lib/Analysis/CFG.cpp - llvm-project/llvm - 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/LoopInfo.h"
 #include "llvm/IR/Dominators.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();
   if (succ_empty(BB))
     return;

   SmallPtrSet<const BasicBlock*, 8> Visited;
   SmallVector<std::pair<const BasicBlock *, const_succ_iterator>, 8> VisitStack;
   SmallPtrSet<const BasicBlock*, 8> InStack;

   Visited.insert(BB);
   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
   InStack.insert(BB);
   do {
     std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
     const BasicBlock *ParentBB = Top.first;
     const_succ_iterator &I = Top.second;

     bool FoundNew = false;
     while (I != succ_end(ParentBB)) {
       BB = *I++;
       if (Visited.insert(BB).second) {
         FoundNew = true;
         break;
       }
       // Successor is in VisitStack, it's a back edge.
       if (InStack.count(BB))
         Result.push_back(std::make_pair(ParentBB, BB));
     }

     if (FoundNew) {
       // Go down one level if there is a unvisited successor.
       InStack.insert(BB);
       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
     } else {
       // Go up one level.
       InStack.erase(VisitStack.pop_back_val().first);
     }
   } 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);
   if (L) {
     while (const Loop *Parent = L->getParentLoop())
       L = Parent;
   }
   return L;
 }

 bool llvm::isPotentiallyReachableFromMany(
     SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
     const LoopInfo *LI) {
   // When the stop block is unreachable, it's dominated from everywhere,
   // regardless of whether there's a path between the two blocks.
   if (DT && !DT->isReachableFromEntry(StopBB))
     DT = nullptr;

   // 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);
     }
   }

   const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;

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

     const Loop *Outer = nullptr;
     if (LI) {
       Outer = 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(Outer))
         Outer = nullptr;
       if (StopLoop && Outer == StopLoop)
         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 (Outer) {
       // 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.
       Outer->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;
 }

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

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

   return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
                                         nullptr, DT, LI);
 }

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

   SmallVector<BasicBlock*, 32> Worklist;

   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 the block is in a loop then we can reach any instruction in the block
     // from any other instruction in the block by going around a backedge.
     if (LI && LI->getLoopFor(BB) != nullptr)
       return true;

     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
     for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
          ++I) {
       if (&*I == B)
         return true;
     }

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

     // Otherwise, continue doing the normal per-BB CFG walk.
     Worklist.append(succ_begin(BB), succ_end(BB));

     if (Worklist.empty()) {
       // We've proven that there's no path!
       return false;
     }
   } else {
     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
   }

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

   return isPotentiallyReachableFromMany(
       Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
 }
	//===-- 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/LoopInfo.h"
	#include "llvm/IR/Dominators.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();
	if (succ_empty(BB))
	return;

	SmallPtrSet<const BasicBlock*, 8> Visited;
	SmallVector<std::pair<const BasicBlock *, const_succ_iterator>, 8> VisitStack;
	SmallPtrSet<const BasicBlock*, 8> InStack;

	Visited.insert(BB);
	VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
	InStack.insert(BB);
	do {
	std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
	const BasicBlock *ParentBB = Top.first;
	const_succ_iterator &I = Top.second;

	bool FoundNew = false;
	while (I != succ_end(ParentBB)) {
	BB = *I++;
	if (Visited.insert(BB).second) {
	FoundNew = true;
	break;
	}
	// Successor is in VisitStack, it's a back edge.
	if (InStack.count(BB))
	Result.push_back(std::make_pair(ParentBB, BB));
	}

	if (FoundNew) {
	// Go down one level if there is a unvisited successor.
	InStack.insert(BB);
	VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
	} else {
	// Go up one level.
	InStack.erase(VisitStack.pop_back_val().first);
	}
	} 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);
	if (L) {
	while (const Loop *Parent = L->getParentLoop())
	L = Parent;
	}
	return L;
	}

	bool llvm::isPotentiallyReachableFromMany(
	SmallVectorImpl<BasicBlock > &Worklist, BasicBlock StopBB,
	const SmallPtrSetImpl<BasicBlock > ExclusionSet, const DominatorTree *DT,
	const LoopInfo *LI) {
	// When the stop block is unreachable, it's dominated from everywhere,
	// regardless of whether there's a path between the two blocks.
	if (DT && !DT->isReachableFromEntry(StopBB))
	DT = nullptr;

	// 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);
	}
	}

	const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;

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

	const Loop *Outer = nullptr;
	if (LI) {
	Outer = 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(Outer))
	Outer = nullptr;
	if (StopLoop && Outer == StopLoop)
	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 (Outer) {
	// 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.
	Outer->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;
	}

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

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

	return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
	nullptr, DT, LI);
	}

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

	SmallVector<BasicBlock*, 32> Worklist;

	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 the block is in a loop then we can reach any instruction in the block
	// from any other instruction in the block by going around a backedge.
	if (LI && LI->getLoopFor(BB) != nullptr)
	return true;

	// Linear scan, start at 'A', see whether we hit 'B' or the end first.
	for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
	++I) {
	if (&*I == B)
	return true;
	}

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

	// Otherwise, continue doing the normal per-BB CFG walk.
	Worklist.append(succ_begin(BB), succ_end(BB));

	if (Worklist.empty()) {
	// We've proven that there's no path!
	return false;
	}
	} else {
	Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
	}

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

	return isPotentiallyReachableFromMany(
	Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
	}