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   /// Returns true if the instruction is loop invariant.
71   /// I.e., all virtual register operands are defined outside of the loop,
72   /// physical registers aren't accessed explicitly, and there are no side
73   /// effects that aren't captured by the operands or other flags.
74   bool isLoopInvariant(MachineInstr &I) const;
75 
76   void dump() const;
77 
78 private:
79   friend class LoopInfoBase<MachineBasicBlock, MachineLoop>;
80 
81   explicit MachineLoop(MachineBasicBlock *MBB)
82     : LoopBase<MachineBasicBlock, MachineLoop>(MBB) {}
83 
84   MachineLoop() = default;
85 };
86 
87 // Implementation in LoopInfoImpl.h
88 extern template class LoopInfoBase<MachineBasicBlock, MachineLoop>;
89 
90 class MachineLoopInfo : public MachineFunctionPass {
91   friend class LoopBase<MachineBasicBlock, MachineLoop>;
92 
93   LoopInfoBase<MachineBasicBlock, MachineLoop> LI;
94 
95 public:
96   static char ID; // Pass identification, replacement for typeid
97 
98   MachineLoopInfo();
99   explicit MachineLoopInfo(MachineDominatorTree &MDT)
100       : MachineFunctionPass(ID) {
101     calculate(MDT);
102   }
103   MachineLoopInfo(const MachineLoopInfo &) = delete;
104   MachineLoopInfo &operator=(const MachineLoopInfo &) = delete;
105 
106   LoopInfoBase<MachineBasicBlock, MachineLoop>& getBase() { return LI; }
107 
108   /// Find the block that either is the loop preheader, or could
109   /// speculatively be used as the preheader. This is e.g. useful to place
110   /// loop setup code. Code that cannot be speculated should not be placed
111   /// here. SpeculativePreheader is controlling whether it also tries to
112   /// find the speculative preheader if the regular preheader is not present.
113   /// With FindMultiLoopPreheader = false, nullptr will be returned if the found
114   /// preheader is the preheader of multiple loops.
115   MachineBasicBlock *
116   findLoopPreheader(MachineLoop *L, bool SpeculativePreheader = false,
117                     bool FindMultiLoopPreheader = false) const;
118 
119   /// The iterator interface to the top-level loops in the current function.
120   using iterator = LoopInfoBase<MachineBasicBlock, MachineLoop>::iterator;
121   inline iterator begin() const { return LI.begin(); }
122   inline iterator end() const { return LI.end(); }
123   bool empty() const { return LI.empty(); }
124 
125   /// Return the innermost loop that BB lives in. If a basic block is in no loop
126   /// (for example the entry node), null is returned.
127   inline MachineLoop *getLoopFor(const MachineBasicBlock *BB) const {
128     return LI.getLoopFor(BB);
129   }
130 
131   /// Same as getLoopFor.
132   inline const MachineLoop *operator[](const MachineBasicBlock *BB) const {
133     return LI.getLoopFor(BB);
134   }
135 
136   /// Return the loop nesting level of the specified block.
137   inline unsigned getLoopDepth(const MachineBasicBlock *BB) const {
138     return LI.getLoopDepth(BB);
139   }
140 
141   /// True if the block is a loop header node.
142   inline bool isLoopHeader(const MachineBasicBlock *BB) const {
143     return LI.isLoopHeader(BB);
144   }
145 
146   /// Calculate the natural loop information.
147   bool runOnMachineFunction(MachineFunction &F) override;
148   void calculate(MachineDominatorTree &MDT);
149 
150   void releaseMemory() override { LI.releaseMemory(); }
151 
152   void getAnalysisUsage(AnalysisUsage &AU) const override;
153 
154   /// This removes the specified top-level loop from this loop info object. The
155   /// loop is not deleted, as it will presumably be inserted into another loop.
156   inline MachineLoop *removeLoop(iterator I) { return LI.removeLoop(I); }
157 
158   /// Change the top-level loop that contains BB to the specified loop. This
159   /// should be used by transformations that restructure the loop hierarchy
160   /// tree.
161   inline void changeLoopFor(MachineBasicBlock *BB, MachineLoop *L) {
162     LI.changeLoopFor(BB, L);
163   }
164 
165   /// Replace the specified loop in the top-level loops list with the indicated
166   /// loop.
167   inline void changeTopLevelLoop(MachineLoop *OldLoop, MachineLoop *NewLoop) {
168     LI.changeTopLevelLoop(OldLoop, NewLoop);
169   }
170 
171   /// This adds the specified loop to the collection of top-level loops.
172   inline void addTopLevelLoop(MachineLoop *New) {
173     LI.addTopLevelLoop(New);
174   }
175 
176   /// This method completely removes BB from all data structures, including all
177   /// of the Loop objects it is nested in and our mapping from
178   /// MachineBasicBlocks to loops.
179   void removeBlock(MachineBasicBlock *BB) {
180     LI.removeBlock(BB);
181   }
182 };
183 
184 // Allow clients to walk the list of nested loops...
185 template <> struct GraphTraits<const MachineLoop*> {
186   using NodeRef = const MachineLoop *;
187   using ChildIteratorType = MachineLoopInfo::iterator;
188 
189   static NodeRef getEntryNode(const 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 template <> struct GraphTraits<MachineLoop*> {
195   using NodeRef = MachineLoop *;
196   using ChildIteratorType = MachineLoopInfo::iterator;
197 
198   static NodeRef getEntryNode(MachineLoop *L) { return L; }
199   static ChildIteratorType child_begin(NodeRef N) { return N->begin(); }
200   static ChildIteratorType child_end(NodeRef N) { return N->end(); }
201 };
202 
203 } // end namespace llvm
204 
205 #endif // LLVM_CODEGEN_MACHINELOOPINFO_H
206