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 21using 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. 28void 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. 73unsigned 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. 88bool 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. 119static const Loop *getOutermostLoop(const 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. 129static bool loopContainsBoth(const 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 136static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist, 137 BasicBlock *StopBB, 138 const DominatorTree *DT, 139 const LoopInfo *LI) { 140 // When the stop block is unreachable, it's dominated from everywhere, 141 // regardless of whether there's a path between the two blocks. 142 if (DT && !DT->isReachableFromEntry(StopBB)) 143 DT = 0; 144 145 // Limit the number of blocks we visit. The goal is to avoid run-away compile 146 // times on large CFGs without hampering sensible code. Arbitrarily chosen. 147 unsigned Limit = 32; 148 SmallSet<const BasicBlock*, 64> Visited; 149 do { 150 BasicBlock *BB = Worklist.pop_back_val(); 151 if (!Visited.insert(BB)) 152 continue; 153 if (BB == StopBB) 154 return true; 155 if (DT && DT->dominates(BB, StopBB)) 156 return true; 157 if (LI && loopContainsBoth(LI, BB, StopBB)) 158 return true; 159 160 if (!--Limit) { 161 // We haven't been able to prove it one way or the other. Conservatively 162 // answer true -- that there is potentially a path. 163 return true; 164 } 165 166 if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) { 167 // All blocks in a single loop are reachable from all other blocks. From 168 // any of these blocks, we can skip directly to the exits of the loop, 169 // ignoring any other blocks inside the loop body. 170 Outer->getExitBlocks(Worklist); 171 } else { 172 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) 173 Worklist.push_back(*I); 174 } 175 } while (!Worklist.empty()); 176 177 // We have exhausted all possible paths and are certain that 'To' can not be 178 // reached from 'From'. 179 return false; 180} 181 182bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B, 183 const DominatorTree *DT, const LoopInfo *LI) { 184 assert(A->getParent() == B->getParent() && 185 "This analysis is function-local!"); 186 187 SmallVector<BasicBlock*, 32> Worklist; 188 Worklist.push_back(const_cast<BasicBlock*>(A)); 189 190 return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B), 191 DT, LI); 192} 193 194bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, 195 const DominatorTree *DT, const LoopInfo *LI) { 196 assert(A->getParent()->getParent() == B->getParent()->getParent() && 197 "This analysis is function-local!"); 198 199 SmallVector<BasicBlock*, 32> Worklist; 200 201 if (A->getParent() == B->getParent()) { 202 // The same block case is special because it's the only time we're looking 203 // within a single block to see which instruction comes first. Once we 204 // start looking at multiple blocks, the first instruction of the block is 205 // reachable, so we only need to determine reachability between whole 206 // blocks. 207 BasicBlock *BB = const_cast<BasicBlock *>(A->getParent()); 208 209 // If the block is in a loop then we can reach any instruction in the block 210 // from any other instruction in the block by going around a backedge. 211 if (LI && LI->getLoopFor(BB) != 0) 212 return true; 213 214 // Linear scan, start at 'A', see whether we hit 'B' or the end first. 215 for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) { 216 if (&*I == B) 217 return true; 218 } 219 220 // Can't be in a loop if it's the entry block -- the entry block may not 221 // have predecessors. 222 if (BB == &BB->getParent()->getEntryBlock()) 223 return false; 224 225 // Otherwise, continue doing the normal per-BB CFG walk. 226 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) 227 Worklist.push_back(*I); 228 229 if (Worklist.empty()) { 230 // We've proven that there's no path! 231 return false; 232 } 233 } else { 234 Worklist.push_back(const_cast<BasicBlock*>(A->getParent())); 235 } 236 237 if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) 238 return true; 239 if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) 240 return false; 241 242 return isPotentiallyReachableInner(Worklist, 243 const_cast<BasicBlock*>(B->getParent()), 244 DT, LI); 245} 246