PeepholeOptimizer.cpp revision adb6bb0f8b18971371f1b459f4e044885411f9a3 
1//===-- PeepholeOptimizer.cpp - Peephole Optimizations --------------------===//
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// Perform peephole optimizations on the machine code:
11//
12// - Optimize Extensions
13//
14//     Optimization of sign / zero extension instructions. It may be extended to
15//     handle other instructions with similar properties.
16//
17//     On some targets, some instructions, e.g. X86 sign / zero extension, may
18//     leave the source value in the lower part of the result. This optimization
19//     will replace some uses of the pre-extension value with uses of the
20//     sub-register of the results.
21//
22// - Optimize Comparisons
23//
24//     Optimization of comparison instructions. For instance, in this code:
25//
26//       sub r1, 1
27//       cmp r1, 0
28//       bz  L1
29//
30//     If the "sub" instruction all ready sets (or could be modified to set) the
31//     same flag that the "cmp" instruction sets and that "bz" uses, then we can
32//     eliminate the "cmp" instruction.
33//
34//===----------------------------------------------------------------------===//
35
36#define DEBUG_TYPE "peephole-opt"
37#include "llvm/CodeGen/Passes.h"
38#include "llvm/CodeGen/MachineDominators.h"
39#include "llvm/CodeGen/MachineInstrBuilder.h"
40#include "llvm/CodeGen/MachineRegisterInfo.h"
41#include "llvm/Target/TargetInstrInfo.h"
42#include "llvm/Target/TargetRegisterInfo.h"
43#include "llvm/Support/CommandLine.h"
44#include "llvm/ADT/SmallPtrSet.h"
45#include "llvm/ADT/Statistic.h"
46using namespace llvm;
47
48// Optimize Extensions
49static cl::opt<bool>
50Aggressive("aggressive-ext-opt", cl::Hidden,
51           cl::desc("Aggressive extension optimization"));
52
53STATISTIC(NumReuse,      "Number of extension results reused");
54STATISTIC(NumEliminated, "Number of compares eliminated");
55
56namespace {
57  class PeepholeOptimizer : public MachineFunctionPass {
58    const TargetMachine   *TM;
59    const TargetInstrInfo *TII;
60    MachineRegisterInfo   *MRI;
61    MachineDominatorTree  *DT;  // Machine dominator tree
62
63  public:
64    static char ID; // Pass identification
65    PeepholeOptimizer() : MachineFunctionPass(ID) {
66      initializePeepholeOptimizerPass(*PassRegistry::getPassRegistry());
67    }
68
69    virtual bool runOnMachineFunction(MachineFunction &MF);
70
71    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
72      AU.setPreservesCFG();
73      MachineFunctionPass::getAnalysisUsage(AU);
74      if (Aggressive) {
75        AU.addRequired<MachineDominatorTree>();
76        AU.addPreserved<MachineDominatorTree>();
77      }
78    }
79
80  private:
81    bool OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB,
82                          MachineBasicBlock::iterator &MII);
83    bool OptimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
84                          SmallPtrSet<MachineInstr*, 8> &LocalMIs);
85  };
86}
87
88char PeepholeOptimizer::ID = 0;
89INITIALIZE_PASS_BEGIN(PeepholeOptimizer, "peephole-opts",
90                "Peephole Optimizations", false, false)
91INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
92INITIALIZE_PASS_END(PeepholeOptimizer, "peephole-opts",
93                "Peephole Optimizations", false, false)
94
95FunctionPass *llvm::createPeepholeOptimizerPass() {
96  return new PeepholeOptimizer();
97}
98
99/// OptimizeExtInstr - If instruction is a copy-like instruction, i.e. it reads
100/// a single register and writes a single register and it does not modify the
101/// source, and if the source value is preserved as a sub-register of the
102/// result, then replace all reachable uses of the source with the subreg of the
103/// result.
104///
105/// Do not generate an EXTRACT that is used only in a debug use, as this changes
106/// the code. Since this code does not currently share EXTRACTs, just ignore all
107/// debug uses.
108bool PeepholeOptimizer::
109OptimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
110                 SmallPtrSet<MachineInstr*, 8> &LocalMIs) {
111  LocalMIs.insert(MI);
112
113  unsigned SrcReg, DstReg, SubIdx;
114  if (!TII->isCoalescableExtInstr(*MI, SrcReg, DstReg, SubIdx))
115    return false;
116
117  if (TargetRegisterInfo::isPhysicalRegister(DstReg) ||
118      TargetRegisterInfo::isPhysicalRegister(SrcReg))
119    return false;
120
121  MachineRegisterInfo::use_nodbg_iterator UI = MRI->use_nodbg_begin(SrcReg);
122  if (++UI == MRI->use_nodbg_end())
123    // No other uses.
124    return false;
125
126  // The source has other uses. See if we can replace the other uses with use of
127  // the result of the extension.
128  SmallPtrSet<MachineBasicBlock*, 4> ReachedBBs;
129  UI = MRI->use_nodbg_begin(DstReg);
130  for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
131       UI != UE; ++UI)
132    ReachedBBs.insert(UI->getParent());
133
134  // Uses that are in the same BB of uses of the result of the instruction.
135  SmallVector<MachineOperand*, 8> Uses;
136
137  // Uses that the result of the instruction can reach.
138  SmallVector<MachineOperand*, 8> ExtendedUses;
139
140  bool ExtendLife = true;
141  UI = MRI->use_nodbg_begin(SrcReg);
142  for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
143       UI != UE; ++UI) {
144    MachineOperand &UseMO = UI.getOperand();
145    MachineInstr *UseMI = &*UI;
146    if (UseMI == MI)
147      continue;
148
149    if (UseMI->isPHI()) {
150      ExtendLife = false;
151      continue;
152    }
153
154    // It's an error to translate this:
155    //
156    //    %reg1025 = <sext> %reg1024
157    //     ...
158    //    %reg1026 = SUBREG_TO_REG 0, %reg1024, 4
159    //
160    // into this:
161    //
162    //    %reg1025 = <sext> %reg1024
163    //     ...
164    //    %reg1027 = COPY %reg1025:4
165    //    %reg1026 = SUBREG_TO_REG 0, %reg1027, 4
166    //
167    // The problem here is that SUBREG_TO_REG is there to assert that an
168    // implicit zext occurs. It doesn't insert a zext instruction. If we allow
169    // the COPY here, it will give us the value after the <sext>, not the
170    // original value of %reg1024 before <sext>.
171    if (UseMI->getOpcode() == TargetOpcode::SUBREG_TO_REG)
172      continue;
173
174    MachineBasicBlock *UseMBB = UseMI->getParent();
175    if (UseMBB == MBB) {
176      // Local uses that come after the extension.
177      if (!LocalMIs.count(UseMI))
178        Uses.push_back(&UseMO);
179    } else if (ReachedBBs.count(UseMBB)) {
180      // Non-local uses where the result of the extension is used. Always
181      // replace these unless it's a PHI.
182      Uses.push_back(&UseMO);
183    } else if (Aggressive && DT->dominates(MBB, UseMBB)) {
184      // We may want to extend the live range of the extension result in order
185      // to replace these uses.
186      ExtendedUses.push_back(&UseMO);
187    } else {
188      // Both will be live out of the def MBB anyway. Don't extend live range of
189      // the extension result.
190      ExtendLife = false;
191      break;
192    }
193  }
194
195  if (ExtendLife && !ExtendedUses.empty())
196    // Extend the liveness of the extension result.
197    std::copy(ExtendedUses.begin(), ExtendedUses.end(),
198              std::back_inserter(Uses));
199
200  // Now replace all uses.
201  bool Changed = false;
202  if (!Uses.empty()) {
203    SmallPtrSet<MachineBasicBlock*, 4> PHIBBs;
204
205    // Look for PHI uses of the extended result, we don't want to extend the
206    // liveness of a PHI input. It breaks all kinds of assumptions down
207    // stream. A PHI use is expected to be the kill of its source values.
208    UI = MRI->use_nodbg_begin(DstReg);
209    for (MachineRegisterInfo::use_nodbg_iterator
210           UE = MRI->use_nodbg_end(); UI != UE; ++UI)
211      if (UI->isPHI())
212        PHIBBs.insert(UI->getParent());
213
214    const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
215    for (unsigned i = 0, e = Uses.size(); i != e; ++i) {
216      MachineOperand *UseMO = Uses[i];
217      MachineInstr *UseMI = UseMO->getParent();
218      MachineBasicBlock *UseMBB = UseMI->getParent();
219      if (PHIBBs.count(UseMBB))
220        continue;
221
222      unsigned NewVR = MRI->createVirtualRegister(RC);
223      BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
224              TII->get(TargetOpcode::COPY), NewVR)
225        .addReg(DstReg, 0, SubIdx);
226
227      UseMO->setReg(NewVR);
228      ++NumReuse;
229      Changed = true;
230    }
231  }
232
233  return Changed;
234}
235
236/// OptimizeCmpInstr - If the instruction is a compare and the previous
237/// instruction it's comparing against all ready sets (or could be modified to
238/// set) the same flag as the compare, then we can remove the comparison and use
239/// the flag from the previous instruction.
240bool PeepholeOptimizer::OptimizeCmpInstr(MachineInstr *MI,
241                                         MachineBasicBlock *MBB,
242                                         MachineBasicBlock::iterator &NextIter){
243  // If this instruction is a comparison against zero and isn't comparing a
244  // physical register, we can try to optimize it.
245  unsigned SrcReg;
246  int CmpMask, CmpValue;
247  if (!TII->AnalyzeCompare(MI, SrcReg, CmpMask, CmpValue) ||
248      TargetRegisterInfo::isPhysicalRegister(SrcReg))
249    return false;
250
251  // Attempt to optimize the comparison instruction.
252  if (TII->OptimizeCompareInstr(MI, SrcReg, CmpMask, CmpValue, MRI, NextIter)) {
253    ++NumEliminated;
254    return true;
255  }
256
257  return false;
258}
259
260bool PeepholeOptimizer::runOnMachineFunction(MachineFunction &MF) {
261  TM  = &MF.getTarget();
262  TII = TM->getInstrInfo();
263  MRI = &MF.getRegInfo();
264  DT  = Aggressive ? &getAnalysis<MachineDominatorTree>() : 0;
265
266  bool Changed = false;
267
268  SmallPtrSet<MachineInstr*, 8> LocalMIs;
269  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
270    MachineBasicBlock *MBB = &*I;
271    LocalMIs.clear();
272
273    for (MachineBasicBlock::iterator
274           MII = I->begin(), MIE = I->end(); MII != MIE; ) {
275      MachineInstr *MI = &*MII;
276
277      if (MI->getDesc().isCompare() &&
278          !MI->getDesc().hasUnmodeledSideEffects()) {
279#if 0
280        if (OptimizeCmpInstr(MI, MBB, MII))
281          Changed = true;
282        else
283#endif
284          ++MII;
285      } else {
286        Changed |= OptimizeExtInstr(MI, MBB, LocalMIs);
287        ++MII;
288      }
289    }
290  }
291
292  return Changed;
293}
294