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285 lines
10 KiB
C++
285 lines
10 KiB
C++
//===-- 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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if (L) {
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while (const Loop *Parent = L->getParentLoop())
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L = Parent;
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}
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return L;
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}
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bool llvm::isPotentiallyReachableFromMany(
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SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
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const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
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const LoopInfo *LI) {
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// When the 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 && !DT->isReachableFromEntry(StopBB))
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DT = nullptr;
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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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const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
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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 (BB == StopBB)
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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 && DT->dominates(BB, StopBB))
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return true;
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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 (StopLoop && Outer == StopLoop)
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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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bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
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const DominatorTree *DT, 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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SmallVector<BasicBlock*, 32> Worklist;
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Worklist.push_back(const_cast<BasicBlock*>(A));
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return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
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nullptr, 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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SmallVector<BasicBlock*, 32> Worklist;
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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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// Linear scan, start at 'A', see whether we hit 'B' or the end first.
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for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
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++I) {
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if (&*I == B)
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return true;
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}
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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 == &BB->getParent()->getEntryBlock())
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return false;
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// Otherwise, continue doing the normal per-BB CFG walk.
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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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} else {
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Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
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}
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if (DT) {
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if (DT->isReachableFromEntry(A->getParent()) &&
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!DT->isReachableFromEntry(B->getParent()))
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return false;
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if (!ExclusionSet || ExclusionSet->empty()) {
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if (A->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
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DT->isReachableFromEntry(B->getParent()))
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return true;
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if (B->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
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DT->isReachableFromEntry(A->getParent()))
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return false;
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}
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}
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return isPotentiallyReachableFromMany(
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Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
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}
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