llvm/lib/Analysis/CFG.cpp

   1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This family of functions performs analyses on basic blocks, and instructions
  10 // contained within basic blocks.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Analysis/CFG.h"
  15 #include "llvm/Analysis/LoopInfo.h"
  16 #include "llvm/IR/Dominators.h"
  17 #include "llvm/Support/CommandLine.h"
  18
  19 using namespace llvm;
  20
  21 // The max number of basic blocks explored during reachability analysis between
  22 // two basic blocks. This is kept reasonably small to limit compile time when
  23 // repeatedly used by clients of this analysis (such as captureTracking).
  24 static cl::opt<unsigned> DefaultMaxBBsToExplore(
  25     "dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
  26     cl::desc("Max number of BBs to explore for reachability analysis"),
  27     cl::init(32));
  28
  29 /// FindFunctionBackedges - Analyze the specified function to find all of the
  30 /// loop backedges in the function and return them.  This is a relatively cheap
  31 /// (compared to computing dominators and loop info) analysis.
  32 ///
  33 /// The output is added to Result, as pairs of <from,to> edge info.
  34 void llvm::FindFunctionBackedges(const Function &F,
  35      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
  36   const BasicBlock *BB = &F.getEntryBlock();
  37   if (succ_empty(BB))
  38     return;
  39
  40   SmallPtrSet<const BasicBlock*, 8> Visited;
  41   SmallVector<std::pair<const BasicBlock *, const_succ_iterator>, 8> VisitStack;
  42   SmallPtrSet<const BasicBlock*, 8> InStack;
  43
  44   Visited.insert(BB);
  45   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
  46   InStack.insert(BB);
  47   do {
  48     std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
  49     const BasicBlock *ParentBB = Top.first;
  50     const_succ_iterator &I = Top.second;
  51
  52     bool FoundNew = false;
  53     while (I != succ_end(ParentBB)) {
  54       BB = *I++;
  55       if (Visited.insert(BB).second) {
  56         FoundNew = true;
  57         break;
  58       }
  59       // Successor is in VisitStack, it's a back edge.
  60       if (InStack.count(BB))
  61         Result.push_back(std::make_pair(ParentBB, BB));
  62     }
  63
  64     if (FoundNew) {
  65       // Go down one level if there is a unvisited successor.
  66       InStack.insert(BB);
  67       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
  68     } else {
  69       // Go up one level.
  70       InStack.erase(VisitStack.pop_back_val().first);
  71     }
  72   } while (!VisitStack.empty());
  73 }
  74
  75 /// GetSuccessorNumber - Search for the specified successor of basic block BB
  76 /// and return its position in the terminator instruction's list of
  77 /// successors.  It is an error to call this with a block that is not a
  78 /// successor.
  79 unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
  80     const BasicBlock *Succ) {
  81   const Instruction *Term = BB->getTerminator();
  82 #ifndef NDEBUG
  83   unsigned e = Term->getNumSuccessors();
  84 #endif
  85   for (unsigned i = 0; ; ++i) {
  86     assert(i != e && "Didn't find edge?");
  87     if (Term->getSuccessor(i) == Succ)
  88       return i;
  89   }
  90 }
  91
  92 /// isCriticalEdge - Return true if the specified edge is a critical edge.
  93 /// Critical edges are edges from a block with multiple successors to a block
  94 /// with multiple predecessors.
  95 bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
  96                           bool AllowIdenticalEdges) {
  97   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
  98   return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
  99 }
 100
 101 bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
 102                           bool AllowIdenticalEdges) {
 103   assert(TI->isTerminator() && "Must be a terminator to have successors!");
 104   if (TI->getNumSuccessors() == 1) return false;
 105
 106   assert(is_contained(predecessors(Dest), TI->getParent()) &&
 107          "No edge between TI's block and Dest.");
 108
 109   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
 110
 111   // If there is more than one predecessor, this is a critical edge...
 112   assert(I != E && "No preds, but we have an edge to the block?");
 113   const BasicBlock *FirstPred = *I;
 114   ++I;        // Skip one edge due to the incoming arc from TI.
 115   if (!AllowIdenticalEdges)
 116     return I != E;
 117
 118   // If AllowIdenticalEdges is true, then we allow this edge to be considered
 119   // non-critical iff all preds come from TI's block.
 120   for (; I != E; ++I)
 121     if (*I != FirstPred)
 122       return true;
 123   return false;
 124 }
 125
 126 // LoopInfo contains a mapping from basic block to the innermost loop. Find
 127 // the outermost loop in the loop nest that contains BB.
 128 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
 129   const Loop *L = LI->getLoopFor(BB);
 130   return L ? L->getOutermostLoop() : nullptr;
 131 }
 132
 133 bool llvm::isPotentiallyReachableFromMany(
 134     SmallVectorImpl<BasicBlock *> &Worklist, const BasicBlock *StopBB,
 135     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
 136     const LoopInfo *LI) {
 137   // When the stop block is unreachable, it's dominated from everywhere,
 138   // regardless of whether there's a path between the two blocks.
 139   if (DT && !DT->isReachableFromEntry(StopBB))
 140     DT = nullptr;
 141
 142   // We can't skip directly from a block that dominates the stop block if the
 143   // exclusion block is potentially in between.
 144   if (ExclusionSet && !ExclusionSet->empty())
 145     DT = nullptr;
 146
 147   // Normally any block in a loop is reachable from any other block in a loop,
 148   // however excluded blocks might partition the body of a loop to make that
 149   // untrue.
 150   SmallPtrSet<const Loop *, 8> LoopsWithHoles;
 151   if (LI && ExclusionSet) {
 152     for (auto *BB : *ExclusionSet) {
 153       if (const Loop *L = getOutermostLoop(LI, BB))
 154         LoopsWithHoles.insert(L);
 155     }
 156   }
 157
 158   const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
 159
 160   unsigned Limit = DefaultMaxBBsToExplore;
 161   SmallPtrSet<const BasicBlock*, 32> Visited;
 162   do {
 163     BasicBlock *BB = Worklist.pop_back_val();
 164     if (!Visited.insert(BB).second)
 165       continue;
 166     if (BB == StopBB)
 167       return true;
 168     if (ExclusionSet && ExclusionSet->count(BB))
 169       continue;
 170     if (DT && DT->dominates(BB, StopBB))
 171       return true;
 172
 173     const Loop *Outer = nullptr;
 174     if (LI) {
 175       Outer = getOutermostLoop(LI, BB);
 176       // If we're in a loop with a hole, not all blocks in the loop are
 177       // reachable from all other blocks. That implies we can't simply jump to
 178       // the loop's exit blocks, as that exit might need to pass through an
 179       // excluded block. Clear Outer so we process BB's successors.
 180       if (LoopsWithHoles.count(Outer))
 181         Outer = nullptr;
 182       if (StopLoop && Outer == StopLoop)
 183         return true;
 184     }
 185
 186     if (!--Limit) {
 187       // We haven't been able to prove it one way or the other. Conservatively
 188       // answer true -- that there is potentially a path.
 189       return true;
 190     }
 191
 192     if (Outer) {
 193       // All blocks in a single loop are reachable from all other blocks. From
 194       // any of these blocks, we can skip directly to the exits of the loop,
 195       // ignoring any other blocks inside the loop body.
 196       Outer->getExitBlocks(Worklist);
 197     } else {
 198       Worklist.append(succ_begin(BB), succ_end(BB));
 199     }
 200   } while (!Worklist.empty());
 201
 202   // We have exhausted all possible paths and are certain that 'To' can not be
 203   // reached from 'From'.
 204   return false;
 205 }
 206
 207 bool llvm::isPotentiallyReachable(
 208     const BasicBlock *A, const BasicBlock *B,
 209     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
 210     const LoopInfo *LI) {
 211   assert(A->getParent() == B->getParent() &&
 212          "This analysis is function-local!");
 213
 214   if (DT) {
 215     if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B))
 216       return false;
 217     if (!ExclusionSet || ExclusionSet->empty()) {
 218       if (A->isEntryBlock() && DT->isReachableFromEntry(B))
 219         return true;
 220       if (B->isEntryBlock() && DT->isReachableFromEntry(A))
 221         return false;
 222     }
 223   }
 224
 225   SmallVector<BasicBlock*, 32> Worklist;
 226   Worklist.push_back(const_cast<BasicBlock*>(A));
 227
 228   return isPotentiallyReachableFromMany(Worklist, B, ExclusionSet, DT, LI);
 229 }
 230
 231 bool llvm::isPotentiallyReachable(
 232     const Instruction *A, const Instruction *B,
 233     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
 234     const LoopInfo *LI) {
 235   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
 236          "This analysis is function-local!");
 237
 238   if (A->getParent() == B->getParent()) {
 239     // The same block case is special because it's the only time we're looking
 240     // within a single block to see which instruction comes first. Once we
 241     // start looking at multiple blocks, the first instruction of the block is
 242     // reachable, so we only need to determine reachability between whole
 243     // blocks.
 244     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
 245
 246     // If the block is in a loop then we can reach any instruction in the block
 247     // from any other instruction in the block by going around a backedge.
 248     if (LI && LI->getLoopFor(BB) != nullptr)
 249       return true;
 250
 251     // If A comes before B, then B is definitively reachable from A.
 252     if (A == B || A->comesBefore(B))
 253       return true;
 254
 255     // Can't be in a loop if it's the entry block -- the entry block may not
 256     // have predecessors.
 257     if (BB->isEntryBlock())
 258       return false;
 259
 260     // Otherwise, continue doing the normal per-BB CFG walk.
 261     SmallVector<BasicBlock*, 32> Worklist;
 262     Worklist.append(succ_begin(BB), succ_end(BB));
 263     if (Worklist.empty()) {
 264       // We've proven that there's no path!
 265       return false;
 266     }
 267
 268     return isPotentiallyReachableFromMany(Worklist, B->getParent(),
 269                                           ExclusionSet, DT, LI);
 270   }
 271
 272   return isPotentiallyReachable(
 273       A->getParent(), B->getParent(), ExclusionSet, DT, LI);
 274 }