//===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- C++ -*-===// | |

// | |

// 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. | |

// | |

//===----------------------------------------------------------------------===// | |

#ifndef LLVM_ANALYSIS_CFG_H | |

#define LLVM_ANALYSIS_CFG_H | |

#include "llvm/IR/BasicBlock.h" | |

#include "llvm/IR/CFG.h" | |

namespace llvm { | |

class BasicBlock; | |

class DominatorTree; | |

class Function; | |

class Instruction; | |

class LoopInfo; | |

/// 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 <from,to> edge info. | |

void FindFunctionBackedges( | |

const Function &F, | |

SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > & | |

Result); | |

/// 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 GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ); | |

/// 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 isCriticalEdge(const Instruction *TI, unsigned SuccNum, | |

bool AllowIdenticalEdges = false); | |

/// Determine whether instruction 'To' is reachable from 'From', | |

/// returning true if uncertain. | |

/// | |

/// Determine whether there is a path from From to To within a single function. | |

/// Returns false only if we can prove that once 'From' has been executed then | |

/// 'To' can not be executed. Conservatively returns true. | |

/// | |

/// This function is linear with respect to the number of blocks in the CFG, | |

/// walking down successors from From to reach To, with a fixed threshold. | |

/// Using DT or LI allows us to answer more quickly. LI reduces the cost of | |

/// an entire loop of any number of blocks to be the same as the cost of a | |

/// single block. DT reduces the cost by allowing the search to terminate when | |

/// we find a block that dominates the block containing 'To'. DT is most useful | |

/// on branchy code but not loops, and LI is most useful on code with loops but | |

/// does not help on branchy code outside loops. | |

bool isPotentiallyReachable(const Instruction *From, const Instruction *To, | |

const DominatorTree *DT = nullptr, | |

const LoopInfo *LI = nullptr); | |

/// Determine whether block 'To' is reachable from 'From', returning | |

/// true if uncertain. | |

/// | |

/// Determine whether there is a path from From to To within a single function. | |

/// Returns false only if we can prove that once 'From' has been reached then | |

/// 'To' can not be executed. Conservatively returns true. | |

bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To, | |

const DominatorTree *DT = nullptr, | |

const LoopInfo *LI = nullptr); | |

/// Determine whether there is at least one path from a block in | |

/// 'Worklist' to 'StopBB', returning true if uncertain. | |

/// | |

/// Determine whether there is a path from at least one block in Worklist to | |

/// StopBB within a single function. Returns false only if we can prove that | |

/// once any block in 'Worklist' has been reached then 'StopBB' can not be | |

/// executed. Conservatively returns true. | |

bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist, | |

BasicBlock *StopBB, | |

const DominatorTree *DT = nullptr, | |

const LoopInfo *LI = nullptr); | |

/// Return true if the control flow in \p RPOTraversal is irreducible. | |

/// | |

/// This is a generic implementation to detect CFG irreducibility based on loop | |

/// info analysis. It can be used for any kind of CFG (Loop, MachineLoop, | |

/// Function, MachineFunction, etc.) by providing an RPO traversal (\p | |

/// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility | |

/// function is only recommended when loop info analysis is available. If loop | |

/// info analysis isn't available, please, don't compute it explicitly for this | |

/// purpose. There are more efficient ways to detect CFG irreducibility that | |

/// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's | |

/// algorithm). | |

/// | |

/// Requirements: | |

/// 1) GraphTraits must be implemented for NodeT type. It is used to access | |

/// NodeT successors. | |

// 2) \p RPOTraversal must be a valid reverse post-order traversal of the | |

/// target CFG with begin()/end() iterator interfaces. | |

/// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop | |

/// analysis information of the CFG. | |

/// | |

/// This algorithm uses the information about reducible loop back-edges already | |

/// computed in \p LI. When a back-edge is found during the RPO traversal, the | |

/// algorithm checks whether the back-edge is one of the reducible back-edges in | |

/// loop info. If it isn't, the CFG is irreducible. For example, for the CFG | |

/// below (canonical irreducible graph) loop info won't contain any loop, so the | |

/// algorithm will return that the CFG is irreducible when checking the B <- | |

/// -> C back-edge. | |

/// | |

/// (A->B, A->C, B->C, C->B, C->D) | |

/// A | |

/// / \ | |

/// B<- ->C | |

/// | | |

/// D | |

/// | |

template <class NodeT, class RPOTraversalT, class LoopInfoT, | |

class GT = GraphTraits<NodeT>> | |

bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) { | |

/// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge | |

/// according to LI. I.e., check if there exists a loop that contains Src and | |

/// where Dst is the loop header. | |

auto isProperBackedge = [&](NodeT Src, NodeT Dst) { | |

for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) { | |

if (Lp->getHeader() == Dst) | |

return true; | |

} | |

return false; | |

}; | |

SmallPtrSet<NodeT, 32> Visited; | |

for (NodeT Node : RPOTraversal) { | |

Visited.insert(Node); | |

for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) { | |

// Succ hasn't been visited yet | |

if (!Visited.count(Succ)) | |

continue; | |

// We already visited Succ, thus Node->Succ must be a backedge. Check that | |

// the head matches what we have in the loop information. Otherwise, we | |

// have an irreducible graph. | |

if (!isProperBackedge(Node, Succ)) | |

return true; | |

} | |

} | |

return false; | |

} | |

} // End llvm namespace | |

#endif |