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/ADT/GraphTraits.h"
18#include "llvm/ADT/SmallPtrSet.h"
19#include <utility>
20
21namespace llvm {
22
23class BasicBlock;
24class DominatorTree;
25class Function;
26class Instruction;
27class LoopInfo;
28template <typename T> class SmallVectorImpl;
29
30/// Analyze the specified function to find all of the loop backedges in the
31/// function and return them. This is a relatively cheap (compared to
32/// computing dominators and loop info) analysis.
33///
34/// The output is added to Result, as pairs of <from,to> edge info.
35void FindFunctionBackedges(
36 const Function &F,
37 SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
38 Result);
39
40/// Search for the specified successor of basic block BB and return its position
41/// in the terminator instruction's list of successors. It is an error to call
42/// this with a block that is not a successor.
43unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
44
45/// Return true if the specified edge is a critical edge. Critical edges are
46/// edges from a block with multiple successors to a block with multiple
47/// predecessors.
48///
49bool isCriticalEdge(const Instruction *TI, unsigned SuccNum,
50 bool AllowIdenticalEdges = false);
51bool isCriticalEdge(const Instruction *TI, const BasicBlock *Succ,
52 bool AllowIdenticalEdges = false);
53
54/// Determine whether instruction 'To' is reachable from 'From', without passing
55/// through any blocks in ExclusionSet, returning true if uncertain.
56///
57/// Determine whether there is a path from From to To within a single function.
58/// Returns false only if we can prove that once 'From' has been executed then
59/// 'To' can not be executed. Conservatively returns true.
60///
61/// This function is linear with respect to the number of blocks in the CFG,
62/// walking down successors from From to reach To, with a fixed threshold.
63/// Using DT or LI allows us to answer more quickly. LI reduces the cost of
64/// an entire loop of any number of blocks to be the same as the cost of a
65/// single block. DT reduces the cost by allowing the search to terminate when
66/// we find a block that dominates the block containing 'To'. DT is most useful
67/// on branchy code but not loops, and LI is most useful on code with loops but
68/// does not help on branchy code outside loops.
69bool isPotentiallyReachable(
70 const Instruction *From, const Instruction *To,
71 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet = nullptr,
72 const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
73
74/// Determine whether block 'To' is reachable from 'From', returning
75/// true if uncertain.
76///
77/// Determine whether there is a path from From to To within a single function.
78/// Returns false only if we can prove that once 'From' has been reached then
79/// 'To' can not be executed. Conservatively returns true.
80bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
81 const DominatorTree *DT = nullptr,
82 const LoopInfo *LI = nullptr);
83
84/// Determine whether there is at least one path from a block in
85/// 'Worklist' to 'StopBB', returning true if uncertain.
86///
87/// Determine whether there is a path from at least one block in Worklist to
88/// StopBB within a single function. Returns false only if we can prove that
89/// once any block in 'Worklist' has been reached then 'StopBB' can not be
90/// executed. Conservatively returns true.
91bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
92 BasicBlock *StopBB,
93 const DominatorTree *DT = nullptr,
94 const LoopInfo *LI = nullptr);
95
96/// Determine whether there is at least one path from a block in
97/// 'Worklist' to 'StopBB' without passing through any blocks in
98/// 'ExclusionSet', returning true if uncertain.
99///
100/// Determine whether there is a path from at least one block in Worklist to
101/// StopBB within a single function without passing through any of the blocks
102/// in 'ExclusionSet'. Returns false only if we can prove that once any block
103/// in 'Worklist' has been reached then 'StopBB' can not be executed.
104/// Conservatively returns true.
105bool isPotentiallyReachableFromMany(
106 SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
107 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
108 const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
109
110/// Return true if the control flow in \p RPOTraversal is irreducible.
111///
112/// This is a generic implementation to detect CFG irreducibility based on loop
113/// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
114/// Function, MachineFunction, etc.) by providing an RPO traversal (\p
115/// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
116/// function is only recommended when loop info analysis is available. If loop
117/// info analysis isn't available, please, don't compute it explicitly for this
118/// purpose. There are more efficient ways to detect CFG irreducibility that
119/// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
120/// algorithm).
121///
122/// Requirements:
123/// 1) GraphTraits must be implemented for NodeT type. It is used to access
124/// NodeT successors.
125// 2) \p RPOTraversal must be a valid reverse post-order traversal of the
126/// target CFG with begin()/end() iterator interfaces.
127/// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
128/// analysis information of the CFG.
129///
130/// This algorithm uses the information about reducible loop back-edges already
131/// computed in \p LI. When a back-edge is found during the RPO traversal, the
132/// algorithm checks whether the back-edge is one of the reducible back-edges in
133/// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
134/// below (canonical irreducible graph) loop info won't contain any loop, so the
135/// algorithm will return that the CFG is irreducible when checking the B <-
136/// -> C back-edge.
137///
138/// (A->B, A->C, B->C, C->B, C->D)
139/// A
140/// / \
141/// B<- ->C
142/// |
143/// D
144///
145template <class NodeT, class RPOTraversalT, class LoopInfoT,
146 class GT = GraphTraits<NodeT>>
147bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
148 /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
149 /// according to LI. I.e., check if there exists a loop that contains Src and
150 /// where Dst is the loop header.
151 auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
152 for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
153 if (Lp->getHeader() == Dst)
154 return true;
155 }
156 return false;
157 };
158
159 SmallPtrSet<NodeT, 32> Visited;
160 for (NodeT Node : RPOTraversal) {
161 Visited.insert(Node);
162 for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
163 // Succ hasn't been visited yet
164 if (!Visited.count(Succ))
165 continue;
166 // We already visited Succ, thus Node->Succ must be a backedge. Check that
167 // the head matches what we have in the loop information. Otherwise, we
168 // have an irreducible graph.
169 if (!isProperBackedge(Node, Succ))
170 return true;
171 }
172 }
173
174 return false;
175}
176} // End llvm namespace
177
178#endif
179