llvm/lib/Analysis/CFG.cpp - llvm-project - Git at Google

 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
 // This family of functions performs analyses on basic blocks, and instructions
 // contained within basic blocks.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/CycleAnalysis.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/Support/CommandLine.h"

 using namespace llvm;

 // The max number of basic blocks explored during reachability analysis between
 // two basic blocks. This is kept reasonably small to limit compile time when
 // repeatedly used by clients of this analysis (such as captureTracking).
 static cl::opt<unsigned> DefaultMaxBBsToExplore(
     "dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
     cl::desc("Max number of BBs to explore for reachability analysis"),
     cl::init(32));

 /// FindFunctionBackedges - Analyze the specified function to find all of the
 /// loop backedges in the function and return them.  This is a relatively cheap
 /// (compared to computing dominators and loop info) analysis.
 ///
 /// The output is added to Result, as pairs of <from,to> edge info.
 void llvm::FindFunctionBackedges(const Function &F,
      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
   const BasicBlock *BB = &F.getEntryBlock();

   // In the DFS traversal, we maintain three states: unvisited, visited in the
   // past, and visited and currently in the DFS stack. If we have an edge to a
   // block in the stack, we have found a backedge.
   enum VisitState : uint8_t { Unvisited = 0, Visited = 1, InStack = 2 };
   SmallVector<VisitState> BlockState(F.getMaxBlockNumber(), Unvisited);
   struct StackEntry {
     const BasicBlock *BB;
     const_succ_iterator SuccIt;
     const_succ_iterator SuccEnd;

     StackEntry(const BasicBlock *BB)
         : BB(BB), SuccIt(nullptr), SuccEnd(nullptr) {
       auto Succs = successors(BB);
       SuccIt = Succs.begin();
       SuccEnd = Succs.end();
     }
   };
   SmallVector<StackEntry, 8> VisitStack;

   BlockState[BB->getNumber()] = InStack;
   VisitStack.emplace_back(BB);
   do {
     StackEntry &Top = VisitStack.back();
     bool FoundNew = false;
     while (Top.SuccIt != Top.SuccEnd) {
       BB = *Top.SuccIt++;
       if (BlockState[BB->getNumber()] == Unvisited) {
         // Unvisited successor => go down one level.
         BlockState[BB->getNumber()] = InStack;
         VisitStack.emplace_back(BB);
         FoundNew = true;
         break;
       }
       // Successor in VisitStack => backedge.
       if (BlockState[BB->getNumber()] == InStack)
         Result.emplace_back(Top.BB, BB);
     }

     // Go up one level.
     if (!FoundNew) {
       BlockState[Top.BB->getNumber()] = Visited;
       VisitStack.pop_back();
     }
   } while (!VisitStack.empty());
 }

 /// GetSuccessorNumber - Search for the specified successor of basic block BB
 /// and return its position in the terminator instruction's list of
 /// successors.  It is an error to call this with a block that is not a
 /// successor.
 unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
     const BasicBlock *Succ) {
   const Instruction *Term = BB->getTerminator();
 #ifndef NDEBUG
   unsigned e = Term->getNumSuccessors();
 #endif
   for (unsigned i = 0; ; ++i) {
     assert(i != e && "Didn't find edge?");
     if (Term->getSuccessor(i) == Succ)
       return i;
   }
 }

 /// isCriticalEdge - Return true if the specified edge is a critical edge.
 /// Critical edges are edges from a block with multiple successors to a block
 /// with multiple predecessors.
 bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
                           bool AllowIdenticalEdges) {
   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
   return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
 }

 bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
                           bool AllowIdenticalEdges) {
   assert(TI->isTerminator() && "Must be a terminator to have successors!");
   if (TI->getNumSuccessors() == 1) return false;

   assert(is_contained(predecessors(Dest), TI->getParent()) &&
          "No edge between TI's block and Dest.");

   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);

   // If there is more than one predecessor, this is a critical edge...
   assert(I != E && "No preds, but we have an edge to the block?");
   const BasicBlock *FirstPred = *I;
   ++I;        // Skip one edge due to the incoming arc from TI.
   if (!AllowIdenticalEdges)
     return I != E;

   // If AllowIdenticalEdges is true, then we allow this edge to be considered
   // non-critical iff all preds come from TI's block.
   for (; I != E; ++I)
     if (*I != FirstPred)
       return true;
   return false;
 }

 // LoopInfo contains a mapping from basic block to the innermost loop. Find
 // the outermost loop in the loop nest that contains BB.
 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
   const Loop *L = LI->getLoopFor(BB);
   return L ? L->getOutermostLoop() : nullptr;
 }

 template <class StopSetT>
 static bool isReachableImpl(SmallVectorImpl<BasicBlock *> &Worklist,
                             const StopSetT &StopSet,
                             const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
                             const DominatorTree *DT, const LoopInfo *LI,
                             const CycleInfo *CI) {
   // If both LI and CI are passed, use CI, which gives us more information.
   if (CI)
     LI = nullptr;

   // When a stop block is unreachable, it's dominated from everywhere,
   // regardless of whether there's a path between the two blocks.
   if (DT) {
     for (auto *BB : StopSet) {
       if (!DT->isReachableFromEntry(BB)) {
         DT = nullptr;
         break;
       }
     }
   }

   // We can't skip directly from a block that dominates the stop block if the
   // exclusion block is potentially in between.
   if (ExclusionSet && !ExclusionSet->empty())
     DT = nullptr;

   // Normally any block in a loop is reachable from any other block in a loop,
   // however excluded blocks might partition the body of a loop to make that
   // untrue.
   SmallPtrSet<const Loop *, 8> LoopsWithHoles;
   if (LI && ExclusionSet) {
     for (auto *BB : *ExclusionSet) {
       if (const Loop *L = getOutermostLoop(LI, BB))
         LoopsWithHoles.insert(L);
     }
   }

   SmallPtrSet<const Cycle *, 8> CyclesWithHoles;
   if (CI && ExclusionSet) {
     for (auto *BB : *ExclusionSet) {
       if (const Cycle *C = CI->getTopLevelParentCycle(BB))
         CyclesWithHoles.insert(C);
     }
   }

   SmallPtrSet<const Loop *, 2> StopLoops;
   if (LI) {
     for (auto *StopSetBB : StopSet) {
       if (const Loop *L = getOutermostLoop(LI, StopSetBB))
         StopLoops.insert(L);
     }
   }

   SmallPtrSet<const Cycle *, 2> StopCycles;
   if (CI) {
     for (auto *StopSetBB : StopSet) {
       if (const Cycle *C = CI->getTopLevelParentCycle(StopSetBB))
         StopCycles.insert(C);
     }
   }

   unsigned Limit = DefaultMaxBBsToExplore;
   SmallPtrSet<const BasicBlock*, 32> Visited;
   do {
     BasicBlock *BB = Worklist.pop_back_val();
     if (!Visited.insert(BB).second)
       continue;
     if (StopSet.contains(BB))
       return true;
     if (ExclusionSet && ExclusionSet->count(BB))
       continue;
     if (DT) {
       if (llvm::any_of(StopSet, [&](const BasicBlock *StopBB) {
             return DT->dominates(BB, StopBB);
           }))
         return true;
     }

     const Loop *OuterL = nullptr;
     if (LI) {
       OuterL = getOutermostLoop(LI, BB);
       // If we're in a loop with a hole, not all blocks in the loop are
       // reachable from all other blocks. That implies we can't simply jump to
       // the loop's exit blocks, as that exit might need to pass through an
       // excluded block. Clear Outer so we process BB's successors.
       if (LoopsWithHoles.count(OuterL))
         OuterL = nullptr;
       else if (StopLoops.contains(OuterL))
         return true;
     }

     const Cycle *OuterC = nullptr;
     if (CI) {
       OuterC = CI->getTopLevelParentCycle(BB);
       if (CyclesWithHoles.count(OuterC))
         OuterC = nullptr;
       else if (StopCycles.contains(OuterC))
         return true;
     }

     if (!--Limit) {
       // We haven't been able to prove it one way or the other. Conservatively
       // answer true -- that there is potentially a path.
       return true;
     }

     if (OuterL) {
       // All blocks in a single loop are reachable from all other blocks. From
       // any of these blocks, we can skip directly to the exits of the loop,
       // ignoring any other blocks inside the loop body.
       OuterL->getExitBlocks(Worklist);
     } else if (OuterC) {
       OuterC->getExitBlocks(Worklist);
     } else {
       Worklist.append(succ_begin(BB), succ_end(BB));
     }
   } while (!Worklist.empty());

   // We have exhausted all possible paths and are certain that 'To' can not be
   // reached from 'From'.
   return false;
 }

 template <class T> class SingleEntrySet {
 public:
   using const_iterator = const T *;

   SingleEntrySet(T Elem) : Elem(Elem) {}

   bool contains(T Other) const { return Elem == Other; }

   const_iterator begin() const { return &Elem; }
   const_iterator end() const { return &Elem + 1; }

 private:
   T Elem;
 };

 bool llvm::isPotentiallyReachableFromMany(
     SmallVectorImpl<BasicBlock *> &Worklist, const BasicBlock *StopBB,
     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
     const LoopInfo *LI, const CycleInfo *CI) {
   return isReachableImpl<SingleEntrySet<const BasicBlock *>>(
       Worklist, SingleEntrySet<const BasicBlock *>(StopBB), ExclusionSet, DT,
       LI, CI);
 }

 bool llvm::isManyPotentiallyReachableFromMany(
     SmallVectorImpl<BasicBlock *> &Worklist,
     const SmallPtrSetImpl<const BasicBlock *> &StopSet,
     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
     const LoopInfo *LI, const CycleInfo *CI) {
   return isReachableImpl<SmallPtrSetImpl<const BasicBlock *>>(
       Worklist, StopSet, ExclusionSet, DT, LI, CI);
 }

 bool llvm::isPotentiallyReachable(
     const BasicBlock *A, const BasicBlock *B,
     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
     const LoopInfo *LI, const CycleInfo *CI) {
   assert(A->getParent() == B->getParent() &&
          "This analysis is function-local!");

   if (DT) {
     if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B))
       return false;
     if (!ExclusionSet || ExclusionSet->empty()) {
       if (A->isEntryBlock() && DT->isReachableFromEntry(B))
         return true;
       if (B->isEntryBlock() && DT->isReachableFromEntry(A))
         return false;
     }
   }

   SmallVector<BasicBlock*, 32> Worklist;
   Worklist.push_back(const_cast<BasicBlock*>(A));

   return isPotentiallyReachableFromMany(Worklist, B, ExclusionSet, DT, LI, CI);
 }

 bool llvm::isPotentiallyReachable(
     const Instruction *A, const Instruction *B,
     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
     const LoopInfo *LI, const CycleInfo *CI) {
   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
          "This analysis is function-local!");

   if (A->getParent() == B->getParent()) {
     // The same block case is special because it's the only time we're looking
     // within a single block to see which instruction comes first. Once we
     // start looking at multiple blocks, the first instruction of the block is
     // reachable, so we only need to determine reachability between whole
     // blocks.
     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());

     // If A comes before B, then B is definitively reachable from A.
     if (A == B || A->comesBefore(B))
       return true;

     // If the block is in a cycle (and there are no excluded blocks), then we
     // can reach any instruction in the block from any other instruction in the
     // block by going around a backedge.
     if (!ExclusionSet || ExclusionSet->empty()) {
       // If cycle info is available, we can know for sure whether or not a
       // block is part of a cycle.
       if (CI)
         return CI->getCycle(BB) != nullptr;

       // If only loop info is available, even if the block is not part of a
       // natural loop, it may still be part of an irreducible cycle.
       if (LI && LI->getLoopFor(BB) != nullptr)
         return true;
     }

     // Can't be in a loop if it's the entry block -- the entry block may not
     // have predecessors.
     if (BB->isEntryBlock())
       return false;

     // Otherwise, continue doing the normal per-BB CFG walk.
     SmallVector<BasicBlock*, 32> Worklist;
     Worklist.append(succ_begin(BB), succ_end(BB));
     if (Worklist.empty()) {
       // We've proven that there's no path!
       return false;
     }

     return isPotentiallyReachableFromMany(Worklist, B->getParent(),
                                           ExclusionSet, DT, LI, CI);
   }

   return isPotentiallyReachable(A->getParent(), B->getParent(), ExclusionSet,
                                 DT, LI, CI);
 }

 static bool instructionDoesNotReturn(const Instruction &I) {
   if (auto *CB = dyn_cast<CallBase>(&I))
     return CB->hasFnAttr(Attribute::NoReturn);
   return false;
 }

 // A basic block can only return if it terminates with a ReturnInst and does not
 // contain calls to noreturn functions.
 static bool basicBlockCanReturn(const BasicBlock &BB) {
   if (!isa<ReturnInst>(BB.getTerminator()))
     return false;
   return none_of(BB, instructionDoesNotReturn);
 }

 // FIXME: this doesn't handle recursion.
 bool llvm::canReturn(const Function &F) {
   SmallVector<const BasicBlock *, 16> Worklist;
   SmallPtrSet<const BasicBlock *, 16> Visited;

   Visited.insert(&F.front());
   Worklist.push_back(&F.front());

   do {
     const BasicBlock *BB = Worklist.pop_back_val();
     if (basicBlockCanReturn(*BB))
       return true;
     for (const BasicBlock *Succ : successors(BB))
       if (Visited.insert(Succ).second)
         Worklist.push_back(Succ);
   } while (!Worklist.empty());

   return false;
 }

 bool llvm::isPresplitCoroSuspendExitEdge(const BasicBlock &Src,
                                          const BasicBlock &Dest) {
   assert(Src.getParent() == Dest.getParent());
   if (!Src.getParent()->isPresplitCoroutine())
     return false;
   if (auto *SW = dyn_cast<SwitchInst>(Src.getTerminator()))
     if (auto *Intr = dyn_cast<IntrinsicInst>(SW->getCondition()))
       return Intr->getIntrinsicID() == Intrinsic::coro_suspend &&
              SW->getDefaultDest() == &Dest;
   return false;
 }
	//===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
	//
	// 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
	//
	//===----------------------------------------------------------------------===//
	//
	// This family of functions performs analyses on basic blocks, and instructions
	// contained within basic blocks.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Analysis/CFG.h"
	#include "llvm/Analysis/CycleAnalysis.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/IR/Dominators.h"
	#include "llvm/IR/IntrinsicInst.h"
	#include "llvm/Support/CommandLine.h"

	using namespace llvm;

	// The max number of basic blocks explored during reachability analysis between
	// two basic blocks. This is kept reasonably small to limit compile time when
	// repeatedly used by clients of this analysis (such as captureTracking).
	static cl::opt<unsigned> DefaultMaxBBsToExplore(
	"dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
	cl::desc("Max number of BBs to explore for reachability analysis"),
	cl::init(32));

	/// FindFunctionBackedges - Analyze the specified function to find all of the
	/// loop backedges in the function and return them. This is a relatively cheap
	/// (compared to computing dominators and loop info) analysis.
	///
	/// The output is added to Result, as pairs of <from,to> edge info.
	void llvm::FindFunctionBackedges(const Function &F,
	SmallVectorImpl<std::pair<const BasicBlock,const BasicBlock> > &Result) {
	const BasicBlock *BB = &F.getEntryBlock();

	// In the DFS traversal, we maintain three states: unvisited, visited in the
	// past, and visited and currently in the DFS stack. If we have an edge to a
	// block in the stack, we have found a backedge.
	enum VisitState : uint8_t { Unvisited = 0, Visited = 1, InStack = 2 };
	SmallVector<VisitState> BlockState(F.getMaxBlockNumber(), Unvisited);
	struct StackEntry {
	const BasicBlock *BB;
	const_succ_iterator SuccIt;
	const_succ_iterator SuccEnd;

	StackEntry(const BasicBlock *BB)
	: BB(BB), SuccIt(nullptr), SuccEnd(nullptr) {
	auto Succs = successors(BB);
	SuccIt = Succs.begin();
	SuccEnd = Succs.end();
	}
	};
	SmallVector<StackEntry, 8> VisitStack;

	BlockState[BB->getNumber()] = InStack;
	VisitStack.emplace_back(BB);
	do {
	StackEntry &Top = VisitStack.back();
	bool FoundNew = false;
	while (Top.SuccIt != Top.SuccEnd) {
	BB = *Top.SuccIt++;
	if (BlockState[BB->getNumber()] == Unvisited) {
	// Unvisited successor => go down one level.
	BlockState[BB->getNumber()] = InStack;
	VisitStack.emplace_back(BB);
	FoundNew = true;
	break;
	}
	// Successor in VisitStack => backedge.
	if (BlockState[BB->getNumber()] == InStack)
	Result.emplace_back(Top.BB, BB);
	}

	// Go up one level.
	if (!FoundNew) {
	BlockState[Top.BB->getNumber()] = Visited;
	VisitStack.pop_back();
	}
	} while (!VisitStack.empty());
	}

	/// GetSuccessorNumber - Search for the specified successor of basic block BB
	/// and return its position in the terminator instruction's list of
	/// successors. It is an error to call this with a block that is not a
	/// successor.
	unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
	const BasicBlock *Succ) {
	const Instruction *Term = BB->getTerminator();
	#ifndef NDEBUG
	unsigned e = Term->getNumSuccessors();
	#endif
	for (unsigned i = 0; ; ++i) {
	assert(i != e && "Didn't find edge?");
	if (Term->getSuccessor(i) == Succ)
	return i;
	}
	}

	/// isCriticalEdge - Return true if the specified edge is a critical edge.
	/// Critical edges are edges from a block with multiple successors to a block
	/// with multiple predecessors.
	bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
	bool AllowIdenticalEdges) {
	assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
	return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
	}

	bool llvm::isCriticalEdge(const Instruction TI, const BasicBlock Dest,
	bool AllowIdenticalEdges) {
	assert(TI->isTerminator() && "Must be a terminator to have successors!");
	if (TI->getNumSuccessors() == 1) return false;

	assert(is_contained(predecessors(Dest), TI->getParent()) &&
	"No edge between TI's block and Dest.");

	const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);

	// If there is more than one predecessor, this is a critical edge...
	assert(I != E && "No preds, but we have an edge to the block?");
	const BasicBlock FirstPred = I;
	++I; // Skip one edge due to the incoming arc from TI.
	if (!AllowIdenticalEdges)
	return I != E;

	// If AllowIdenticalEdges is true, then we allow this edge to be considered
	// non-critical iff all preds come from TI's block.
	for (; I != E; ++I)
	if (*I != FirstPred)
	return true;
	return false;
	}

	// LoopInfo contains a mapping from basic block to the innermost loop. Find
	// the outermost loop in the loop nest that contains BB.
	static const Loop getOutermostLoop(const LoopInfo LI, const BasicBlock *BB) {
	const Loop *L = LI->getLoopFor(BB);
	return L ? L->getOutermostLoop() : nullptr;
	}

	template <class StopSetT>
	static bool isReachableImpl(SmallVectorImpl<BasicBlock *> &Worklist,
	const StopSetT &StopSet,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet,
	const DominatorTree DT, const LoopInfo LI,
	const CycleInfo *CI) {
	// If both LI and CI are passed, use CI, which gives us more information.
	if (CI)
	LI = nullptr;

	// When a stop block is unreachable, it's dominated from everywhere,
	// regardless of whether there's a path between the two blocks.
	if (DT) {
	for (auto *BB : StopSet) {
	if (!DT->isReachableFromEntry(BB)) {
	DT = nullptr;
	break;
	}
	}
	}

	// We can't skip directly from a block that dominates the stop block if the
	// exclusion block is potentially in between.
	if (ExclusionSet && !ExclusionSet->empty())
	DT = nullptr;

	// Normally any block in a loop is reachable from any other block in a loop,
	// however excluded blocks might partition the body of a loop to make that
	// untrue.
	SmallPtrSet<const Loop *, 8> LoopsWithHoles;
	if (LI && ExclusionSet) {
	for (auto BB : ExclusionSet) {
	if (const Loop *L = getOutermostLoop(LI, BB))
	LoopsWithHoles.insert(L);
	}
	}

	SmallPtrSet<const Cycle *, 8> CyclesWithHoles;
	if (CI && ExclusionSet) {
	for (auto BB : ExclusionSet) {
	if (const Cycle *C = CI->getTopLevelParentCycle(BB))
	CyclesWithHoles.insert(C);
	}
	}

	SmallPtrSet<const Loop *, 2> StopLoops;
	if (LI) {
	for (auto *StopSetBB : StopSet) {
	if (const Loop *L = getOutermostLoop(LI, StopSetBB))
	StopLoops.insert(L);
	}
	}

	SmallPtrSet<const Cycle *, 2> StopCycles;
	if (CI) {
	for (auto *StopSetBB : StopSet) {
	if (const Cycle *C = CI->getTopLevelParentCycle(StopSetBB))
	StopCycles.insert(C);
	}
	}

	unsigned Limit = DefaultMaxBBsToExplore;
	SmallPtrSet<const BasicBlock*, 32> Visited;
	do {
	BasicBlock *BB = Worklist.pop_back_val();
	if (!Visited.insert(BB).second)
	continue;
	if (StopSet.contains(BB))
	return true;
	if (ExclusionSet && ExclusionSet->count(BB))
	continue;
	if (DT) {
	if (llvm::any_of(StopSet, [&](const BasicBlock *StopBB) {
	return DT->dominates(BB, StopBB);
	}))
	return true;
	}

	const Loop *OuterL = nullptr;
	if (LI) {
	OuterL = getOutermostLoop(LI, BB);
	// If we're in a loop with a hole, not all blocks in the loop are
	// reachable from all other blocks. That implies we can't simply jump to
	// the loop's exit blocks, as that exit might need to pass through an
	// excluded block. Clear Outer so we process BB's successors.
	if (LoopsWithHoles.count(OuterL))
	OuterL = nullptr;
	else if (StopLoops.contains(OuterL))
	return true;
	}

	const Cycle *OuterC = nullptr;
	if (CI) {
	OuterC = CI->getTopLevelParentCycle(BB);
	if (CyclesWithHoles.count(OuterC))
	OuterC = nullptr;
	else if (StopCycles.contains(OuterC))
	return true;
	}

	if (!--Limit) {
	// We haven't been able to prove it one way or the other. Conservatively
	// answer true -- that there is potentially a path.
	return true;
	}

	if (OuterL) {
	// All blocks in a single loop are reachable from all other blocks. From
	// any of these blocks, we can skip directly to the exits of the loop,
	// ignoring any other blocks inside the loop body.
	OuterL->getExitBlocks(Worklist);
	} else if (OuterC) {
	OuterC->getExitBlocks(Worklist);
	} else {
	Worklist.append(succ_begin(BB), succ_end(BB));
	}
	} while (!Worklist.empty());

	// We have exhausted all possible paths and are certain that 'To' can not be
	// reached from 'From'.
	return false;
	}

	template <class T> class SingleEntrySet {
	public:
	using const_iterator = const T *;

	SingleEntrySet(T Elem) : Elem(Elem) {}

	bool contains(T Other) const { return Elem == Other; }

	const_iterator begin() const { return &Elem; }
	const_iterator end() const { return &Elem + 1; }

	private:
	T Elem;
	};

	bool llvm::isPotentiallyReachableFromMany(
	SmallVectorImpl<BasicBlock > &Worklist, const BasicBlock StopBB,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet, const DominatorTree *DT,
	const LoopInfo LI, const CycleInfo CI) {
	return isReachableImpl<SingleEntrySet<const BasicBlock *>>(
	Worklist, SingleEntrySet<const BasicBlock *>(StopBB), ExclusionSet, DT,
	LI, CI);
	}

	bool llvm::isManyPotentiallyReachableFromMany(
	SmallVectorImpl<BasicBlock *> &Worklist,
	const SmallPtrSetImpl<const BasicBlock *> &StopSet,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet, const DominatorTree *DT,
	const LoopInfo LI, const CycleInfo CI) {
	return isReachableImpl<SmallPtrSetImpl<const BasicBlock *>>(
	Worklist, StopSet, ExclusionSet, DT, LI, CI);
	}

	bool llvm::isPotentiallyReachable(
	const BasicBlock A, const BasicBlock B,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet, const DominatorTree *DT,
	const LoopInfo LI, const CycleInfo CI) {
	assert(A->getParent() == B->getParent() &&
	"This analysis is function-local!");

	if (DT) {
	if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B))
	return false;
	if (!ExclusionSet \|\| ExclusionSet->empty()) {
	if (A->isEntryBlock() && DT->isReachableFromEntry(B))
	return true;
	if (B->isEntryBlock() && DT->isReachableFromEntry(A))
	return false;
	}
	}

	SmallVector<BasicBlock*, 32> Worklist;
	Worklist.push_back(const_cast<BasicBlock*>(A));

	return isPotentiallyReachableFromMany(Worklist, B, ExclusionSet, DT, LI, CI);
	}

	bool llvm::isPotentiallyReachable(
	const Instruction A, const Instruction B,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet, const DominatorTree *DT,
	const LoopInfo LI, const CycleInfo CI) {
	assert(A->getParent()->getParent() == B->getParent()->getParent() &&
	"This analysis is function-local!");

	if (A->getParent() == B->getParent()) {
	// The same block case is special because it's the only time we're looking
	// within a single block to see which instruction comes first. Once we
	// start looking at multiple blocks, the first instruction of the block is
	// reachable, so we only need to determine reachability between whole
	// blocks.
	BasicBlock BB = const_cast<BasicBlock >(A->getParent());

	// If A comes before B, then B is definitively reachable from A.
	if (A == B \|\| A->comesBefore(B))
	return true;

	// If the block is in a cycle (and there are no excluded blocks), then we
	// can reach any instruction in the block from any other instruction in the
	// block by going around a backedge.
	if (!ExclusionSet \|\| ExclusionSet->empty()) {
	// If cycle info is available, we can know for sure whether or not a
	// block is part of a cycle.
	if (CI)
	return CI->getCycle(BB) != nullptr;

	// If only loop info is available, even if the block is not part of a
	// natural loop, it may still be part of an irreducible cycle.
	if (LI && LI->getLoopFor(BB) != nullptr)
	return true;
	}

	// Can't be in a loop if it's the entry block -- the entry block may not
	// have predecessors.
	if (BB->isEntryBlock())
	return false;

	// Otherwise, continue doing the normal per-BB CFG walk.
	SmallVector<BasicBlock*, 32> Worklist;
	Worklist.append(succ_begin(BB), succ_end(BB));
	if (Worklist.empty()) {
	// We've proven that there's no path!
	return false;
	}

	return isPotentiallyReachableFromMany(Worklist, B->getParent(),
	ExclusionSet, DT, LI, CI);
	}

	return isPotentiallyReachable(A->getParent(), B->getParent(), ExclusionSet,
	DT, LI, CI);
	}

	static bool instructionDoesNotReturn(const Instruction &I) {
	if (auto *CB = dyn_cast<CallBase>(&I))
	return CB->hasFnAttr(Attribute::NoReturn);
	return false;
	}

	// A basic block can only return if it terminates with a ReturnInst and does not
	// contain calls to noreturn functions.
	static bool basicBlockCanReturn(const BasicBlock &BB) {
	if (!isa<ReturnInst>(BB.getTerminator()))
	return false;
	return none_of(BB, instructionDoesNotReturn);
	}

	// FIXME: this doesn't handle recursion.
	bool llvm::canReturn(const Function &F) {
	SmallVector<const BasicBlock *, 16> Worklist;
	SmallPtrSet<const BasicBlock *, 16> Visited;

	Visited.insert(&F.front());
	Worklist.push_back(&F.front());

	do {
	const BasicBlock *BB = Worklist.pop_back_val();
	if (basicBlockCanReturn(*BB))
	return true;
	for (const BasicBlock *Succ : successors(BB))
	if (Visited.insert(Succ).second)
	Worklist.push_back(Succ);
	} while (!Worklist.empty());

	return false;
	}

	bool llvm::isPresplitCoroSuspendExitEdge(const BasicBlock &Src,
	const BasicBlock &Dest) {
	assert(Src.getParent() == Dest.getParent());
	if (!Src.getParent()->isPresplitCoroutine())
	return false;
	if (auto *SW = dyn_cast<SwitchInst>(Src.getTerminator()))
	if (auto *Intr = dyn_cast<IntrinsicInst>(SW->getCondition()))
	return Intr->getIntrinsicID() == Intrinsic::coro_suspend &&
	SW->getDefaultDest() == &Dest;
	return false;
	}