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 TerminatorInst *TI, unsigned SuccNum,
  89                           bool AllowIdenticalEdges) {
  90   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
  91   if (TI->getNumSuccessors() == 1) return false;
  92
  93   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
  94   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
  95
  96   // If there is more than one predecessor, this is a critical edge...
  97   assert(I != E && "No preds, but we have an edge to the block?");
  98   const BasicBlock *FirstPred = *I;
  99   ++I;        // Skip one edge due to the incoming arc from TI.
 100   if (!AllowIdenticalEdges)
 101     return I != E;
 102
 103   // If AllowIdenticalEdges is true, then we allow this edge to be considered
 104   // non-critical iff all preds come from TI's block.
 105   for (; I != E; ++I)
 106     if (*I != FirstPred)
 107       return true;
 108   return false;
 109 }
 110
 111 // LoopInfo contains a mapping from basic block to the innermost loop. Find
 112 // the outermost loop in the loop nest that contains BB.
 113 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
 114   const Loop *L = LI->getLoopFor(BB);
 115   if (L) {
 116     while (const Loop *Parent = L->getParentLoop())
 117       L = Parent;
 118   }
 119   return L;
 120 }
 121
 122 // True if there is a loop which contains both BB1 and BB2.
 123 static bool loopContainsBoth(const LoopInfo *LI,
 124                              const BasicBlock *BB1, const BasicBlock *BB2) {
 125   const Loop *L1 = getOutermostLoop(LI, BB1);
 126   const Loop *L2 = getOutermostLoop(LI, BB2);
 127   return L1 != nullptr && L1 == L2;
 128 }
 129
 130 bool llvm::isPotentiallyReachableFromMany(
 131     SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
 132     const DominatorTree *DT, const LoopInfo *LI) {
 133   // When the stop block is unreachable, it's dominated from everywhere,
 134   // regardless of whether there's a path between the two blocks.
 135   if (DT && !DT->isReachableFromEntry(StopBB))
 136     DT = nullptr;
 137
 138   // Limit the number of blocks we visit. The goal is to avoid run-away compile
 139   // times on large CFGs without hampering sensible code. Arbitrarily chosen.
 140   unsigned Limit = 32;
 141   SmallPtrSet<const BasicBlock*, 32> Visited;
 142   do {
 143     BasicBlock *BB = Worklist.pop_back_val();
 144     if (!Visited.insert(BB).second)
 145       continue;
 146     if (BB == StopBB)
 147       return true;
 148     if (DT && DT->dominates(BB, StopBB))
 149       return true;
 150     if (LI && loopContainsBoth(LI, BB, StopBB))
 151       return true;
 152
 153     if (!--Limit) {
 154       // We haven't been able to prove it one way or the other. Conservatively
 155       // answer true -- that there is potentially a path.
 156       return true;
 157     }
 158
 159     if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : nullptr) {
 160       // All blocks in a single loop are reachable from all other blocks. From
 161       // any of these blocks, we can skip directly to the exits of the loop,
 162       // ignoring any other blocks inside the loop body.
 163       Outer->getExitBlocks(Worklist);
 164     } else {
 165       Worklist.append(succ_begin(BB), succ_end(BB));
 166     }
 167   } while (!Worklist.empty());
 168
 169   // We have exhausted all possible paths and are certain that 'To' can not be
 170   // reached from 'From'.
 171   return false;
 172 }
 173
 174 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
 175                                   const DominatorTree *DT, const LoopInfo *LI) {
 176   assert(A->getParent() == B->getParent() &&
 177          "This analysis is function-local!");
 178
 179   SmallVector<BasicBlock*, 32> Worklist;
 180   Worklist.push_back(const_cast<BasicBlock*>(A));
 181
 182   return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
 183                                         DT, LI);
 184 }
 185
 186 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
 187                                   const DominatorTree *DT, const LoopInfo *LI) {
 188   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
 189          "This analysis is function-local!");
 190
 191   SmallVector<BasicBlock*, 32> Worklist;
 192
 193   if (A->getParent() == B->getParent()) {
 194     // The same block case is special because it's the only time we're looking
 195     // within a single block to see which instruction comes first. Once we
 196     // start looking at multiple blocks, the first instruction of the block is
 197     // reachable, so we only need to determine reachability between whole
 198     // blocks.
 199     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
 200
 201     // If the block is in a loop then we can reach any instruction in the block
 202     // from any other instruction in the block by going around a backedge.
 203     if (LI && LI->getLoopFor(BB) != nullptr)
 204       return true;
 205
 206     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
 207     for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
 208          ++I) {
 209       if (&*I == B)
 210         return true;
 211     }
 212
 213     // Can't be in a loop if it's the entry block -- the entry block may not
 214     // have predecessors.
 215     if (BB == &BB->getParent()->getEntryBlock())
 216       return false;
 217
 218     // Otherwise, continue doing the normal per-BB CFG walk.
 219     Worklist.append(succ_begin(BB), succ_end(BB));
 220
 221     if (Worklist.empty()) {
 222       // We've proven that there's no path!
 223       return false;
 224     }
 225   } else {
 226     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
 227   }
 228
 229   if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
 230     return true;
 231   if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
 232     return false;
 233
 234   return isPotentiallyReachableFromMany(
 235       Worklist, const_cast<BasicBlock *>(B->getParent()), DT, LI);
 236 }