//===-- CFG.cpp - BasicBlock analysis --------------------------------------==// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This family of functions performs analyses on basic blocks, and instructions // contained within basic blocks. // //===----------------------------------------------------------------------===// #include "llvm/Analysis/CFG.h" #include "llvm/ADT/SmallSet.h" #include "llvm/Analysis/Dominators.h" #include "llvm/Analysis/LoopInfo.h" using namespace llvm; /// FindFunctionBackedges - Analyze the specified function to find all of the /// loop backedges in the function and return them. This is a relatively cheap /// (compared to computing dominators and loop info) analysis. /// /// The output is added to Result, as pairs of edge info. void llvm::FindFunctionBackedges(const Function &F, SmallVectorImpl > &Result) { const BasicBlock *BB = &F.getEntryBlock(); if (succ_begin(BB) == succ_end(BB)) return; SmallPtrSet Visited; SmallVector, 8> VisitStack; SmallPtrSet InStack; Visited.insert(BB); VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); InStack.insert(BB); do { std::pair &Top = VisitStack.back(); const BasicBlock *ParentBB = Top.first; succ_const_iterator &I = Top.second; bool FoundNew = false; while (I != succ_end(ParentBB)) { BB = *I++; if (Visited.insert(BB)) { FoundNew = true; break; } // Successor is in VisitStack, it's a back edge. if (InStack.count(BB)) Result.push_back(std::make_pair(ParentBB, BB)); } if (FoundNew) { // Go down one level if there is a unvisited successor. InStack.insert(BB); VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); } else { // Go up one level. InStack.erase(VisitStack.pop_back_val().first); } } while (!VisitStack.empty()); } /// GetSuccessorNumber - Search for the specified successor of basic block BB /// and return its position in the terminator instruction's list of /// successors. It is an error to call this with a block that is not a /// successor. unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) { TerminatorInst *Term = BB->getTerminator(); #ifndef NDEBUG unsigned e = Term->getNumSuccessors(); #endif for (unsigned i = 0; ; ++i) { assert(i != e && "Didn't find edge?"); if (Term->getSuccessor(i) == Succ) return i; } } /// isCriticalEdge - Return true if the specified edge is a critical edge. /// Critical edges are edges from a block with multiple successors to a block /// with multiple predecessors. bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, bool AllowIdenticalEdges) { assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!"); if (TI->getNumSuccessors() == 1) return false; const BasicBlock *Dest = TI->getSuccessor(SuccNum); const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest); // If there is more than one predecessor, this is a critical edge... assert(I != E && "No preds, but we have an edge to the block?"); const BasicBlock *FirstPred = *I; ++I; // Skip one edge due to the incoming arc from TI. if (!AllowIdenticalEdges) return I != E; // If AllowIdenticalEdges is true, then we allow this edge to be considered // non-critical iff all preds come from TI's block. while (I != E) { const BasicBlock *P = *I; if (P != FirstPred) return true; // Note: leave this as is until no one ever compiles with either gcc 4.0.1 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207 E = pred_end(P); ++I; } return false; } // LoopInfo contains a mapping from basic block to the innermost loop. Find // the outermost loop in the loop nest that contains BB. static const Loop *getOutermostLoop(LoopInfo *LI, const BasicBlock *BB) { const Loop *L = LI->getLoopFor(BB); if (L) { while (const Loop *Parent = L->getParentLoop()) L = Parent; } return L; } // True if there is a loop which contains both BB1 and BB2. static bool loopContainsBoth(LoopInfo *LI, const BasicBlock *BB1, const BasicBlock *BB2) { const Loop *L1 = getOutermostLoop(LI, BB1); const Loop *L2 = getOutermostLoop(LI, BB2); return L1 != NULL && L1 == L2; } static bool isPotentiallyReachableSameBlock(const Instruction *A, const Instruction *B, LoopInfo *LI) { // The same block case is special because it's the only time we're looking // within a single block to see which comes first. Once we start looking at // multiple blocks, the first instruction of the block is reachable, so we // only need to determine reachability between whole blocks. const BasicBlock *BB = A->getParent(); // If the block is in a loop then we can reach any instruction in the block // from any other instruction in the block by going around the backedge. // Check whether we're in a loop (or aren't sure). // Can't be in a loop if it's the entry block -- the entry block may not // have predecessors. bool HasLoop = BB != &BB->getParent()->getEntryBlock(); // Can't be in a loop if LoopInfo doesn't know about it. if (LI && HasLoop) { HasLoop = LI->getLoopFor(BB) != 0; } if (HasLoop) return true; // Linear scan, start at 'A', see whether we hit 'B' or the end first. for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) { if (&*I == B) return true; } return false; } bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, DominatorTree *DT, LoopInfo *LI) { assert(A->getParent()->getParent() == B->getParent()->getParent() && "This analysis is function-local!"); const BasicBlock *StopBB = B->getParent(); if (A->getParent() == B->getParent()) return isPotentiallyReachableSameBlock(A, B, LI); if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) return true; if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) return false; // When the stop block is unreachable, it's dominated from everywhere, // regardless of whether there's a path between the two blocks. if (DT && !DT->isReachableFromEntry(StopBB)) DT = 0; // Limit the number of blocks we visit. The goal is to avoid run-away compile // times on large CFGs without hampering sensible code. Arbitrarily chosen. unsigned Limit = 32; SmallSet Visited; SmallVector Worklist; Worklist.push_back(const_cast(A->getParent())); do { BasicBlock *BB = Worklist.pop_back_val(); if (!Visited.insert(BB)) continue; if (BB == StopBB) return true; if (DT && DT->dominates(BB, StopBB)) return true; if (LI && loopContainsBoth(LI, BB, StopBB)) return true; if (!--Limit) { // We haven't been able to prove it one way or the other. Conservatively // answer true -- that there is potentially a path. return true; } if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) { // All blocks in a single loop are reachable from all other blocks. From // any of these blocks, we can skip directly to the exits of the loop, // ignoring any other blocks inside the loop body. Outer->getExitBlocks(Worklist); } else { for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) Worklist.push_back(*I); } } while (!Worklist.empty()); // We have exhaustived all possible paths and are certain that 'To' can not // be reached from 'From'. return false; }