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 17 #include "llvm/ADT/SmallSet.h" 18 #include "llvm/Analysis/Dominators.h" 19 #include "llvm/Analysis/LoopInfo.h" 20 21 using namespace llvm; 22 23 /// FindFunctionBackedges - Analyze the specified function to find all of the 24 /// loop backedges in the function and return them. This is a relatively cheap 25 /// (compared to computing dominators and loop info) analysis. 26 /// 27 /// The output is added to Result, as pairs of <from,to> edge info. 28 void llvm::FindFunctionBackedges(const Function &F, 29 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { 30 const BasicBlock *BB = &F.getEntryBlock(); 31 if (succ_begin(BB) == succ_end(BB)) 32 return; 33 34 SmallPtrSet<const BasicBlock*, 8> Visited; 35 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; 36 SmallPtrSet<const BasicBlock*, 8> InStack; 37 38 Visited.insert(BB); 39 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 40 InStack.insert(BB); 41 do { 42 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); 43 const BasicBlock *ParentBB = Top.first; 44 succ_const_iterator &I = Top.second; 45 46 bool FoundNew = false; 47 while (I != succ_end(ParentBB)) { 48 BB = *I++; 49 if (Visited.insert(BB)) { 50 FoundNew = true; 51 break; 52 } 53 // Successor is in VisitStack, it's a back edge. 54 if (InStack.count(BB)) 55 Result.push_back(std::make_pair(ParentBB, BB)); 56 } 57 58 if (FoundNew) { 59 // Go down one level if there is a unvisited successor. 60 InStack.insert(BB); 61 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 62 } else { 63 // Go up one level. 64 InStack.erase(VisitStack.pop_back_val().first); 65 } 66 } while (!VisitStack.empty()); 67 } 68 69 /// GetSuccessorNumber - Search for the specified successor of basic block BB 70 /// and return its position in the terminator instruction's list of 71 /// successors. It is an error to call this with a block that is not a 72 /// successor. 73 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) { 74 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 while (I != E) { 106 const BasicBlock *P = *I; 107 if (P != FirstPred) 108 return true; 109 // Note: leave this as is until no one ever compiles with either gcc 4.0.1 110 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207 111 E = pred_end(P); 112 ++I; 113 } 114 return false; 115 } 116 117 // LoopInfo contains a mapping from basic block to the innermost loop. Find 118 // the outermost loop in the loop nest that contains BB. 119 static const Loop *getOutermostLoop(LoopInfo *LI, const BasicBlock *BB) { 120 const Loop *L = LI->getLoopFor(BB); 121 if (L) { 122 while (const Loop *Parent = L->getParentLoop()) 123 L = Parent; 124 } 125 return L; 126 } 127 128 // True if there is a loop which contains both BB1 and BB2. 129 static bool loopContainsBoth(LoopInfo *LI, 130 const BasicBlock *BB1, const BasicBlock *BB2) { 131 const Loop *L1 = getOutermostLoop(LI, BB1); 132 const Loop *L2 = getOutermostLoop(LI, BB2); 133 return L1 != NULL && L1 == L2; 134 } 135 136 static bool isPotentiallyReachableSameBlock(const Instruction *A, 137 const Instruction *B, 138 LoopInfo *LI) { 139 // The same block case is special because it's the only time we're looking 140 // within a single block to see which comes first. Once we start looking at 141 // multiple blocks, the first instruction of the block is reachable, so we 142 // only need to determine reachability between whole blocks. 143 144 const BasicBlock *BB = A->getParent(); 145 // If the block is in a loop then we can reach any instruction in the block 146 // from any other instruction in the block by going around the backedge. 147 // Check whether we're in a loop (or aren't sure). 148 149 // Can't be in a loop if it's the entry block -- the entry block may not 150 // have predecessors. 151 bool HasLoop = BB != &BB->getParent()->getEntryBlock(); 152 153 // Can't be in a loop if LoopInfo doesn't know about it. 154 if (LI && HasLoop) { 155 HasLoop = LI->getLoopFor(BB) != 0; 156 } 157 if (HasLoop) 158 return true; 159 160 // Linear scan, start at 'A', see whether we hit 'B' or the end first. 161 for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) { 162 if (&*I == B) 163 return true; 164 } 165 return false; 166 } 167 168 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, 169 DominatorTree *DT, LoopInfo *LI) { 170 assert(A->getParent()->getParent() == B->getParent()->getParent() && 171 "This analysis is function-local!"); 172 173 const BasicBlock *StopBB = B->getParent(); 174 175 if (A->getParent() == B->getParent()) 176 return isPotentiallyReachableSameBlock(A, B, LI); 177 178 if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) 179 return true; 180 if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) 181 return false; 182 183 // When the stop block is unreachable, it's dominated from everywhere, 184 // regardless of whether there's a path between the two blocks. 185 if (DT && !DT->isReachableFromEntry(StopBB)) 186 DT = 0; 187 188 // Limit the number of blocks we visit. The goal is to avoid run-away compile 189 // times on large CFGs without hampering sensible code. Arbitrarily chosen. 190 unsigned Limit = 32; 191 192 SmallSet<const BasicBlock*, 64> Visited; 193 SmallVector<BasicBlock*, 32> Worklist; 194 Worklist.push_back(const_cast<BasicBlock*>(A->getParent())); 195 196 do { 197 BasicBlock *BB = Worklist.pop_back_val(); 198 if (!Visited.insert(BB)) 199 continue; 200 if (BB == StopBB) 201 return true; 202 if (DT && DT->dominates(BB, StopBB)) 203 return true; 204 if (LI && loopContainsBoth(LI, BB, StopBB)) 205 return true; 206 207 if (!--Limit) { 208 // We haven't been able to prove it one way or the other. Conservatively 209 // answer true -- that there is potentially a path. 210 return true; 211 } 212 213 if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) { 214 // All blocks in a single loop are reachable from all other blocks. From 215 // any of these blocks, we can skip directly to the exits of the loop, 216 // ignoring any other blocks inside the loop body. 217 Outer->getExitBlocks(Worklist); 218 } else { 219 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) 220 Worklist.push_back(*I); 221 } 222 } while (!Worklist.empty()); 223 224 // We have exhaustived all possible paths and are certain that 'To' can not 225 // be reached from 'From'. 226 return false; 227 } 228
