X86FixupLEAs.cpp revision a5e5ba611f787f518fd3f7349343f8c4ae863fc2 
1//===-- X86FixupLEAs.cpp - use or replace LEA instructions -----------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines the pass which will find  instructions  which
11// can be re-written as LEA instructions in order to reduce pipeline
12// delays for some models of the Intel Atom family.
13//
14//===----------------------------------------------------------------------===//
15
16#define DEBUG_TYPE "x86-fixup-LEAs"
17#include "X86.h"
18#include "X86InstrInfo.h"
19#include "X86Subtarget.h"
20#include "llvm/ADT/Statistic.h"
21#include "llvm/CodeGen/LiveVariables.h"
22#include "llvm/CodeGen/MachineFunctionPass.h"
23#include "llvm/CodeGen/MachineInstrBuilder.h"
24#include "llvm/CodeGen/MachineRegisterInfo.h"
25#include "llvm/CodeGen/Passes.h"
26#include "llvm/Support/Debug.h"
27#include "llvm/Support/raw_ostream.h"
28#include "llvm/Target/TargetInstrInfo.h"
29using namespace llvm;
30
31STATISTIC(NumLEAs, "Number of LEA instructions created");
32
33namespace {
34  class FixupLEAPass : public MachineFunctionPass {
35    enum RegUsageState { RU_NotUsed, RU_Write, RU_Read };
36    static char ID;
37    /// \brief Loop over all of the instructions in the basic block
38    /// replacing applicable instructions with LEA instructions,
39    /// where appropriate.
40    bool processBasicBlock(MachineFunction &MF, MachineFunction::iterator MFI);
41
42    virtual const char *getPassName() const { return "X86 Atom LEA Fixup";}
43
44    /// \brief Given a machine register, look for the instruction
45    /// which writes it in the current basic block. If found,
46    /// try to replace it with an equivalent LEA instruction.
47    /// If replacement succeeds, then also process the the newly created
48    /// instruction.
49    void  seekLEAFixup(MachineOperand& p, MachineBasicBlock::iterator& I,
50                      MachineFunction::iterator MFI);
51
52    /// \brief Given a memory access or LEA instruction
53    /// whose address mode uses a base and/or index register, look for
54    /// an opportunity to replace the instruction which sets the base or index
55    /// register with an equivalent LEA instruction.
56    void processInstruction(MachineBasicBlock::iterator& I,
57                            MachineFunction::iterator MFI);
58
59    /// \brief Determine if an instruction references a machine register
60    /// and, if so, whether it reads or writes the register.
61    RegUsageState usesRegister(MachineOperand& p,
62                               MachineBasicBlock::iterator I);
63
64    /// \brief Step backwards through a basic block, looking
65    /// for an instruction which writes a register within
66    /// a maximum of INSTR_DISTANCE_THRESHOLD instruction latency cycles.
67    MachineBasicBlock::iterator searchBackwards(MachineOperand& p,
68                                                MachineBasicBlock::iterator& I,
69                                                MachineFunction::iterator MFI);
70
71    /// \brief if an instruction can be converted to an
72    /// equivalent LEA, insert the new instruction into the basic block
73    /// and return a pointer to it. Otherwise, return zero.
74    MachineInstr* postRAConvertToLEA(MachineFunction::iterator &MFI,
75                                     MachineBasicBlock::iterator &MBBI) const;
76
77  public:
78    FixupLEAPass() : MachineFunctionPass(ID) {}
79
80    /// \brief Loop over all of the basic blocks,
81    /// replacing instructions by equivalent LEA instructions
82    /// if needed and when possible.
83    virtual bool runOnMachineFunction(MachineFunction &MF);
84
85  private:
86    MachineFunction *MF;
87    const TargetMachine *TM;
88    const TargetInstrInfo *TII; // Machine instruction info.
89
90  };
91  char FixupLEAPass::ID = 0;
92}
93
94MachineInstr *
95FixupLEAPass::postRAConvertToLEA(MachineFunction::iterator &MFI,
96                                 MachineBasicBlock::iterator &MBBI) const {
97  MachineInstr* MI = MBBI;
98  MachineInstr* NewMI;
99  switch (MI->getOpcode()) {
100  case X86::MOV32rr:
101  case X86::MOV64rr: {
102    const MachineOperand& Src = MI->getOperand(1);
103    const MachineOperand& Dest = MI->getOperand(0);
104    NewMI = BuildMI(*MF, MI->getDebugLoc(),
105      TII->get( MI->getOpcode() == X86::MOV32rr ? X86::LEA32r : X86::LEA64r))
106      .addOperand(Dest)
107      .addOperand(Src).addImm(1).addReg(0).addImm(0).addReg(0);
108    MFI->insert(MBBI, NewMI);   // Insert the new inst
109    return NewMI;
110  }
111  case X86::ADD64ri32:
112  case X86::ADD64ri8:
113  case X86::ADD64ri32_DB:
114  case X86::ADD64ri8_DB:
115  case X86::ADD32ri:
116  case X86::ADD32ri8:
117  case X86::ADD32ri_DB:
118  case X86::ADD32ri8_DB:
119  case X86::ADD16ri:
120  case X86::ADD16ri8:
121  case X86::ADD16ri_DB:
122  case X86::ADD16ri8_DB:
123    if (!MI->getOperand(2).isImm()) {
124      // convertToThreeAddress will call getImm()
125      // which requires isImm() to be true
126      return 0;
127    }
128  }
129  return TII->convertToThreeAddress(MFI, MBBI, 0);
130}
131
132FunctionPass *llvm::createX86FixupLEAs() {
133  return new FixupLEAPass();
134}
135
136bool FixupLEAPass::runOnMachineFunction(MachineFunction &Func) {
137  MF = &Func;
138  TM = &MF->getTarget();
139  TII = TM->getInstrInfo();
140
141  DEBUG(dbgs() << "Start X86FixupLEAs\n";);
142  // Process all basic blocks.
143  for (MachineFunction::iterator I = Func.begin(), E = Func.end(); I != E; ++I)
144    processBasicBlock(Func, I);
145  DEBUG(dbgs() << "End X86FixupLEAs\n";);
146
147  return true;
148}
149
150FixupLEAPass::RegUsageState FixupLEAPass::usesRegister(MachineOperand& p,
151                                MachineBasicBlock::iterator I) {
152  RegUsageState RegUsage = RU_NotUsed;
153  MachineInstr* MI = I;
154
155  for (unsigned int i = 0; i < MI->getNumOperands(); ++i) {
156    MachineOperand& opnd = MI->getOperand(i);
157    if (opnd.isReg() && opnd.getReg() == p.getReg()){
158      if (opnd.isDef())
159        return RU_Write;
160      RegUsage = RU_Read;
161    }
162  }
163  return RegUsage;
164}
165
166/// getPreviousInstr - Given a reference to an instruction in a basic
167/// block, return a reference to the previous instruction in the block,
168/// wrapping around to the last instruction of the block if the block
169/// branches to itself.
170static inline bool getPreviousInstr(MachineBasicBlock::iterator& I,
171                                    MachineFunction::iterator MFI) {
172  if (I == MFI->begin()) {
173    if (MFI->isPredecessor(MFI)) {
174      I = --MFI->end();
175      return true;
176    }
177    else
178      return false;
179  }
180  --I;
181  return true;
182}
183
184MachineBasicBlock::iterator FixupLEAPass::searchBackwards(MachineOperand& p,
185                                   MachineBasicBlock::iterator& I,
186                                   MachineFunction::iterator MFI) {
187  int InstrDistance = 1;
188  MachineBasicBlock::iterator CurInst;
189  static const int INSTR_DISTANCE_THRESHOLD = 5;
190
191  CurInst = I;
192  bool Found;
193  Found = getPreviousInstr(CurInst, MFI);
194  while( Found && I != CurInst) {
195    if (CurInst->isCall() || CurInst->isInlineAsm())
196      break;
197    if (InstrDistance > INSTR_DISTANCE_THRESHOLD)
198      break; // too far back to make a difference
199    if (usesRegister(p, CurInst) == RU_Write){
200      return CurInst;
201    }
202    InstrDistance += TII->getInstrLatency(TM->getInstrItineraryData(), CurInst);
203    Found = getPreviousInstr(CurInst, MFI);
204  }
205  return 0;
206}
207
208void FixupLEAPass::processInstruction(MachineBasicBlock::iterator& I,
209                                      MachineFunction::iterator MFI) {
210  // Process a load, store, or LEA instruction.
211  MachineInstr *MI = I;
212  int opcode = MI->getOpcode();
213  const MCInstrDesc& Desc = MI->getDesc();
214  int AddrOffset = X86II::getMemoryOperandNo(Desc.TSFlags, opcode);
215  if (AddrOffset >= 0) {
216    AddrOffset += X86II::getOperandBias(Desc);
217    MachineOperand& p = MI->getOperand(AddrOffset + X86::AddrBaseReg);
218    if (p.isReg() && p.getReg() != X86::ESP) {
219      seekLEAFixup(p, I, MFI);
220    }
221    MachineOperand& q = MI->getOperand(AddrOffset + X86::AddrIndexReg);
222    if (q.isReg() && q.getReg() != X86::ESP) {
223      seekLEAFixup(q, I, MFI);
224    }
225  }
226}
227
228void FixupLEAPass::seekLEAFixup(MachineOperand& p,
229                                MachineBasicBlock::iterator& I,
230                                MachineFunction::iterator MFI) {
231  MachineBasicBlock::iterator MBI = searchBackwards(p, I, MFI);
232  if (MBI) {
233    MachineInstr* NewMI = postRAConvertToLEA(MFI, MBI);
234    if (NewMI) {
235      ++NumLEAs;
236      DEBUG(dbgs() << "Candidate to replace:"; MBI->dump(););
237      // now to replace with an equivalent LEA...
238      DEBUG(dbgs() << "Replaced by: "; NewMI->dump(););
239      MFI->erase(MBI);
240      MachineBasicBlock::iterator J =
241                             static_cast<MachineBasicBlock::iterator> (NewMI);
242      processInstruction(J, MFI);
243    }
244  }
245}
246
247bool FixupLEAPass::processBasicBlock(MachineFunction &MF,
248                                     MachineFunction::iterator MFI) {
249
250  for (MachineBasicBlock::iterator I = MFI->begin(); I != MFI->end(); ++I)
251    processInstruction(I, MFI);
252  return false;
253}
254