1//===- LiveRangeShrink.cpp - Move instructions to shrink live range -------===//
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 pass moves instructions close to the definition of its operands to
11/// shrink live range of the def instruction. The code motion is limited within
12/// the basic block. The moved instruction should have 1 def, and more than one
13/// uses, all of which are the only use of the def.
14///
15///===---------------------------------------------------------------------===//
16
17#include "llvm/ADT/DenseMap.h"
18#include "llvm/ADT/Statistic.h"
19#include "llvm/ADT/iterator_range.h"
20#include "llvm/CodeGen/MachineBasicBlock.h"
21#include "llvm/CodeGen/MachineFunction.h"
22#include "llvm/CodeGen/MachineFunctionPass.h"
23#include "llvm/CodeGen/MachineInstr.h"
24#include "llvm/CodeGen/MachineOperand.h"
25#include "llvm/CodeGen/MachineRegisterInfo.h"
26#include "llvm/CodeGen/TargetRegisterInfo.h"
27#include "llvm/InitializePasses.h"
28#include "llvm/Pass.h"
29#include "llvm/Support/Debug.h"
30#include "llvm/Support/raw_ostream.h"
31#include <iterator>
32#include <utility>
33
34using namespace llvm;
35
36#define DEBUG_TYPE "lrshrink"
37
38STATISTIC(NumInstrsHoistedToShrinkLiveRange,
39          "Number of insructions hoisted to shrink live range.");
40
41namespace {
42
43class LiveRangeShrink : public MachineFunctionPass {
44public:
45  static char ID;
46
47  LiveRangeShrink() : MachineFunctionPass(ID) {
48    initializeLiveRangeShrinkPass(*PassRegistry::getPassRegistry());
49  }
50
51  void getAnalysisUsage(AnalysisUsage &AU) const override {
52    AU.setPreservesCFG();
53    MachineFunctionPass::getAnalysisUsage(AU);
54  }
55
56  StringRef getPassName() const override { return "Live Range Shrink"; }
57
58  bool runOnMachineFunction(MachineFunction &MF) override;
59};
60
61} // end anonymous namespace
62
63char LiveRangeShrink::ID = 0;
64
65char &llvm::LiveRangeShrinkID = LiveRangeShrink::ID;
66
67INITIALIZE_PASS(LiveRangeShrink, "lrshrink", "Live Range Shrink Pass", false,
68                false)
69
70using InstOrderMap = DenseMap<MachineInstr *, unsigned>;
71
72/// Returns \p New if it's dominated by \p Old, otherwise return \p Old.
73/// \p M maintains a map from instruction to its dominating order that satisfies
74/// M[A] > M[B] guarantees that A is dominated by B.
75/// If \p New is not in \p M, return \p Old. Otherwise if \p Old is null, return
76/// \p New.
77static MachineInstr *FindDominatedInstruction(MachineInstr &New,
78                                              MachineInstr *Old,
79                                              const InstOrderMap &M) {
80  auto NewIter = M.find(&New);
81  if (NewIter == M.end())
82    return Old;
83  if (Old == nullptr)
84    return &New;
85  unsigned OrderOld = M.find(Old)->second;
86  unsigned OrderNew = NewIter->second;
87  if (OrderOld != OrderNew)
88    return OrderOld < OrderNew ? &New : Old;
89  // OrderOld == OrderNew, we need to iterate down from Old to see if it
90  // can reach New, if yes, New is dominated by Old.
91  for (MachineInstr *I = Old->getNextNode(); M.find(I)->second == OrderNew;
92       I = I->getNextNode())
93    if (I == &New)
94      return &New;
95  return Old;
96}
97
98/// Builds Instruction to its dominating order number map \p M by traversing
99/// from instruction \p Start.
100static void BuildInstOrderMap(MachineBasicBlock::iterator Start,
101                              InstOrderMap &M) {
102  M.clear();
103  unsigned i = 0;
104  for (MachineInstr &I : make_range(Start, Start->getParent()->end()))
105    M[&I] = i++;
106}
107
108bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
109  if (skipFunction(MF.getFunction()))
110    return false;
111
112  MachineRegisterInfo &MRI = MF.getRegInfo();
113
114  LLVM_DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
115
116  InstOrderMap IOM;
117  // Map from register to instruction order (value of IOM) where the
118  // register is used last. When moving instructions up, we need to
119  // make sure all its defs (including dead def) will not cross its
120  // last use when moving up.
121  DenseMap<unsigned, std::pair<unsigned, MachineInstr *>> UseMap;
122
123  for (MachineBasicBlock &MBB : MF) {
124    if (MBB.empty())
125      continue;
126    bool SawStore = false;
127    BuildInstOrderMap(MBB.begin(), IOM);
128    UseMap.clear();
129
130    for (MachineBasicBlock::iterator Next = MBB.begin(); Next != MBB.end();) {
131      MachineInstr &MI = *Next;
132      ++Next;
133      if (MI.isPHI() || MI.isDebugInstr())
134        continue;
135      if (MI.mayStore())
136        SawStore = true;
137
138      unsigned CurrentOrder = IOM[&MI];
139      unsigned Barrier = 0;
140      MachineInstr *BarrierMI = nullptr;
141      for (const MachineOperand &MO : MI.operands()) {
142        if (!MO.isReg() || MO.isDebug())
143          continue;
144        if (MO.isUse())
145          UseMap[MO.getReg()] = std::make_pair(CurrentOrder, &MI);
146        else if (MO.isDead() && UseMap.count(MO.getReg()))
147          // Barrier is the last instruction where MO get used. MI should not
148          // be moved above Barrier.
149          if (Barrier < UseMap[MO.getReg()].first) {
150            Barrier = UseMap[MO.getReg()].first;
151            BarrierMI = UseMap[MO.getReg()].second;
152          }
153      }
154
155      if (!MI.isSafeToMove(nullptr, SawStore)) {
156        // If MI has side effects, it should become a barrier for code motion.
157        // IOM is rebuild from the next instruction to prevent later
158        // instructions from being moved before this MI.
159        if (MI.hasUnmodeledSideEffects() && Next != MBB.end()) {
160          BuildInstOrderMap(Next, IOM);
161          SawStore = false;
162        }
163        continue;
164      }
165
166      const MachineOperand *DefMO = nullptr;
167      MachineInstr *Insert = nullptr;
168
169      // Number of live-ranges that will be shortened. We do not count
170      // live-ranges that are defined by a COPY as it could be coalesced later.
171      unsigned NumEligibleUse = 0;
172
173      for (const MachineOperand &MO : MI.operands()) {
174        if (!MO.isReg() || MO.isDead() || MO.isDebug())
175          continue;
176        Register Reg = MO.getReg();
177        // Do not move the instruction if it def/uses a physical register,
178        // unless it is a constant physical register or a noreg.
179        if (!Register::isVirtualRegister(Reg)) {
180          if (!Reg || MRI.isConstantPhysReg(Reg))
181            continue;
182          Insert = nullptr;
183          break;
184        }
185        if (MO.isDef()) {
186          // Do not move if there is more than one def.
187          if (DefMO) {
188            Insert = nullptr;
189            break;
190          }
191          DefMO = &MO;
192        } else if (MRI.hasOneNonDBGUse(Reg) && MRI.hasOneDef(Reg) && DefMO &&
193                   MRI.getRegClass(DefMO->getReg()) ==
194                       MRI.getRegClass(MO.getReg())) {
195          // The heuristic does not handle different register classes yet
196          // (registers of different sizes, looser/tighter constraints). This
197          // is because it needs more accurate model to handle register
198          // pressure correctly.
199          MachineInstr &DefInstr = *MRI.def_instr_begin(Reg);
200          if (!DefInstr.isCopy())
201            NumEligibleUse++;
202          Insert = FindDominatedInstruction(DefInstr, Insert, IOM);
203        } else {
204          Insert = nullptr;
205          break;
206        }
207      }
208
209      // If Barrier equals IOM[I], traverse forward to find if BarrierMI is
210      // after Insert, if yes, then we should not hoist.
211      for (MachineInstr *I = Insert; I && IOM[I] == Barrier;
212           I = I->getNextNode())
213        if (I == BarrierMI) {
214          Insert = nullptr;
215          break;
216        }
217      // Move the instruction when # of shrunk live range > 1.
218      if (DefMO && Insert && NumEligibleUse > 1 && Barrier <= IOM[Insert]) {
219        MachineBasicBlock::iterator I = std::next(Insert->getIterator());
220        // Skip all the PHI and debug instructions.
221        while (I != MBB.end() && (I->isPHI() || I->isDebugInstr()))
222          I = std::next(I);
223        if (I == MI.getIterator())
224          continue;
225
226        // Update the dominator order to be the same as the insertion point.
227        // We do this to maintain a non-decreasing order without need to update
228        // all instruction orders after the insertion point.
229        unsigned NewOrder = IOM[&*I];
230        IOM[&MI] = NewOrder;
231        NumInstrsHoistedToShrinkLiveRange++;
232
233        // Find MI's debug value following MI.
234        MachineBasicBlock::iterator EndIter = std::next(MI.getIterator());
235        if (MI.getOperand(0).isReg())
236          for (; EndIter != MBB.end() && EndIter->isDebugValue() &&
237                 EndIter->getOperand(0).isReg() &&
238                 EndIter->getOperand(0).getReg() == MI.getOperand(0).getReg();
239               ++EndIter, ++Next)
240            IOM[&*EndIter] = NewOrder;
241        MBB.splice(I, &MBB, MI.getIterator(), EndIter);
242      }
243    }
244  }
245  return false;
246}
247