	BasicBlock::iterator Indirect = Target->begin(),
	End = Target->getFirstNonPHI()->getIterator();
	BasicBlock::iterator Direct = DirectSucc->begin();
	BasicBlock::iterator MergeInsert = BodyBlock->getFirstInsertionPt();

	assert(&*End == Target->getTerminator() &&
	"Block was expected to only contain PHIs");

	while (Indirect != End) {
	PHINode *DirPHI = cast<PHINode>(Direct);
	PHINode *IndPHI = cast<PHINode>(Indirect);

	// Now, clean up - the direct block shouldn't get the indirect value,
	// and vice versa.
	DirPHI->removeIncomingValue(IBRPred);
	Direct++;

	// Advance the pointer here, to avoid invalidation issues when the old
	// PHI is erased.
	Indirect++;

	PHINode *NewIndPHI = PHINode::Create(IndPHI->getType(), 1, "ind", IndPHI);
	NewIndPHI->addIncoming(IndPHI->getIncomingValueForBlock(IBRPred),
	IBRPred);

	// Create a PHI in the body block, to merge the direct and indirect
	// predecessors.
	PHINode *MergePHI =
	PHINode::Create(IndPHI->getType(), 2, "merge", &*MergeInsert);
	MergePHI->addIncoming(NewIndPHI, Target);
	MergePHI->addIncoming(DirPHI, DirectSucc);

	IndPHI->replaceAllUsesWith(MergePHI);
	IndPHI->eraseFromParent();
	}

	Changed = true;
	}

	return Changed;
	}