1 //===- llvm/CodeGen/MachineLoopInfo.h - Natural Loop Calculator -*- 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 file defines the MachineLoopInfo class that is used to identify natural
10 // loops and determine the loop depth of various nodes of the CFG.  Note that
11 // natural loops may actually be several loops that share the same header node.
12 //
13 // This analysis calculates the nesting structure of loops in a function.  For
14 // each natural loop identified, this analysis identifies natural loops
15 // contained entirely within the loop and the basic blocks the make up the loop.
16 //
17 // It can calculate on the fly various bits of information, for example:
18 //
19 //  * whether there is a preheader for the loop
20 //  * the number of back edges to the header
21 //  * whether or not a particular block branches out of the loop
22 //  * the successor blocks of the loop
23 //  * the loop depth
24 //  * the trip count
25 //  * etc...
26 //
27 //===----------------------------------------------------------------------===//
28 
29 #ifndef LLVM_CODEGEN_MACHINELOOPINFO_H
30 #define LLVM_CODEGEN_MACHINELOOPINFO_H
31 
32 #include "llvm/Analysis/LoopInfo.h"
33 #include "llvm/CodeGen/MachineBasicBlock.h"
34 #include "llvm/CodeGen/MachineFunctionPass.h"
35 #include "llvm/IR/DebugLoc.h"
36 #include "llvm/Pass.h"
37 
38 namespace llvm {
39 
40 class MachineDominatorTree;
41 // Implementation in LoopInfoImpl.h
42 class MachineLoop;
43 extern template class LoopBase<MachineBasicBlock, MachineLoop>;
44 
45 class MachineLoop : public LoopBase<MachineBasicBlock, MachineLoop> {
46 public:
47   /// Return the "top" block in the loop, which is the first block in the linear
48   /// layout, ignoring any parts of the loop not contiguous with the part that
49   /// contains the header.
50   MachineBasicBlock *getTopBlock();
51 
52   /// Return the "bottom" block in the loop, which is the last block in the
53   /// linear layout, ignoring any parts of the loop not contiguous with the part
54   /// that contains the header.
55   MachineBasicBlock *getBottomBlock();
56 
57   /// Find the block that contains the loop control variable and the
58   /// loop test. This will return the latch block if it's one of the exiting
59   /// blocks. Otherwise, return the exiting block. Return 'null' when
60   /// multiple exiting blocks are present.
61   MachineBasicBlock *findLoopControlBlock();
62 
63   /// Return the debug location of the start of this loop.
64   /// This looks for a BB terminating instruction with a known debug
65   /// location by looking at the preheader and header blocks. If it
66   /// cannot find a terminating instruction with location information,
67   /// it returns an unknown location.
68   DebugLoc getStartLoc() const;
69 
70   void dump() const;
71 
72 private:
73   friend class LoopInfoBase<MachineBasicBlock, MachineLoop>;
74 
75   explicit MachineLoop(MachineBasicBlock *MBB)
76     : LoopBase<MachineBasicBlock, MachineLoop>(MBB) {}
77 
78   MachineLoop() = default;
79 };
80 
81 // Implementation in LoopInfoImpl.h
82 extern template class LoopInfoBase<MachineBasicBlock, MachineLoop>;
83 
84 class MachineLoopInfo : public MachineFunctionPass {
85   friend class LoopBase<MachineBasicBlock, MachineLoop>;
86 
87   LoopInfoBase<MachineBasicBlock, MachineLoop> LI;
88 
89 public:
90   static char ID; // Pass identification, replacement for typeid
91 
92   MachineLoopInfo();
93   explicit MachineLoopInfo(MachineDominatorTree &MDT)
94       : MachineFunctionPass(ID) {
95     calculate(MDT);
96   }
97   MachineLoopInfo(const MachineLoopInfo &) = delete;
98   MachineLoopInfo &operator=(const MachineLoopInfo &) = delete;
99 
100   LoopInfoBase<MachineBasicBlock, MachineLoop>& getBase() { return LI; }
101 
102   /// Find the block that either is the loop preheader, or could
103   /// speculatively be used as the preheader. This is e.g. useful to place
104   /// loop setup code. Code that cannot be speculated should not be placed
105   /// here. SpeculativePreheader is controlling whether it also tries to
106   /// find the speculative preheader if the regular preheader is not present.
107   MachineBasicBlock *findLoopPreheader(MachineLoop *L,
108                                        bool SpeculativePreheader = false) const;
109 
110   /// The iterator interface to the top-level loops in the current function.
111   using iterator = LoopInfoBase<MachineBasicBlock, MachineLoop>::iterator;
112   inline iterator begin() const { return LI.begin(); }
113   inline iterator end() const { return LI.end(); }
114   bool empty() const { return LI.empty(); }
115 
116   /// Return the innermost loop that BB lives in. If a basic block is in no loop
117   /// (for example the entry node), null is returned.
118   inline MachineLoop *getLoopFor(const MachineBasicBlock *BB) const {
119     return LI.getLoopFor(BB);
120   }
121 
122   /// Same as getLoopFor.
123   inline const MachineLoop *operator[](const MachineBasicBlock *BB) const {
124     return LI.getLoopFor(BB);
125   }
126 
127   /// Return the loop nesting level of the specified block.
128   inline unsigned getLoopDepth(const MachineBasicBlock *BB) const {
129     return LI.getLoopDepth(BB);
130   }
131 
132   /// True if the block is a loop header node.
133   inline bool isLoopHeader(const MachineBasicBlock *BB) const {
134     return LI.isLoopHeader(BB);
135   }
136 
137   /// Calculate the natural loop information.
138   bool runOnMachineFunction(MachineFunction &F) override;
139   void calculate(MachineDominatorTree &MDT);
140 
141   void releaseMemory() override { LI.releaseMemory(); }
142 
143   void getAnalysisUsage(AnalysisUsage &AU) const override;
144 
145   /// This removes the specified top-level loop from this loop info object. The
146   /// loop is not deleted, as it will presumably be inserted into another loop.
147   inline MachineLoop *removeLoop(iterator I) { return LI.removeLoop(I); }
148 
149   /// Change the top-level loop that contains BB to the specified loop. This
150   /// should be used by transformations that restructure the loop hierarchy
151   /// tree.
152   inline void changeLoopFor(MachineBasicBlock *BB, MachineLoop *L) {
153     LI.changeLoopFor(BB, L);
154   }
155 
156   /// Replace the specified loop in the top-level loops list with the indicated
157   /// loop.
158   inline void changeTopLevelLoop(MachineLoop *OldLoop, MachineLoop *NewLoop) {
159     LI.changeTopLevelLoop(OldLoop, NewLoop);
160   }
161 
162   /// This adds the specified loop to the collection of top-level loops.
163   inline void addTopLevelLoop(MachineLoop *New) {
164     LI.addTopLevelLoop(New);
165   }
166 
167   /// This method completely removes BB from all data structures, including all
168   /// of the Loop objects it is nested in and our mapping from
169   /// MachineBasicBlocks to loops.
170   void removeBlock(MachineBasicBlock *BB) {
171     LI.removeBlock(BB);
172   }
173 };
174 
175 // Allow clients to walk the list of nested loops...
176 template <> struct GraphTraits<const MachineLoop*> {
177   using NodeRef = const MachineLoop *;
178   using ChildIteratorType = MachineLoopInfo::iterator;
179 
180   static NodeRef getEntryNode(const MachineLoop *L) { return L; }
181   static ChildIteratorType child_begin(NodeRef N) { return N->begin(); }
182   static ChildIteratorType child_end(NodeRef N) { return N->end(); }
183 };
184 
185 template <> struct GraphTraits<MachineLoop*> {
186   using NodeRef = MachineLoop *;
187   using ChildIteratorType = MachineLoopInfo::iterator;
188 
189   static NodeRef getEntryNode(MachineLoop *L) { return L; }
190   static ChildIteratorType child_begin(NodeRef N) { return N->begin(); }
191   static ChildIteratorType child_end(NodeRef N) { return N->end(); }
192 };
193 
194 } // end namespace llvm
195 
196 #endif // LLVM_CODEGEN_MACHINELOOPINFO_H
197