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 } 237
