llvm-project

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//===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This family of functions performs analyses on basic blocks, and instructions
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// contained within basic blocks.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/Support/CommandLine.h"
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using namespace llvm;
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// The max number of basic blocks explored during reachability analysis between
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// two basic blocks. This is kept reasonably small to limit compile time when
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// repeatedly used by clients of this analysis (such as captureTracking).
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static cl::opt<unsigned> DefaultMaxBBsToExplore(
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    "dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
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    cl::desc("Max number of BBs to explore for reachability analysis"),
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    cl::init(32));
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/// FindFunctionBackedges - Analyze the specified function to find all of the
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/// loop backedges in the function and return them.  This is a relatively cheap
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/// (compared to computing dominators and loop info) analysis.
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///
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/// The output is added to Result, as pairs of <from,to> edge info.
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void llvm::FindFunctionBackedges(const Function &F,
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     SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
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  const BasicBlock *BB = &F.getEntryBlock();
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  if (succ_empty(BB))
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    return;
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  SmallPtrSet<const BasicBlock*, 8> Visited;
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  SmallVector<std::pair<const BasicBlock *, const_succ_iterator>, 8> VisitStack;
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  SmallPtrSet<const BasicBlock*, 8> InStack;
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  Visited.insert(BB);
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  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
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  InStack.insert(BB);
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  do {
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    std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
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    const BasicBlock *ParentBB = Top.first;
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    const_succ_iterator &I = Top.second;
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    bool FoundNew = false;
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    while (I != succ_end(ParentBB)) {
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      BB = *I++;
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      if (Visited.insert(BB).second) {
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        FoundNew = true;
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        break;
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      }
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      // Successor is in VisitStack, it's a back edge.
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      if (InStack.count(BB))
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        Result.push_back(std::make_pair(ParentBB, BB));
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    }
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    if (FoundNew) {
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      // Go down one level if there is a unvisited successor.
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      InStack.insert(BB);
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      VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
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    } else {
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      // Go up one level.
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      InStack.erase(VisitStack.pop_back_val().first);
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    }
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  } while (!VisitStack.empty());
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}
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/// GetSuccessorNumber - Search for the specified successor of basic block BB
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/// and return its position in the terminator instruction's list of
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/// successors.  It is an error to call this with a block that is not a
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/// successor.
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unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
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    const BasicBlock *Succ) {
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  const Instruction *Term = BB->getTerminator();
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#ifndef NDEBUG
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  unsigned e = Term->getNumSuccessors();
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#endif
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  for (unsigned i = 0; ; ++i) {
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    assert(i != e && "Didn't find edge?");
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    if (Term->getSuccessor(i) == Succ)
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      return i;
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  }
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}
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/// isCriticalEdge - Return true if the specified edge is a critical edge.
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/// Critical edges are edges from a block with multiple successors to a block
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/// with multiple predecessors.
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bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
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                          bool AllowIdenticalEdges) {
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  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
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  return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
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}
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bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
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                          bool AllowIdenticalEdges) {
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  assert(TI->isTerminator() && "Must be a terminator to have successors!");
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  if (TI->getNumSuccessors() == 1) return false;
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  assert(is_contained(predecessors(Dest), TI->getParent()) &&
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         "No edge between TI's block and Dest.");
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  const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
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  // If there is more than one predecessor, this is a critical edge...
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  assert(I != E && "No preds, but we have an edge to the block?");
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  const BasicBlock *FirstPred = *I;
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  ++I;        // Skip one edge due to the incoming arc from TI.
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  if (!AllowIdenticalEdges)
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    return I != E;
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  // If AllowIdenticalEdges is true, then we allow this edge to be considered
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  // non-critical iff all preds come from TI's block.
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  for (; I != E; ++I)
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    if (*I != FirstPred)
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      return true;
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  return false;
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}
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// LoopInfo contains a mapping from basic block to the innermost loop. Find
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// the outermost loop in the loop nest that contains BB.
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static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
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  const Loop *L = LI->getLoopFor(BB);
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  return L ? L->getOutermostLoop() : nullptr;
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}
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template <class StopSetT>
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static bool isReachableImpl(SmallVectorImpl<BasicBlock *> &Worklist,
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                            const StopSetT &StopSet,
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                            const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
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                            const DominatorTree *DT, const LoopInfo *LI) {
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  // When a stop block is unreachable, it's dominated from everywhere,
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  // regardless of whether there's a path between the two blocks.
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  if (DT) {
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    for (auto *BB : StopSet) {
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      if (!DT->isReachableFromEntry(BB)) {
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        DT = nullptr;
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        break;
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      }
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    }
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  }
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  // We can't skip directly from a block that dominates the stop block if the
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  // exclusion block is potentially in between.
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  if (ExclusionSet && !ExclusionSet->empty())
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    DT = nullptr;
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  // Normally any block in a loop is reachable from any other block in a loop,
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  // however excluded blocks might partition the body of a loop to make that
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  // untrue.
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  SmallPtrSet<const Loop *, 8> LoopsWithHoles;
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  if (LI && ExclusionSet) {
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    for (auto *BB : *ExclusionSet) {
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      if (const Loop *L = getOutermostLoop(LI, BB))
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        LoopsWithHoles.insert(L);
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    }
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  }
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  SmallPtrSet<const Loop *, 2> StopLoops;
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  if (LI) {
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    for (auto *StopSetBB : StopSet) {
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      if (const Loop *L = getOutermostLoop(LI, StopSetBB))
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        StopLoops.insert(L);
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    }
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  }
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  unsigned Limit = DefaultMaxBBsToExplore;
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  SmallPtrSet<const BasicBlock*, 32> Visited;
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  do {
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    BasicBlock *BB = Worklist.pop_back_val();
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    if (!Visited.insert(BB).second)
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      continue;
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    if (StopSet.contains(BB))
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      return true;
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    if (ExclusionSet && ExclusionSet->count(BB))
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      continue;
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    if (DT) {
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      if (llvm::any_of(StopSet, [&](const BasicBlock *StopBB) {
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            return DT->dominates(BB, StopBB);
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          }))
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        return true;
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    }
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    const Loop *Outer = nullptr;
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    if (LI) {
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      Outer = getOutermostLoop(LI, BB);
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      // If we're in a loop with a hole, not all blocks in the loop are
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      // reachable from all other blocks. That implies we can't simply jump to
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      // the loop's exit blocks, as that exit might need to pass through an
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      // excluded block. Clear Outer so we process BB's successors.
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      if (LoopsWithHoles.count(Outer))
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        Outer = nullptr;
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      if (StopLoops.contains(Outer))
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        return true;
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    }
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    if (!--Limit) {
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      // We haven't been able to prove it one way or the other. Conservatively
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      // answer true -- that there is potentially a path.
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      return true;
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    }
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    if (Outer) {
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      // All blocks in a single loop are reachable from all other blocks. From
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      // any of these blocks, we can skip directly to the exits of the loop,
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      // ignoring any other blocks inside the loop body.
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      Outer->getExitBlocks(Worklist);
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    } else {
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      Worklist.append(succ_begin(BB), succ_end(BB));
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    }
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  } while (!Worklist.empty());
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  // We have exhausted all possible paths and are certain that 'To' can not be
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  // reached from 'From'.
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  return false;
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}
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template <class T> class SingleEntrySet {
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public:
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  using const_iterator = const T *;
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  SingleEntrySet(T Elem) : Elem(Elem) {}
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  bool contains(T Other) const { return Elem == Other; }
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  const_iterator begin() const { return &Elem; }
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  const_iterator end() const { return &Elem + 1; }
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private:
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  T Elem;
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};
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bool llvm::isPotentiallyReachableFromMany(
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    SmallVectorImpl<BasicBlock *> &Worklist, const BasicBlock *StopBB,
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    const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
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    const LoopInfo *LI) {
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  return isReachableImpl<SingleEntrySet<const BasicBlock *>>(
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      Worklist, SingleEntrySet<const BasicBlock *>(StopBB), ExclusionSet, DT,
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      LI);
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}
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bool llvm::isManyPotentiallyReachableFromMany(
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    SmallVectorImpl<BasicBlock *> &Worklist,
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    const SmallPtrSetImpl<const BasicBlock *> &StopSet,
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    const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
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    const LoopInfo *LI) {
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  return isReachableImpl<SmallPtrSetImpl<const BasicBlock *>>(
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      Worklist, StopSet, ExclusionSet, DT, LI);
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}
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bool llvm::isPotentiallyReachable(
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    const BasicBlock *A, const BasicBlock *B,
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    const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
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    const LoopInfo *LI) {
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  assert(A->getParent() == B->getParent() &&
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         "This analysis is function-local!");
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  if (DT) {
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    if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B))
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      return false;
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    if (!ExclusionSet || ExclusionSet->empty()) {
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      if (A->isEntryBlock() && DT->isReachableFromEntry(B))
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        return true;
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      if (B->isEntryBlock() && DT->isReachableFromEntry(A))
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        return false;
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    }
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  }
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  SmallVector<BasicBlock*, 32> Worklist;
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  Worklist.push_back(const_cast<BasicBlock*>(A));
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  return isPotentiallyReachableFromMany(Worklist, B, ExclusionSet, DT, LI);
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}
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bool llvm::isPotentiallyReachable(
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    const Instruction *A, const Instruction *B,
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    const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
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    const LoopInfo *LI) {
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  assert(A->getParent()->getParent() == B->getParent()->getParent() &&
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         "This analysis is function-local!");
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  if (A->getParent() == B->getParent()) {
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    // The same block case is special because it's the only time we're looking
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    // within a single block to see which instruction comes first. Once we
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    // start looking at multiple blocks, the first instruction of the block is
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    // reachable, so we only need to determine reachability between whole
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    // blocks.
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    BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
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    // If the block is in a loop then we can reach any instruction in the block
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    // from any other instruction in the block by going around a backedge.
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    if (LI && LI->getLoopFor(BB) != nullptr)
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      return true;
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    // If A comes before B, then B is definitively reachable from A.
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    if (A == B || A->comesBefore(B))
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      return true;
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    // Can't be in a loop if it's the entry block -- the entry block may not
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    // have predecessors.
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    if (BB->isEntryBlock())
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      return false;
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    // Otherwise, continue doing the normal per-BB CFG walk.
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    SmallVector<BasicBlock*, 32> Worklist;
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    Worklist.append(succ_begin(BB), succ_end(BB));
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    if (Worklist.empty()) {
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      // We've proven that there's no path!
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      return false;
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    }
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    return isPotentiallyReachableFromMany(Worklist, B->getParent(),
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                                          ExclusionSet, DT, LI);
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  }
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  return isPotentiallyReachable(
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      A->getParent(), B->getParent(), ExclusionSet, DT, LI);
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}
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