MachineCopyPropagation.cpp revision fb9ebbf236974beac31705eaeb9f50ab585af6ab 
1//===- MachineCopyPropagation.cpp - Machine Copy Propagation Pass ---------===//
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 is an extremely simple MachineInstr-level copy propagation pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "codegen-cp"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/Pass.h"
17#include "llvm/CodeGen/MachineFunction.h"
18#include "llvm/CodeGen/MachineFunctionPass.h"
19#include "llvm/CodeGen/MachineRegisterInfo.h"
20#include "llvm/Target/TargetRegisterInfo.h"
21#include "llvm/Support/Debug.h"
22#include "llvm/Support/ErrorHandling.h"
23#include "llvm/Support/raw_ostream.h"
24#include "llvm/ADT/BitVector.h"
25#include "llvm/ADT/DenseMap.h"
26#include "llvm/ADT/SetVector.h"
27#include "llvm/ADT/SmallVector.h"
28#include "llvm/ADT/Statistic.h"
29using namespace llvm;
30
31STATISTIC(NumDeletes, "Number of dead copies deleted");
32
33namespace {
34  class MachineCopyPropagation : public MachineFunctionPass {
35    const TargetRegisterInfo *TRI;
36    MachineRegisterInfo *MRI;
37
38  public:
39    static char ID; // Pass identification, replacement for typeid
40    MachineCopyPropagation() : MachineFunctionPass(ID) {
41     initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
42    }
43
44    virtual bool runOnMachineFunction(MachineFunction &MF);
45
46  private:
47    typedef SmallVector<unsigned, 4> DestList;
48    typedef DenseMap<unsigned, DestList> SourceMap;
49
50    void SourceNoLongerAvailable(unsigned Reg,
51                                 SourceMap &SrcMap,
52                                 DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
53    bool CopyPropagateBlock(MachineBasicBlock &MBB);
54  };
55}
56char MachineCopyPropagation::ID = 0;
57char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
58
59INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
60                "Machine Copy Propagation Pass", false, false)
61
62void
63MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
64                              SourceMap &SrcMap,
65                              DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
66  for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
67    SourceMap::iterator SI = SrcMap.find(*AI);
68    if (SI != SrcMap.end()) {
69      const DestList& Defs = SI->second;
70      for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
71           I != E; ++I) {
72        unsigned MappedDef = *I;
73        // Source of copy is no longer available for propagation.
74        if (AvailCopyMap.erase(MappedDef)) {
75          for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
76            AvailCopyMap.erase(*SR);
77        }
78      }
79    }
80  }
81}
82
83static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
84                                    const MachineInstr *MI) {
85  const MachineBasicBlock *MBB = CopyMI->getParent();
86  if (MI->getParent() != MBB)
87    return false;
88  MachineBasicBlock::const_iterator I = CopyMI;
89  MachineBasicBlock::const_iterator E = MBB->end();
90  MachineBasicBlock::const_iterator E2 = MI;
91
92  ++I;
93  while (I != E && I != E2) {
94    if (I->hasUnmodeledSideEffects() || I->isCall() ||
95        I->isTerminator())
96      return false;
97    ++I;
98  }
99  return true;
100}
101
102/// isNopCopy - Return true if the specified copy is really a nop. That is
103/// if the source of the copy is the same of the definition of the copy that
104/// supplied the source. If the source of the copy is a sub-register than it
105/// must check the sub-indices match. e.g.
106/// ecx = mov eax
107/// al  = mov cl
108/// But not
109/// ecx = mov eax
110/// al  = mov ch
111static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
112                      const TargetRegisterInfo *TRI) {
113  unsigned SrcSrc = CopyMI->getOperand(1).getReg();
114  if (Def == SrcSrc)
115    return true;
116  if (TRI->isSubRegister(SrcSrc, Def)) {
117    unsigned SrcDef = CopyMI->getOperand(0).getReg();
118    unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
119    if (!SubIdx)
120      return false;
121    return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
122  }
123
124  return false;
125}
126
127bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
128  SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
129  DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
130  DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
131  SourceMap SrcMap; // Src -> Def map
132
133  bool Changed = false;
134  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
135    MachineInstr *MI = &*I;
136    ++I;
137
138    if (MI->isCopy()) {
139      unsigned Def = MI->getOperand(0).getReg();
140      unsigned Src = MI->getOperand(1).getReg();
141
142      if (TargetRegisterInfo::isVirtualRegister(Def) ||
143          TargetRegisterInfo::isVirtualRegister(Src))
144        report_fatal_error("MachineCopyPropagation should be run after"
145                           " register allocation!");
146
147      DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
148      if (CI != AvailCopyMap.end()) {
149        MachineInstr *CopyMI = CI->second;
150        if (!MRI->isReserved(Def) &&
151            (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
152            isNopCopy(CopyMI, Def, Src, TRI)) {
153          // The two copies cancel out and the source of the first copy
154          // hasn't been overridden, eliminate the second one. e.g.
155          //  %ECX<def> = COPY %EAX<kill>
156          //  ... nothing clobbered EAX.
157          //  %EAX<def> = COPY %ECX
158          // =>
159          //  %ECX<def> = COPY %EAX
160          //
161          // Also avoid eliminating a copy from reserved registers unless the
162          // definition is proven not clobbered. e.g.
163          // %RSP<def> = COPY %RAX
164          // CALL
165          // %RAX<def> = COPY %RSP
166
167          // Clear any kills of Def between CopyMI and MI. This extends the
168          // live range.
169          for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
170            I->clearRegisterKills(Def, TRI);
171
172          MI->eraseFromParent();
173          Changed = true;
174          ++NumDeletes;
175          continue;
176        }
177      }
178
179      // If Src is defined by a previous copy, it cannot be eliminated.
180      for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
181        CI = CopyMap.find(*AI);
182        if (CI != CopyMap.end())
183          MaybeDeadCopies.remove(CI->second);
184      }
185
186      // Copy is now a candidate for deletion.
187      MaybeDeadCopies.insert(MI);
188
189      // If 'Src' is previously source of another copy, then this earlier copy's
190      // source is no longer available. e.g.
191      // %xmm9<def> = copy %xmm2
192      // ...
193      // %xmm2<def> = copy %xmm0
194      // ...
195      // %xmm2<def> = copy %xmm9
196      SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
197
198      // Remember Def is defined by the copy.
199      // ... Make sure to clear the def maps of aliases first.
200      for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
201        CopyMap.erase(*AI);
202        AvailCopyMap.erase(*AI);
203      }
204      CopyMap[Def] = MI;
205      AvailCopyMap[Def] = MI;
206      for (MCSubRegIterator SR(Def, TRI); SR.isValid(); ++SR) {
207        CopyMap[*SR] = MI;
208        AvailCopyMap[*SR] = MI;
209      }
210
211      // Remember source that's copied to Def. Once it's clobbered, then
212      // it's no longer available for copy propagation.
213      if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
214          SrcMap[Src].end()) {
215        SrcMap[Src].push_back(Def);
216      }
217
218      continue;
219    }
220
221    // Not a copy.
222    SmallVector<unsigned, 2> Defs;
223    int RegMaskOpNum = -1;
224    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
225      MachineOperand &MO = MI->getOperand(i);
226      if (MO.isRegMask())
227        RegMaskOpNum = i;
228      if (!MO.isReg())
229        continue;
230      unsigned Reg = MO.getReg();
231      if (!Reg)
232        continue;
233
234      if (TargetRegisterInfo::isVirtualRegister(Reg))
235        report_fatal_error("MachineCopyPropagation should be run after"
236                           " register allocation!");
237
238      if (MO.isDef()) {
239        Defs.push_back(Reg);
240        continue;
241      }
242
243      // If 'Reg' is defined by a copy, the copy is no longer a candidate
244      // for elimination.
245      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
246        DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
247        if (CI != CopyMap.end())
248          MaybeDeadCopies.remove(CI->second);
249      }
250    }
251
252    // The instruction has a register mask operand which means that it clobbers
253    // a large set of registers.  It is possible to use the register mask to
254    // prune the available copies, but treat it like a basic block boundary for
255    // now.
256    if (RegMaskOpNum >= 0) {
257      // Erase any MaybeDeadCopies whose destination register is clobbered.
258      const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
259      for (SmallSetVector<MachineInstr*, 8>::iterator
260           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
261           DI != DE; ++DI) {
262        unsigned Reg = (*DI)->getOperand(0).getReg();
263        if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
264          continue;
265        (*DI)->eraseFromParent();
266        Changed = true;
267        ++NumDeletes;
268      }
269
270      // Clear all data structures as if we were beginning a new basic block.
271      MaybeDeadCopies.clear();
272      AvailCopyMap.clear();
273      CopyMap.clear();
274      SrcMap.clear();
275      continue;
276    }
277
278    for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
279      unsigned Reg = Defs[i];
280
281      // No longer defined by a copy.
282      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
283        CopyMap.erase(*AI);
284        AvailCopyMap.erase(*AI);
285      }
286
287      // If 'Reg' is previously source of a copy, it is no longer available for
288      // copy propagation.
289      SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
290    }
291  }
292
293  // If MBB doesn't have successors, delete the copies whose defs are not used.
294  // If MBB does have successors, then conservative assume the defs are live-out
295  // since we don't want to trust live-in lists.
296  if (MBB.succ_empty()) {
297    for (SmallSetVector<MachineInstr*, 8>::iterator
298           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
299         DI != DE; ++DI) {
300      if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
301        (*DI)->eraseFromParent();
302        Changed = true;
303        ++NumDeletes;
304      }
305    }
306  }
307
308  return Changed;
309}
310
311bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
312  bool Changed = false;
313
314  TRI = MF.getTarget().getRegisterInfo();
315  MRI = &MF.getRegInfo();
316
317  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
318    Changed |= CopyPropagateBlock(*I);
319
320  return Changed;
321}
322