1259698Sdim//===-- CFG.cpp - BasicBlock analysis --------------------------------------==// 2259698Sdim// 3259698Sdim// The LLVM Compiler Infrastructure 4259698Sdim// 5259698Sdim// This file is distributed under the University of Illinois Open Source 6259698Sdim// License. See LICENSE.TXT for details. 7259698Sdim// 8259698Sdim//===----------------------------------------------------------------------===// 9259698Sdim// 10259698Sdim// This family of functions performs analyses on basic blocks, and instructions 11259698Sdim// contained within basic blocks. 12259698Sdim// 13259698Sdim//===----------------------------------------------------------------------===// 14259698Sdim 15259698Sdim#include "llvm/Analysis/CFG.h" 16259698Sdim 17259698Sdim#include "llvm/ADT/SmallSet.h" 18259698Sdim#include "llvm/Analysis/Dominators.h" 19259698Sdim#include "llvm/Analysis/LoopInfo.h" 20259698Sdim 21259698Sdimusing namespace llvm; 22259698Sdim 23259698Sdim/// FindFunctionBackedges - Analyze the specified function to find all of the 24259698Sdim/// loop backedges in the function and return them. This is a relatively cheap 25259698Sdim/// (compared to computing dominators and loop info) analysis. 26259698Sdim/// 27259698Sdim/// The output is added to Result, as pairs of <from,to> edge info. 28259698Sdimvoid llvm::FindFunctionBackedges(const Function &F, 29259698Sdim SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { 30259698Sdim const BasicBlock *BB = &F.getEntryBlock(); 31259698Sdim if (succ_begin(BB) == succ_end(BB)) 32259698Sdim return; 33259698Sdim 34259698Sdim SmallPtrSet<const BasicBlock*, 8> Visited; 35259698Sdim SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; 36259698Sdim SmallPtrSet<const BasicBlock*, 8> InStack; 37259698Sdim 38259698Sdim Visited.insert(BB); 39259698Sdim VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 40259698Sdim InStack.insert(BB); 41259698Sdim do { 42259698Sdim std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); 43259698Sdim const BasicBlock *ParentBB = Top.first; 44259698Sdim succ_const_iterator &I = Top.second; 45259698Sdim 46259698Sdim bool FoundNew = false; 47259698Sdim while (I != succ_end(ParentBB)) { 48259698Sdim BB = *I++; 49259698Sdim if (Visited.insert(BB)) { 50259698Sdim FoundNew = true; 51259698Sdim break; 52259698Sdim } 53259698Sdim // Successor is in VisitStack, it's a back edge. 54259698Sdim if (InStack.count(BB)) 55259698Sdim Result.push_back(std::make_pair(ParentBB, BB)); 56259698Sdim } 57259698Sdim 58259698Sdim if (FoundNew) { 59259698Sdim // Go down one level if there is a unvisited successor. 60259698Sdim InStack.insert(BB); 61259698Sdim VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 62259698Sdim } else { 63259698Sdim // Go up one level. 64259698Sdim InStack.erase(VisitStack.pop_back_val().first); 65259698Sdim } 66259698Sdim } while (!VisitStack.empty()); 67259698Sdim} 68259698Sdim 69259698Sdim/// GetSuccessorNumber - Search for the specified successor of basic block BB 70259698Sdim/// and return its position in the terminator instruction's list of 71259698Sdim/// successors. It is an error to call this with a block that is not a 72259698Sdim/// successor. 73259698Sdimunsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) { 74259698Sdim TerminatorInst *Term = BB->getTerminator(); 75259698Sdim#ifndef NDEBUG 76259698Sdim unsigned e = Term->getNumSuccessors(); 77259698Sdim#endif 78259698Sdim for (unsigned i = 0; ; ++i) { 79259698Sdim assert(i != e && "Didn't find edge?"); 80259698Sdim if (Term->getSuccessor(i) == Succ) 81259698Sdim return i; 82259698Sdim } 83259698Sdim} 84259698Sdim 85259698Sdim/// isCriticalEdge - Return true if the specified edge is a critical edge. 86259698Sdim/// Critical edges are edges from a block with multiple successors to a block 87259698Sdim/// with multiple predecessors. 88259698Sdimbool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, 89259698Sdim bool AllowIdenticalEdges) { 90259698Sdim assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!"); 91259698Sdim if (TI->getNumSuccessors() == 1) return false; 92259698Sdim 93259698Sdim const BasicBlock *Dest = TI->getSuccessor(SuccNum); 94259698Sdim const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest); 95259698Sdim 96259698Sdim // If there is more than one predecessor, this is a critical edge... 97259698Sdim assert(I != E && "No preds, but we have an edge to the block?"); 98259698Sdim const BasicBlock *FirstPred = *I; 99259698Sdim ++I; // Skip one edge due to the incoming arc from TI. 100259698Sdim if (!AllowIdenticalEdges) 101259698Sdim return I != E; 102259698Sdim 103259698Sdim // If AllowIdenticalEdges is true, then we allow this edge to be considered 104259698Sdim // non-critical iff all preds come from TI's block. 105259698Sdim while (I != E) { 106259698Sdim const BasicBlock *P = *I; 107259698Sdim if (P != FirstPred) 108259698Sdim return true; 109259698Sdim // Note: leave this as is until no one ever compiles with either gcc 4.0.1 110259698Sdim // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207 111259698Sdim E = pred_end(P); 112259698Sdim ++I; 113259698Sdim } 114259698Sdim return false; 115259698Sdim} 116259698Sdim 117259698Sdim// LoopInfo contains a mapping from basic block to the innermost loop. Find 118259698Sdim// the outermost loop in the loop nest that contains BB. 119259698Sdimstatic const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) { 120259698Sdim const Loop *L = LI->getLoopFor(BB); 121259698Sdim if (L) { 122259698Sdim while (const Loop *Parent = L->getParentLoop()) 123259698Sdim L = Parent; 124259698Sdim } 125259698Sdim return L; 126259698Sdim} 127259698Sdim 128259698Sdim// True if there is a loop which contains both BB1 and BB2. 129259698Sdimstatic bool loopContainsBoth(const LoopInfo *LI, 130259698Sdim const BasicBlock *BB1, const BasicBlock *BB2) { 131259698Sdim const Loop *L1 = getOutermostLoop(LI, BB1); 132259698Sdim const Loop *L2 = getOutermostLoop(LI, BB2); 133259698Sdim return L1 != NULL && L1 == L2; 134259698Sdim} 135259698Sdim 136259698Sdimstatic bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist, 137259698Sdim BasicBlock *StopBB, 138259698Sdim const DominatorTree *DT, 139259698Sdim const LoopInfo *LI) { 140259698Sdim // When the stop block is unreachable, it's dominated from everywhere, 141259698Sdim // regardless of whether there's a path between the two blocks. 142259698Sdim if (DT && !DT->isReachableFromEntry(StopBB)) 143259698Sdim DT = 0; 144259698Sdim 145259698Sdim // Limit the number of blocks we visit. The goal is to avoid run-away compile 146259698Sdim // times on large CFGs without hampering sensible code. Arbitrarily chosen. 147259698Sdim unsigned Limit = 32; 148259698Sdim SmallSet<const BasicBlock*, 64> Visited; 149259698Sdim do { 150259698Sdim BasicBlock *BB = Worklist.pop_back_val(); 151259698Sdim if (!Visited.insert(BB)) 152259698Sdim continue; 153259698Sdim if (BB == StopBB) 154259698Sdim return true; 155259698Sdim if (DT && DT->dominates(BB, StopBB)) 156259698Sdim return true; 157259698Sdim if (LI && loopContainsBoth(LI, BB, StopBB)) 158259698Sdim return true; 159259698Sdim 160259698Sdim if (!--Limit) { 161259698Sdim // We haven't been able to prove it one way or the other. Conservatively 162259698Sdim // answer true -- that there is potentially a path. 163259698Sdim return true; 164259698Sdim } 165259698Sdim 166259698Sdim if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) { 167259698Sdim // All blocks in a single loop are reachable from all other blocks. From 168259698Sdim // any of these blocks, we can skip directly to the exits of the loop, 169259698Sdim // ignoring any other blocks inside the loop body. 170259698Sdim Outer->getExitBlocks(Worklist); 171259698Sdim } else { 172259698Sdim for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) 173259698Sdim Worklist.push_back(*I); 174259698Sdim } 175259698Sdim } while (!Worklist.empty()); 176259698Sdim 177259698Sdim // We have exhausted all possible paths and are certain that 'To' can not be 178259698Sdim // reached from 'From'. 179259698Sdim return false; 180259698Sdim} 181259698Sdim 182259698Sdimbool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B, 183259698Sdim const DominatorTree *DT, const LoopInfo *LI) { 184259698Sdim assert(A->getParent() == B->getParent() && 185259698Sdim "This analysis is function-local!"); 186259698Sdim 187259698Sdim SmallVector<BasicBlock*, 32> Worklist; 188259698Sdim Worklist.push_back(const_cast<BasicBlock*>(A)); 189259698Sdim 190259698Sdim return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B), 191259698Sdim DT, LI); 192259698Sdim} 193259698Sdim 194259698Sdimbool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, 195259698Sdim const DominatorTree *DT, const LoopInfo *LI) { 196259698Sdim assert(A->getParent()->getParent() == B->getParent()->getParent() && 197259698Sdim "This analysis is function-local!"); 198259698Sdim 199259698Sdim SmallVector<BasicBlock*, 32> Worklist; 200259698Sdim 201259698Sdim if (A->getParent() == B->getParent()) { 202259698Sdim // The same block case is special because it's the only time we're looking 203259698Sdim // within a single block to see which instruction comes first. Once we 204259698Sdim // start looking at multiple blocks, the first instruction of the block is 205259698Sdim // reachable, so we only need to determine reachability between whole 206259698Sdim // blocks. 207259698Sdim BasicBlock *BB = const_cast<BasicBlock *>(A->getParent()); 208259698Sdim 209259698Sdim // If the block is in a loop then we can reach any instruction in the block 210259698Sdim // from any other instruction in the block by going around a backedge. 211259698Sdim if (LI && LI->getLoopFor(BB) != 0) 212259698Sdim return true; 213259698Sdim 214259698Sdim // Linear scan, start at 'A', see whether we hit 'B' or the end first. 215259698Sdim for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) { 216259698Sdim if (&*I == B) 217259698Sdim return true; 218259698Sdim } 219259698Sdim 220259698Sdim // Can't be in a loop if it's the entry block -- the entry block may not 221259698Sdim // have predecessors. 222259698Sdim if (BB == &BB->getParent()->getEntryBlock()) 223259698Sdim return false; 224259698Sdim 225259698Sdim // Otherwise, continue doing the normal per-BB CFG walk. 226259698Sdim for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) 227259698Sdim Worklist.push_back(*I); 228259698Sdim 229259698Sdim if (Worklist.empty()) { 230259698Sdim // We've proven that there's no path! 231259698Sdim return false; 232259698Sdim } 233259698Sdim } else { 234259698Sdim Worklist.push_back(const_cast<BasicBlock*>(A->getParent())); 235259698Sdim } 236259698Sdim 237259698Sdim if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) 238259698Sdim return true; 239259698Sdim if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) 240259698Sdim return false; 241259698Sdim 242259698Sdim return isPotentiallyReachableInner(Worklist, 243259698Sdim const_cast<BasicBlock*>(B->getParent()), 244259698Sdim DT, LI); 245259698Sdim} 246