lib/Analysis/CFG.cpp

   1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This family of functions performs analyses on basic blocks, and instructions
  11 // contained within basic blocks.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/Analysis/CFG.h"
  16 #include "llvm/ADT/SmallSet.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 #include "llvm/IR/Dominators.h"
  19
  20 using namespace llvm;
  21
  22 /// FindFunctionBackedges - Analyze the specified function to find all of the
  23 /// loop backedges in the function and return them.  This is a relatively cheap
  24 /// (compared to computing dominators and loop info) analysis.
  25 ///
  26 /// The output is added to Result, as pairs of <from,to> edge info.
  27 void llvm::FindFunctionBackedges(const Function &F,
  28      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
  29   const BasicBlock *BB = &F.getEntryBlock();
  30   if (succ_empty(BB))
  31     return;
  32
  33   SmallPtrSet<const BasicBlock*, 8> Visited;
  34   SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
  35   SmallPtrSet<const BasicBlock*, 8> InStack;
  36
  37   Visited.insert(BB);
  38   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
  39   InStack.insert(BB);
  40   do {
  41     std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
  42     const BasicBlock *ParentBB = Top.first;
  43     succ_const_iterator &I = Top.second;
  44
  45     bool FoundNew = false;
  46     while (I != succ_end(ParentBB)) {
  47       BB = *I++;
  48       if (Visited.insert(BB).second) {
  49         FoundNew = true;
  50         break;
  51       }
  52       // Successor is in VisitStack, it's a back edge.
  53       if (InStack.count(BB))
  54         Result.push_back(std::make_pair(ParentBB, BB));
  55     }
  56
  57     if (FoundNew) {
  58       // Go down one level if there is a unvisited successor.
  59       InStack.insert(BB);
  60       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
  61     } else {
  62       // Go up one level.
  63       InStack.erase(VisitStack.pop_back_val().first);
  64     }
  65   } while (!VisitStack.empty());
  66 }
  67
  68 /// GetSuccessorNumber - Search for the specified successor of basic block BB
  69 /// and return its position in the terminator instruction's list of
  70 /// successors.  It is an error to call this with a block that is not a
  71 /// successor.
  72 unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
  73     const BasicBlock *Succ) {
  74   const TerminatorInst *Term = BB->getTerminator();
  75 #ifndef NDEBUG
  76   unsigned e = Term->getNumSuccessors();
  77 #endif
  78   for (unsigned i = 0; ; ++i) {
  79     assert(i != e && "Didn't find edge?");
  80     if (Term->getSuccessor(i) == Succ)
  81       return i;
  82   }
  83 }
  84
  85 /// isCriticalEdge - Return true if the specified edge is a critical edge.
  86 /// Critical edges are edges from a block with multiple successors to a block
  87 /// with multiple predecessors.
  88 bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
  89                           bool AllowIdenticalEdges) {
  90   assert(TI->isTerminator() && "Must be a terminator to have successors!");
  91   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
  92   if (TI->getNumSuccessors() == 1) return false;
  93
  94   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
  95   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
  96
  97   // If there is more than one predecessor, this is a critical edge...
  98   assert(I != E && "No preds, but we have an edge to the block?");
  99   const BasicBlock *FirstPred = *I;
 100   ++I;        // Skip one edge due to the incoming arc from TI.
 101   if (!AllowIdenticalEdges)
 102     return I != E;
 103
 104   // If AllowIdenticalEdges is true, then we allow this edge to be considered
 105   // non-critical iff all preds come from TI's block.
 106   for (; I != E; ++I)
 107     if (*I != FirstPred)
 108       return true;
 109   return false;
 110 }
 111
 112 // LoopInfo contains a mapping from basic block to the innermost loop. Find
 113 // the outermost loop in the loop nest that contains BB.
 114 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
 115   const Loop *L = LI->getLoopFor(BB);
 116   if (L) {
 117     while (const Loop *Parent = L->getParentLoop())
 118       L = Parent;
 119   }
 120   return L;
 121 }
 122
 123 // True if there is a loop which contains both BB1 and BB2.
 124 static bool loopContainsBoth(const LoopInfo *LI,
 125                              const BasicBlock *BB1, const BasicBlock *BB2) {
 126   const Loop *L1 = getOutermostLoop(LI, BB1);
 127   const Loop *L2 = getOutermostLoop(LI, BB2);
 128   return L1 != nullptr && L1 == L2;
 129 }
 130
 131 bool llvm::isPotentiallyReachableFromMany(
 132     SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
 133     const DominatorTree *DT, const LoopInfo *LI) {
 134   // When the stop block is unreachable, it's dominated from everywhere,
 135   // regardless of whether there's a path between the two blocks.
 136   if (DT && !DT->isReachableFromEntry(StopBB))
 137     DT = nullptr;
 138
 139   // Limit the number of blocks we visit. The goal is to avoid run-away compile
 140   // times on large CFGs without hampering sensible code. Arbitrarily chosen.
 141   unsigned Limit = 32;
 142   SmallPtrSet<const BasicBlock*, 32> Visited;
 143   do {
 144     BasicBlock *BB = Worklist.pop_back_val();
 145     if (!Visited.insert(BB).second)
 146       continue;
 147     if (BB == StopBB)
 148       return true;
 149     if (DT && DT->dominates(BB, StopBB))
 150       return true;
 151     if (LI && loopContainsBoth(LI, BB, StopBB))
 152       return true;
 153
 154     if (!--Limit) {
 155       // We haven't been able to prove it one way or the other. Conservatively
 156       // answer true -- that there is potentially a path.
 157       return true;
 158     }
 159
 160     if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : nullptr) {
 161       // All blocks in a single loop are reachable from all other blocks. From
 162       // any of these blocks, we can skip directly to the exits of the loop,
 163       // ignoring any other blocks inside the loop body.
 164       Outer->getExitBlocks(Worklist);
 165     } else {
 166       Worklist.append(succ_begin(BB), succ_end(BB));
 167     }
 168   } while (!Worklist.empty());
 169
 170   // We have exhausted all possible paths and are certain that 'To' can not be
 171   // reached from 'From'.
 172   return false;
 173 }
 174
 175 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
 176                                   const DominatorTree *DT, const LoopInfo *LI) {
 177   assert(A->getParent() == B->getParent() &&
 178          "This analysis is function-local!");
 179
 180   SmallVector<BasicBlock*, 32> Worklist;
 181   Worklist.push_back(const_cast<BasicBlock*>(A));
 182
 183   return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
 184                                         DT, LI);
 185 }
 186
 187 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
 188                                   const DominatorTree *DT, const LoopInfo *LI) {
 189   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
 190          "This analysis is function-local!");
 191
 192   SmallVector<BasicBlock*, 32> Worklist;
 193
 194   if (A->getParent() == B->getParent()) {
 195     // The same block case is special because it's the only time we're looking
 196     // within a single block to see which instruction comes first. Once we
 197     // start looking at multiple blocks, the first instruction of the block is
 198     // reachable, so we only need to determine reachability between whole
 199     // blocks.
 200     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
 201
 202     // If the block is in a loop then we can reach any instruction in the block
 203     // from any other instruction in the block by going around a backedge.
 204     if (LI && LI->getLoopFor(BB) != nullptr)
 205       return true;
 206
 207     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
 208     for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
 209          ++I) {
 210       if (&*I == B)
 211         return true;
 212     }
 213
 214     // Can't be in a loop if it's the entry block -- the entry block may not
 215     // have predecessors.
 216     if (BB == &BB->getParent()->getEntryBlock())
 217       return false;
 218
 219     // Otherwise, continue doing the normal per-BB CFG walk.
 220     Worklist.append(succ_begin(BB), succ_end(BB));
 221
 222     if (Worklist.empty()) {
 223       // We've proven that there's no path!
 224       return false;
 225     }
 226   } else {
 227     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
 228   }
 229
 230   if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
 231     return true;
 232   if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
 233     return false;
 234
 235   return isPotentiallyReachableFromMany(
 236       Worklist, const_cast<BasicBlock *>(B->getParent()), DT, LI);
 237 }