include/llvm/Analysis/CFG.h

   1 //===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This family of functions performs analyses on basic blocks, and instructions
  10 // contained within basic blocks.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #ifndef LLVM_ANALYSIS_CFG_H
  15 #define LLVM_ANALYSIS_CFG_H
  16
  17 #include "llvm/IR/BasicBlock.h"
  18 #include "llvm/IR/CFG.h"
  19
  20 namespace llvm {
  21
  22 class BasicBlock;
  23 class DominatorTree;
  24 class Function;
  25 class Instruction;
  26 class LoopInfo;
  27
  28 /// Analyze the specified function to find all of the loop backedges in the
  29 /// function and return them.  This is a relatively cheap (compared to
  30 /// computing dominators and loop info) analysis.
  31 ///
  32 /// The output is added to Result, as pairs of <from,to> edge info.
  33 void FindFunctionBackedges(
  34     const Function &F,
  35     SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
  36         Result);
  37
  38 /// Search for the specified successor of basic block BB and return its position
  39 /// in the terminator instruction's list of successors.  It is an error to call
  40 /// this with a block that is not a successor.
  41 unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
  42
  43 /// Return true if the specified edge is a critical edge. Critical edges are
  44 /// edges from a block with multiple successors to a block with multiple
  45 /// predecessors.
  46 ///
  47 bool isCriticalEdge(const Instruction *TI, unsigned SuccNum,
  48                     bool AllowIdenticalEdges = false);
  49 bool isCriticalEdge(const Instruction *TI, const BasicBlock *Succ,
  50                     bool AllowIdenticalEdges = false);
  51
  52 /// Determine whether instruction 'To' is reachable from 'From', without passing
  53 /// through any blocks in ExclusionSet, returning true if uncertain.
  54 ///
  55 /// Determine whether there is a path from From to To within a single function.
  56 /// Returns false only if we can prove that once 'From' has been executed then
  57 /// 'To' can not be executed. Conservatively returns true.
  58 ///
  59 /// This function is linear with respect to the number of blocks in the CFG,
  60 /// walking down successors from From to reach To, with a fixed threshold.
  61 /// Using DT or LI allows us to answer more quickly. LI reduces the cost of
  62 /// an entire loop of any number of blocks to be the same as the cost of a
  63 /// single block. DT reduces the cost by allowing the search to terminate when
  64 /// we find a block that dominates the block containing 'To'. DT is most useful
  65 /// on branchy code but not loops, and LI is most useful on code with loops but
  66 /// does not help on branchy code outside loops.
  67 bool isPotentiallyReachable(
  68     const Instruction *From, const Instruction *To,
  69     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet = nullptr,
  70     const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
  71
  72 /// Determine whether block 'To' is reachable from 'From', returning
  73 /// true if uncertain.
  74 ///
  75 /// Determine whether there is a path from From to To within a single function.
  76 /// Returns false only if we can prove that once 'From' has been reached then
  77 /// 'To' can not be executed. Conservatively returns true.
  78 bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
  79                             const DominatorTree *DT = nullptr,
  80                             const LoopInfo *LI = nullptr);
  81
  82 /// Determine whether there is at least one path from a block in
  83 /// 'Worklist' to 'StopBB', returning true if uncertain.
  84 ///
  85 /// Determine whether there is a path from at least one block in Worklist to
  86 /// StopBB within a single function. Returns false only if we can prove that
  87 /// once any block in 'Worklist' has been reached then 'StopBB' can not be
  88 /// executed. Conservatively returns true.
  89 bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
  90                                     BasicBlock *StopBB,
  91                                     const DominatorTree *DT = nullptr,
  92                                     const LoopInfo *LI = nullptr);
  93
  94 /// Determine whether there is at least one path from a block in
  95 /// 'Worklist' to 'StopBB' without passing through any blocks in
  96 /// 'ExclusionSet', returning true if uncertain.
  97 ///
  98 /// Determine whether there is a path from at least one block in Worklist to
  99 /// StopBB within a single function without passing through any of the blocks
 100 /// in 'ExclusionSet'. Returns false only if we can prove that once any block
 101 /// in 'Worklist' has been reached then 'StopBB' can not be executed.
 102 /// Conservatively returns true.
 103 bool isPotentiallyReachableFromMany(
 104     SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
 105     const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
 106     const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
 107
 108 /// Return true if the control flow in \p RPOTraversal is irreducible.
 109 ///
 110 /// This is a generic implementation to detect CFG irreducibility based on loop
 111 /// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
 112 /// Function, MachineFunction, etc.) by providing an RPO traversal (\p
 113 /// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
 114 /// function is only recommended when loop info analysis is available. If loop
 115 /// info analysis isn't available, please, don't compute it explicitly for this
 116 /// purpose. There are more efficient ways to detect CFG irreducibility that
 117 /// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
 118 /// algorithm).
 119 ///
 120 /// Requirements:
 121 ///   1) GraphTraits must be implemented for NodeT type. It is used to access
 122 ///      NodeT successors.
 123 //    2) \p RPOTraversal must be a valid reverse post-order traversal of the
 124 ///      target CFG with begin()/end() iterator interfaces.
 125 ///   3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
 126 ///      analysis information of the CFG.
 127 ///
 128 /// This algorithm uses the information about reducible loop back-edges already
 129 /// computed in \p LI. When a back-edge is found during the RPO traversal, the
 130 /// algorithm checks whether the back-edge is one of the reducible back-edges in
 131 /// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
 132 /// below (canonical irreducible graph) loop info won't contain any loop, so the
 133 /// algorithm will return that the CFG is irreducible when checking the B <-
 134 /// -> C back-edge.
 135 ///
 136 /// (A->B, A->C, B->C, C->B, C->D)
 137 ///    A
 138 ///  /   \
 139 /// B<- ->C
 140 ///       |
 141 ///       D
 142 ///
 143 template <class NodeT, class RPOTraversalT, class LoopInfoT,
 144           class GT = GraphTraits<NodeT>>
 145 bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
 146   /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
 147   /// according to LI. I.e., check if there exists a loop that contains Src and
 148   /// where Dst is the loop header.
 149   auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
 150     for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
 151       if (Lp->getHeader() == Dst)
 152         return true;
 153     }
 154     return false;
 155   };
 156
 157   SmallPtrSet<NodeT, 32> Visited;
 158   for (NodeT Node : RPOTraversal) {
 159     Visited.insert(Node);
 160     for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
 161       // Succ hasn't been visited yet
 162       if (!Visited.count(Succ))
 163         continue;
 164       // We already visited Succ, thus Node->Succ must be a backedge. Check that
 165       // the head matches what we have in the loop information. Otherwise, we
 166       // have an irreducible graph.
 167       if (!isProperBackedge(Node, Succ))
 168         return true;
 169     }
 170   }
 171
 172   return false;
 173 }
 174 } // End llvm namespace
 175
 176 #endif