1 //=- MachineLoopUtils.cpp - Functions for manipulating loops ----------------=//
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 #include "llvm/CodeGen/MachineLoopInfo.h"
10 #include "llvm/CodeGen/MachineLoopUtils.h"
11 #include "llvm/CodeGen/MachineBasicBlock.h"
12 #include "llvm/CodeGen/MachineRegisterInfo.h"
13 #include "llvm/CodeGen/TargetInstrInfo.h"
14 using namespace llvm;
15 
16 namespace {
17 // MI's parent and BB are clones of each other. Find the equivalent copy of MI
18 // in BB.
19 MachineInstr &findEquivalentInstruction(MachineInstr &MI,
20                                         MachineBasicBlock *BB) {
21   MachineBasicBlock *PB = MI.getParent();
22   unsigned Offset = std::distance(PB->instr_begin(), MachineBasicBlock::instr_iterator(MI));
23   return *std::next(BB->instr_begin(), Offset);
24 }
25 } // namespace
26 
27 MachineBasicBlock *llvm::PeelSingleBlockLoop(LoopPeelDirection Direction,
28                                              MachineBasicBlock *Loop,
29                                              MachineRegisterInfo &MRI,
30                                              const TargetInstrInfo *TII) {
31   MachineFunction &MF = *Loop->getParent();
32   MachineBasicBlock *Preheader = *Loop->pred_begin();
33   if (Preheader == Loop)
34     Preheader = *std::next(Loop->pred_begin());
35   MachineBasicBlock *Exit = *Loop->succ_begin();
36   if (Exit == Loop)
37     Exit = *std::next(Loop->succ_begin());
38 
39   MachineBasicBlock *NewBB = MF.CreateMachineBasicBlock(Loop->getBasicBlock());
40   if (Direction == LPD_Front)
41     MF.insert(Loop->getIterator(), NewBB);
42   else
43     MF.insert(std::next(Loop->getIterator()), NewBB);
44 
45   // FIXME: Add DenseMapInfo trait for Register so we can use it as a key.
46   DenseMap<unsigned, Register> Remaps;
47   auto InsertPt = NewBB->end();
48   for (MachineInstr &MI : *Loop) {
49     MachineInstr *NewMI = MF.CloneMachineInstr(&MI);
50     NewBB->insert(InsertPt, NewMI);
51     for (MachineOperand &MO : NewMI->defs()) {
52       Register OrigR = MO.getReg();
53       if (OrigR.isPhysical())
54         continue;
55       Register &R = Remaps[OrigR];
56       R = MRI.createVirtualRegister(MRI.getRegClass(OrigR));
57       MO.setReg(R);
58 
59       if (Direction == LPD_Back) {
60         // Replace all uses outside the original loop with the new register.
61         // FIXME: is the use_iterator stable enough to mutate register uses
62         // while iterating?
63         SmallVector<MachineOperand *, 4> Uses;
64         for (auto &Use : MRI.use_operands(OrigR))
65           if (Use.getParent()->getParent() != Loop)
66             Uses.push_back(&Use);
67         for (auto *Use : Uses) {
68           MRI.constrainRegClass(R, MRI.getRegClass(Use->getReg()));
69           Use->setReg(R);
70         }
71       }
72     }
73   }
74 
75   for (auto I = NewBB->getFirstNonPHI(); I != NewBB->end(); ++I)
76     for (MachineOperand &MO : I->uses())
77       if (MO.isReg() && Remaps.count(MO.getReg()))
78         MO.setReg(Remaps[MO.getReg()]);
79 
80   for (auto I = NewBB->begin(); I->isPHI(); ++I) {
81     MachineInstr &MI = *I;
82     unsigned LoopRegIdx = 3, InitRegIdx = 1;
83     if (MI.getOperand(2).getMBB() != Preheader)
84       std::swap(LoopRegIdx, InitRegIdx);
85     MachineInstr &OrigPhi = findEquivalentInstruction(MI, Loop);
86     assert(OrigPhi.isPHI());
87     if (Direction == LPD_Front) {
88       // When peeling front, we are only left with the initial value from the
89       // preheader.
90       Register R = MI.getOperand(LoopRegIdx).getReg();
91       if (Remaps.count(R))
92         R = Remaps[R];
93       OrigPhi.getOperand(InitRegIdx).setReg(R);
94       MI.RemoveOperand(LoopRegIdx + 1);
95       MI.RemoveOperand(LoopRegIdx + 0);
96     } else {
97       // When peeling back, the initial value is the loop-carried value from
98       // the original loop.
99       Register LoopReg = OrigPhi.getOperand(LoopRegIdx).getReg();
100       MI.getOperand(LoopRegIdx).setReg(LoopReg);
101       MI.RemoveOperand(InitRegIdx + 1);
102       MI.RemoveOperand(InitRegIdx + 0);
103     }
104   }
105 
106   DebugLoc DL;
107   if (Direction == LPD_Front) {
108     Preheader->replaceSuccessor(Loop, NewBB);
109     NewBB->addSuccessor(Loop);
110     Loop->replacePhiUsesWith(Preheader, NewBB);
111     if (TII->removeBranch(*Preheader) > 0)
112       TII->insertBranch(*Preheader, NewBB, nullptr, {}, DL);
113     TII->removeBranch(*NewBB);
114     TII->insertBranch(*NewBB, Loop, nullptr, {}, DL);
115   } else {
116     Loop->replaceSuccessor(Exit, NewBB);
117     Exit->replacePhiUsesWith(Loop, NewBB);
118     NewBB->addSuccessor(Exit);
119 
120     MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
121     SmallVector<MachineOperand, 4> Cond;
122     bool CanAnalyzeBr = !TII->analyzeBranch(*Loop, TBB, FBB, Cond);
123     (void)CanAnalyzeBr;
124     assert(CanAnalyzeBr && "Must be able to analyze the loop branch!");
125     TII->removeBranch(*Loop);
126     TII->insertBranch(*Loop, TBB == Exit ? NewBB : TBB,
127                       FBB == Exit ? NewBB : FBB, Cond, DL);
128     if (TII->removeBranch(*NewBB) > 0)
129       TII->insertBranch(*NewBB, Exit, nullptr, {}, DL);
130   }
131 
132   return NewBB;
133 }
134 
135 bool llvm::isRegLiveInExitBlocks(MachineLoop *Loop, int PhysReg) {
136   SmallVector<MachineBasicBlock *, 4> ExitBlocks;
137   Loop->getExitBlocks(ExitBlocks);
138 
139   for (auto *MBB : ExitBlocks)
140     if (MBB->isLiveIn(PhysReg))
141       return true;
142 
143   return false;
144 }
145