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#include "llvm/CodeGen/Passes.h"
15#include "llvm/ADT/DenseMap.h"
16#include "llvm/ADT/SetVector.h"
17#include "llvm/ADT/SmallVector.h"
18#include "llvm/ADT/Statistic.h"
19#include "llvm/CodeGen/MachineFunction.h"
20#include "llvm/CodeGen/MachineFunctionPass.h"
21#include "llvm/CodeGen/MachineRegisterInfo.h"
22#include "llvm/Pass.h"
23#include "llvm/Support/Debug.h"
24#include "llvm/Support/ErrorHandling.h"
25#include "llvm/Support/raw_ostream.h"
26#include "llvm/Target/TargetInstrInfo.h"
27#include "llvm/Target/TargetRegisterInfo.h"
28#include "llvm/Target/TargetSubtargetInfo.h"
29using namespace llvm;
30
31#define DEBUG_TYPE "codegen-cp"
32
33STATISTIC(NumDeletes, "Number of dead copies deleted");
34
35namespace {
36  class MachineCopyPropagation : public MachineFunctionPass {
37    const TargetRegisterInfo *TRI;
38    const TargetInstrInfo *TII;
39    MachineRegisterInfo *MRI;
40
41  public:
42    static char ID; // Pass identification, replacement for typeid
43    MachineCopyPropagation() : MachineFunctionPass(ID) {
44     initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
45    }
46
47    bool runOnMachineFunction(MachineFunction &MF) override;
48
49  private:
50    typedef SmallVector<unsigned, 4> DestList;
51    typedef DenseMap<unsigned, DestList> SourceMap;
52
53    void SourceNoLongerAvailable(unsigned Reg,
54                                 SourceMap &SrcMap,
55                                 DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
56    bool CopyPropagateBlock(MachineBasicBlock &MBB);
57  };
58}
59char MachineCopyPropagation::ID = 0;
60char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
61
62INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
63                "Machine Copy Propagation Pass", false, false)
64
65void
66MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
67                              SourceMap &SrcMap,
68                              DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
69  for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
70    SourceMap::iterator SI = SrcMap.find(*AI);
71    if (SI != SrcMap.end()) {
72      const DestList& Defs = SI->second;
73      for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
74           I != E; ++I) {
75        unsigned MappedDef = *I;
76        // Source of copy is no longer available for propagation.
77        AvailCopyMap.erase(MappedDef);
78        for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
79          AvailCopyMap.erase(*SR);
80      }
81    }
82  }
83}
84
85static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
86                                    const MachineInstr *MI) {
87  const MachineBasicBlock *MBB = CopyMI->getParent();
88  if (MI->getParent() != MBB)
89    return false;
90  MachineBasicBlock::const_iterator I = CopyMI;
91  MachineBasicBlock::const_iterator E = MBB->end();
92  MachineBasicBlock::const_iterator E2 = MI;
93
94  ++I;
95  while (I != E && I != E2) {
96    if (I->hasUnmodeledSideEffects() || I->isCall() ||
97        I->isTerminator())
98      return false;
99    ++I;
100  }
101  return true;
102}
103
104/// isNopCopy - Return true if the specified copy is really a nop. That is
105/// if the source of the copy is the same of the definition of the copy that
106/// supplied the source. If the source of the copy is a sub-register than it
107/// must check the sub-indices match. e.g.
108/// ecx = mov eax
109/// al  = mov cl
110/// But not
111/// ecx = mov eax
112/// al  = mov ch
113static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
114                      const TargetRegisterInfo *TRI) {
115  unsigned SrcSrc = CopyMI->getOperand(1).getReg();
116  if (Def == SrcSrc)
117    return true;
118  if (TRI->isSubRegister(SrcSrc, Def)) {
119    unsigned SrcDef = CopyMI->getOperand(0).getReg();
120    unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
121    if (!SubIdx)
122      return false;
123    return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
124  }
125
126  return false;
127}
128
129bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
130  SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
131  DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
132  DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
133  SourceMap SrcMap; // Src -> Def map
134
135  DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
136
137  bool Changed = false;
138  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
139    MachineInstr *MI = &*I;
140    ++I;
141
142    if (MI->isCopy()) {
143      unsigned Def = MI->getOperand(0).getReg();
144      unsigned Src = MI->getOperand(1).getReg();
145
146      if (TargetRegisterInfo::isVirtualRegister(Def) ||
147          TargetRegisterInfo::isVirtualRegister(Src))
148        report_fatal_error("MachineCopyPropagation should be run after"
149                           " register allocation!");
150
151      DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
152      if (CI != AvailCopyMap.end()) {
153        MachineInstr *CopyMI = CI->second;
154        if (!MRI->isReserved(Def) &&
155            (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
156            isNopCopy(CopyMI, Def, Src, TRI)) {
157          // The two copies cancel out and the source of the first copy
158          // hasn't been overridden, eliminate the second one. e.g.
159          //  %ECX<def> = COPY %EAX<kill>
160          //  ... nothing clobbered EAX.
161          //  %EAX<def> = COPY %ECX
162          // =>
163          //  %ECX<def> = COPY %EAX
164          //
165          // Also avoid eliminating a copy from reserved registers unless the
166          // definition is proven not clobbered. e.g.
167          // %RSP<def> = COPY %RAX
168          // CALL
169          // %RAX<def> = COPY %RSP
170
171          DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
172
173          // Clear any kills of Def between CopyMI and MI. This extends the
174          // live range.
175          for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
176            I->clearRegisterKills(Def, TRI);
177
178          MI->eraseFromParent();
179          Changed = true;
180          ++NumDeletes;
181          continue;
182        }
183      }
184
185      // If Src is defined by a previous copy, it cannot be eliminated.
186      for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
187        CI = CopyMap.find(*AI);
188        if (CI != CopyMap.end()) {
189          DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
190          MaybeDeadCopies.remove(CI->second);
191        }
192      }
193
194      DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
195
196      // Copy is now a candidate for deletion.
197      MaybeDeadCopies.insert(MI);
198
199      // If 'Src' is previously source of another copy, then this earlier copy's
200      // source is no longer available. e.g.
201      // %xmm9<def> = copy %xmm2
202      // ...
203      // %xmm2<def> = copy %xmm0
204      // ...
205      // %xmm2<def> = copy %xmm9
206      SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
207
208      // Remember Def is defined by the copy.
209      // ... Make sure to clear the def maps of aliases first.
210      for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
211        CopyMap.erase(*AI);
212        AvailCopyMap.erase(*AI);
213      }
214      for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
215           ++SR) {
216        CopyMap[*SR] = MI;
217        AvailCopyMap[*SR] = MI;
218      }
219
220      // Remember source that's copied to Def. Once it's clobbered, then
221      // it's no longer available for copy propagation.
222      if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
223          SrcMap[Src].end()) {
224        SrcMap[Src].push_back(Def);
225      }
226
227      continue;
228    }
229
230    // Not a copy.
231    SmallVector<unsigned, 2> Defs;
232    int RegMaskOpNum = -1;
233    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
234      MachineOperand &MO = MI->getOperand(i);
235      if (MO.isRegMask())
236        RegMaskOpNum = i;
237      if (!MO.isReg())
238        continue;
239      unsigned Reg = MO.getReg();
240      if (!Reg)
241        continue;
242
243      if (TargetRegisterInfo::isVirtualRegister(Reg))
244        report_fatal_error("MachineCopyPropagation should be run after"
245                           " register allocation!");
246
247      if (MO.isDef()) {
248        Defs.push_back(Reg);
249        continue;
250      }
251
252      // If 'Reg' is defined by a copy, the copy is no longer a candidate
253      // for elimination.
254      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
255        DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
256        if (CI != CopyMap.end()) {
257          DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
258          MaybeDeadCopies.remove(CI->second);
259        }
260      }
261      // Treat undef use like defs for copy propagation but not for
262      // dead copy. We would need to do a liveness check to be sure the copy
263      // is dead for undef uses.
264      // The backends are allowed to do whatever they want with undef value
265      // and we cannot be sure this register will not be rewritten to break
266      // some false dependencies for the hardware for instance.
267      if (MO.isUndef())
268        Defs.push_back(Reg);
269    }
270
271    // The instruction has a register mask operand which means that it clobbers
272    // a large set of registers.  It is possible to use the register mask to
273    // prune the available copies, but treat it like a basic block boundary for
274    // now.
275    if (RegMaskOpNum >= 0) {
276      // Erase any MaybeDeadCopies whose destination register is clobbered.
277      const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
278      for (SmallSetVector<MachineInstr*, 8>::iterator
279           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
280           DI != DE; ++DI) {
281        unsigned Reg = (*DI)->getOperand(0).getReg();
282        if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
283          continue;
284        DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
285              (*DI)->dump());
286        (*DI)->eraseFromParent();
287        Changed = true;
288        ++NumDeletes;
289      }
290
291      // Clear all data structures as if we were beginning a new basic block.
292      MaybeDeadCopies.clear();
293      AvailCopyMap.clear();
294      CopyMap.clear();
295      SrcMap.clear();
296      continue;
297    }
298
299    for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
300      unsigned Reg = Defs[i];
301
302      // No longer defined by a copy.
303      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
304        CopyMap.erase(*AI);
305        AvailCopyMap.erase(*AI);
306      }
307
308      // If 'Reg' is previously source of a copy, it is no longer available for
309      // copy propagation.
310      SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
311    }
312  }
313
314  // If MBB doesn't have successors, delete the copies whose defs are not used.
315  // If MBB does have successors, then conservative assume the defs are live-out
316  // since we don't want to trust live-in lists.
317  if (MBB.succ_empty()) {
318    for (SmallSetVector<MachineInstr*, 8>::iterator
319           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
320         DI != DE; ++DI) {
321      if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
322        (*DI)->eraseFromParent();
323        Changed = true;
324        ++NumDeletes;
325      }
326    }
327  }
328
329  return Changed;
330}
331
332bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
333  if (skipOptnoneFunction(*MF.getFunction()))
334    return false;
335
336  bool Changed = false;
337
338  TRI = MF.getSubtarget().getRegisterInfo();
339  TII = MF.getSubtarget().getInstrInfo();
340  MRI = &MF.getRegInfo();
341
342  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
343    Changed |= CopyPropagateBlock(*I);
344
345  return Changed;
346}
347