1 //===- RegisterScavenging.h - Machine register scavenging -------*- 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 /// \file
10 /// This file declares the machine register scavenger class. It can provide
11 /// information such as unused register at any point in a machine basic block.
12 /// It also provides a mechanism to make registers available by evicting them
13 /// to spill slots.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #ifndef LLVM_CODEGEN_REGISTERSCAVENGING_H
18 #define LLVM_CODEGEN_REGISTERSCAVENGING_H
19 
20 #include "llvm/ADT/BitVector.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/CodeGen/LiveRegUnits.h"
23 #include "llvm/CodeGen/MachineBasicBlock.h"
24 #include "llvm/CodeGen/MachineRegisterInfo.h"
25 #include "llvm/MC/LaneBitmask.h"
26 
27 namespace llvm {
28 
29 class MachineInstr;
30 class TargetInstrInfo;
31 class TargetRegisterClass;
32 class TargetRegisterInfo;
33 
34 class RegScavenger {
35   const TargetRegisterInfo *TRI = nullptr;
36   const TargetInstrInfo *TII = nullptr;
37   MachineRegisterInfo *MRI = nullptr;
38   MachineBasicBlock *MBB = nullptr;
39   MachineBasicBlock::iterator MBBI;
40 
41   /// Information on scavenged registers (held in a spill slot).
42   struct ScavengedInfo {
43     ScavengedInfo(int FI = -1) : FrameIndex(FI) {}
44 
45     /// A spill slot used for scavenging a register post register allocation.
46     int FrameIndex;
47 
48     /// If non-zero, the specific register is currently being
49     /// scavenged. That is, it is spilled to this scavenging stack slot.
50     Register Reg;
51 
52     /// The instruction that restores the scavenged register from stack.
53     const MachineInstr *Restore = nullptr;
54   };
55 
56   /// A vector of information on scavenged registers.
57   SmallVector<ScavengedInfo, 2> Scavenged;
58 
59   LiveRegUnits LiveUnits;
60 
61 public:
62   RegScavenger() = default;
63 
64   /// Record that \p Reg is in use at scavenging index \p FI. This is for
65   /// targets which need to directly manage the spilling process, and need to
66   /// update the scavenger's internal state.  It's expected this be called a
67   /// second time with \p Restore set to a non-null value, so that the
68   /// externally inserted restore instruction resets the scavenged slot
69   /// liveness when encountered.
70   void assignRegToScavengingIndex(int FI, Register Reg,
71                                   MachineInstr *Restore = nullptr) {
72     for (ScavengedInfo &Slot : Scavenged) {
73       if (Slot.FrameIndex == FI) {
74         assert(!Slot.Reg || Slot.Reg == Reg);
75         Slot.Reg = Reg;
76         Slot.Restore = Restore;
77         return;
78       }
79     }
80 
81     llvm_unreachable("did not find scavenging index");
82   }
83 
84   /// Start tracking liveness from the begin of basic block \p MBB.
85   void enterBasicBlock(MachineBasicBlock &MBB);
86 
87   /// Start tracking liveness from the end of basic block \p MBB.
88   /// Use backward() to move towards the beginning of the block.
89   void enterBasicBlockEnd(MachineBasicBlock &MBB);
90 
91   /// Update internal register state and move MBB iterator backwards. This
92   /// method gives precise results even in the absence of kill flags.
93   void backward();
94 
95   /// Call backward() to update internal register state to just before \p *I.
96   void backward(MachineBasicBlock::iterator I) {
97     while (MBBI != I)
98       backward();
99   }
100 
101   /// Return if a specific register is currently used.
102   bool isRegUsed(Register Reg, bool includeReserved = true) const;
103 
104   /// Return all available registers in the register class in Mask.
105   BitVector getRegsAvailable(const TargetRegisterClass *RC);
106 
107   /// Find an unused register of the specified register class.
108   /// Return 0 if none is found.
109   Register FindUnusedReg(const TargetRegisterClass *RC) const;
110 
111   /// Add a scavenging frame index.
112   void addScavengingFrameIndex(int FI) {
113     Scavenged.push_back(ScavengedInfo(FI));
114   }
115 
116   /// Query whether a frame index is a scavenging frame index.
117   bool isScavengingFrameIndex(int FI) const {
118     for (const ScavengedInfo &SI : Scavenged)
119       if (SI.FrameIndex == FI)
120         return true;
121 
122     return false;
123   }
124 
125   /// Get an array of scavenging frame indices.
126   void getScavengingFrameIndices(SmallVectorImpl<int> &A) const {
127     for (const ScavengedInfo &I : Scavenged)
128       if (I.FrameIndex >= 0)
129         A.push_back(I.FrameIndex);
130   }
131 
132   /// Make a register of the specific register class available from the current
133   /// position backwards to the place before \p To. If \p RestoreAfter is true
134   /// this includes the instruction following the current position.
135   /// SPAdj is the stack adjustment due to call frame, it's passed along to
136   /// eliminateFrameIndex().
137   /// Returns the scavenged register.
138   ///
139   /// If \p AllowSpill is false, fail if a spill is required to make the
140   /// register available, and return NoRegister.
141   Register scavengeRegisterBackwards(const TargetRegisterClass &RC,
142                                      MachineBasicBlock::iterator To,
143                                      bool RestoreAfter, int SPAdj,
144                                      bool AllowSpill = true);
145 
146   /// Tell the scavenger a register is used.
147   void setRegUsed(Register Reg, LaneBitmask LaneMask = LaneBitmask::getAll());
148 
149 private:
150   /// Returns true if a register is reserved. It is never "unused".
151   bool isReserved(Register Reg) const { return MRI->isReserved(Reg); }
152 
153   /// Initialize RegisterScavenger.
154   void init(MachineBasicBlock &MBB);
155 
156   /// Spill a register after position \p After and reload it before position
157   /// \p UseMI.
158   ScavengedInfo &spill(Register Reg, const TargetRegisterClass &RC, int SPAdj,
159                        MachineBasicBlock::iterator Before,
160                        MachineBasicBlock::iterator &UseMI);
161 };
162 
163 /// Replaces all frame index virtual registers with physical registers. Uses the
164 /// register scavenger to find an appropriate register to use.
165 void scavengeFrameVirtualRegs(MachineFunction &MF, RegScavenger &RS);
166 
167 } // end namespace llvm
168 
169 #endif // LLVM_CODEGEN_REGISTERSCAVENGING_H
170