//===-- CFG.cpp - BasicBlock analysis --------------------------------------==// // // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. // See https://llvm.org/LICENSE.txt for license information. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception // //===----------------------------------------------------------------------===// // // This family of functions performs analyses on basic blocks, and instructions // contained within basic blocks. // //===----------------------------------------------------------------------===// #include "llvm/Analysis/CFG.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/IR/Dominators.h" #include "llvm/Support/CommandLine.h" using namespace llvm; // The max number of basic blocks explored during reachability analysis between // two basic blocks. This is kept reasonably small to limit compile time when // repeatedly used by clients of this analysis (such as captureTracking). static cl::opt DefaultMaxBBsToExplore( "dom-tree-reachability-max-bbs-to-explore", cl::Hidden, cl::desc("Max number of BBs to explore for reachability analysis"), cl::init(32)); /// 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_empty(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; const_succ_iterator &I = Top.second; bool FoundNew = false; while (I != succ_end(ParentBB)) { BB = *I++; if (Visited.insert(BB).second) { 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(const BasicBlock *BB, const BasicBlock *Succ) { const Instruction *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 Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges) { assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!"); return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges); } bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest, bool AllowIdenticalEdges) { assert(TI->isTerminator() && "Must be a terminator to have successors!"); if (TI->getNumSuccessors() == 1) return false; assert(is_contained(predecessors(Dest), TI->getParent()) && "No edge between TI's block and Dest."); 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. for (; I != E; ++I) if (*I != FirstPred) return true; 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(const LoopInfo *LI, const BasicBlock *BB) { const Loop *L = LI->getLoopFor(BB); if (L) { while (const Loop *Parent = L->getParentLoop()) L = Parent; } return L; } bool llvm::isPotentiallyReachableFromMany( SmallVectorImpl &Worklist, BasicBlock *StopBB, const SmallPtrSetImpl *ExclusionSet, const DominatorTree *DT, const LoopInfo *LI) { // 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 = nullptr; // We can't skip directly from a block that dominates the stop block if the // exclusion block is potentially in between. if (ExclusionSet && !ExclusionSet->empty()) DT = nullptr; // Normally any block in a loop is reachable from any other block in a loop, // however excluded blocks might partition the body of a loop to make that // untrue. SmallPtrSet LoopsWithHoles; if (LI && ExclusionSet) { for (auto BB : *ExclusionSet) { if (const Loop *L = getOutermostLoop(LI, BB)) LoopsWithHoles.insert(L); } } const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr; unsigned Limit = DefaultMaxBBsToExplore; SmallPtrSet Visited; do { BasicBlock *BB = Worklist.pop_back_val(); if (!Visited.insert(BB).second) continue; if (BB == StopBB) return true; if (ExclusionSet && ExclusionSet->count(BB)) continue; if (DT && DT->dominates(BB, StopBB)) return true; const Loop *Outer = nullptr; if (LI) { Outer = getOutermostLoop(LI, BB); // If we're in a loop with a hole, not all blocks in the loop are // reachable from all other blocks. That implies we can't simply jump to // the loop's exit blocks, as that exit might need to pass through an // excluded block. Clear Outer so we process BB's successors. if (LoopsWithHoles.count(Outer)) Outer = nullptr; if (StopLoop && Outer == StopLoop) 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 (Outer) { // 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 { Worklist.append(succ_begin(BB), succ_end(BB)); } } while (!Worklist.empty()); // We have exhausted all possible paths and are certain that 'To' can not be // reached from 'From'. return false; } bool llvm::isPotentiallyReachable( const BasicBlock *A, const BasicBlock *B, const SmallPtrSetImpl *ExclusionSet, const DominatorTree *DT, const LoopInfo *LI) { assert(A->getParent() == B->getParent() && "This analysis is function-local!"); if (DT) { if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B)) return false; if (!ExclusionSet || ExclusionSet->empty()) { if (A->isEntryBlock() && DT->isReachableFromEntry(B)) return true; if (B->isEntryBlock() && DT->isReachableFromEntry(A)) return false; } } SmallVector Worklist; Worklist.push_back(const_cast(A)); return isPotentiallyReachableFromMany(Worklist, const_cast(B), ExclusionSet, DT, LI); } bool llvm::isPotentiallyReachable( const Instruction *A, const Instruction *B, const SmallPtrSetImpl *ExclusionSet, const DominatorTree *DT, const LoopInfo *LI) { assert(A->getParent()->getParent() == B->getParent()->getParent() && "This analysis is function-local!"); if (A->getParent() == B->getParent()) { // The same block case is special because it's the only time we're looking // within a single block to see which instruction 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. BasicBlock *BB = const_cast(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 a backedge. if (LI && LI->getLoopFor(BB) != nullptr) return true; // If A comes before B, then B is definitively reachable from A. if (A == B || A->comesBefore(B)) return true; // Can't be in a loop if it's the entry block -- the entry block may not // have predecessors. if (BB->isEntryBlock()) return false; // Otherwise, continue doing the normal per-BB CFG walk. SmallVector Worklist; Worklist.append(succ_begin(BB), succ_end(BB)); if (Worklist.empty()) { // We've proven that there's no path! return false; } return isPotentiallyReachableFromMany( Worklist, const_cast(B->getParent()), ExclusionSet, DT, LI); } return isPotentiallyReachable( A->getParent(), B->getParent(), ExclusionSet, DT, LI); }