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"
28using namespace llvm;
29
30#define DEBUG_TYPE "codegen-cp"
31
32STATISTIC(NumDeletes, "Number of dead copies deleted");
33
34namespace {
35  class MachineCopyPropagation : public MachineFunctionPass {
36    const TargetRegisterInfo *TRI;
37    const TargetInstrInfo *TII;
38    MachineRegisterInfo *MRI;
39
40  public:
41    static char ID; // Pass identification, replacement for typeid
42    MachineCopyPropagation() : MachineFunctionPass(ID) {
43     initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
44    }
45
46    bool runOnMachineFunction(MachineFunction &MF) override;
47
48  private:
49    typedef SmallVector<unsigned, 4> DestList;
50    typedef DenseMap<unsigned, DestList> SourceMap;
51
52    void SourceNoLongerAvailable(unsigned Reg,
53                                 SourceMap &SrcMap,
54                                 DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
55    bool CopyPropagateBlock(MachineBasicBlock &MBB);
56    void removeCopy(MachineInstr *MI);
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        if (AvailCopyMap.erase(MappedDef)) {
78          for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
79            AvailCopyMap.erase(*SR);
80        }
81      }
82    }
83  }
84}
85
86static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
87                                    const MachineInstr *MI) {
88  const MachineBasicBlock *MBB = CopyMI->getParent();
89  if (MI->getParent() != MBB)
90    return false;
91  MachineBasicBlock::const_iterator I = CopyMI;
92  MachineBasicBlock::const_iterator E = MBB->end();
93  MachineBasicBlock::const_iterator E2 = MI;
94
95  ++I;
96  while (I != E && I != E2) {
97    if (I->hasUnmodeledSideEffects() || I->isCall() ||
98        I->isTerminator())
99      return false;
100    ++I;
101  }
102  return true;
103}
104
105/// isNopCopy - Return true if the specified copy is really a nop. That is
106/// if the source of the copy is the same of the definition of the copy that
107/// supplied the source. If the source of the copy is a sub-register than it
108/// must check the sub-indices match. e.g.
109/// ecx = mov eax
110/// al  = mov cl
111/// But not
112/// ecx = mov eax
113/// al  = mov ch
114static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
115                      const TargetRegisterInfo *TRI) {
116  unsigned SrcSrc = CopyMI->getOperand(1).getReg();
117  if (Def == SrcSrc)
118    return true;
119  if (TRI->isSubRegister(SrcSrc, Def)) {
120    unsigned SrcDef = CopyMI->getOperand(0).getReg();
121    unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
122    if (!SubIdx)
123      return false;
124    return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
125  }
126
127  return false;
128}
129
130// Remove MI from the function because it has been determined it is dead.
131// Turn it into a noop KILL instruction as opposed to removing it to
132// maintain imp-use/imp-def chains.
133void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
134  MI->setDesc(TII->get(TargetOpcode::KILL));
135}
136
137bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
138  SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;  // Candidates for deletion
139  DenseMap<unsigned, MachineInstr*> AvailCopyMap;    // Def -> available copies map
140  DenseMap<unsigned, MachineInstr*> CopyMap;         // Def -> copies map
141  SourceMap SrcMap; // Src -> Def map
142
143  DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
144
145  bool Changed = false;
146  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
147    MachineInstr *MI = &*I;
148    ++I;
149
150    if (MI->isCopy()) {
151      unsigned Def = MI->getOperand(0).getReg();
152      unsigned Src = MI->getOperand(1).getReg();
153
154      if (TargetRegisterInfo::isVirtualRegister(Def) ||
155          TargetRegisterInfo::isVirtualRegister(Src))
156        report_fatal_error("MachineCopyPropagation should be run after"
157                           " register allocation!");
158
159      DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
160      if (CI != AvailCopyMap.end()) {
161        MachineInstr *CopyMI = CI->second;
162        if (!MRI->isReserved(Def) &&
163            (!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
164            isNopCopy(CopyMI, Def, Src, TRI)) {
165          // The two copies cancel out and the source of the first copy
166          // hasn't been overridden, eliminate the second one. e.g.
167          //  %ECX<def> = COPY %EAX<kill>
168          //  ... nothing clobbered EAX.
169          //  %EAX<def> = COPY %ECX
170          // =>
171          //  %ECX<def> = COPY %EAX
172          //
173          // Also avoid eliminating a copy from reserved registers unless the
174          // definition is proven not clobbered. e.g.
175          // %RSP<def> = COPY %RAX
176          // CALL
177          // %RAX<def> = COPY %RSP
178
179          DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
180
181          // Clear any kills of Def between CopyMI and MI. This extends the
182          // live range.
183          for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
184            I->clearRegisterKills(Def, TRI);
185
186          removeCopy(MI);
187          Changed = true;
188          ++NumDeletes;
189          continue;
190        }
191      }
192
193      // If Src is defined by a previous copy, it cannot be eliminated.
194      for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
195        CI = CopyMap.find(*AI);
196        if (CI != CopyMap.end()) {
197          DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
198          MaybeDeadCopies.remove(CI->second);
199        }
200      }
201
202      DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
203
204      // Copy is now a candidate for deletion.
205      MaybeDeadCopies.insert(MI);
206
207      // If 'Src' is previously source of another copy, then this earlier copy's
208      // source is no longer available. e.g.
209      // %xmm9<def> = copy %xmm2
210      // ...
211      // %xmm2<def> = copy %xmm0
212      // ...
213      // %xmm2<def> = copy %xmm9
214      SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
215
216      // Remember Def is defined by the copy.
217      // ... Make sure to clear the def maps of aliases first.
218      for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
219        CopyMap.erase(*AI);
220        AvailCopyMap.erase(*AI);
221      }
222      for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
223           ++SR) {
224        CopyMap[*SR] = MI;
225        AvailCopyMap[*SR] = MI;
226      }
227
228      // Remember source that's copied to Def. Once it's clobbered, then
229      // it's no longer available for copy propagation.
230      if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
231          SrcMap[Src].end()) {
232        SrcMap[Src].push_back(Def);
233      }
234
235      continue;
236    }
237
238    // Not a copy.
239    SmallVector<unsigned, 2> Defs;
240    int RegMaskOpNum = -1;
241    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
242      MachineOperand &MO = MI->getOperand(i);
243      if (MO.isRegMask())
244        RegMaskOpNum = i;
245      if (!MO.isReg())
246        continue;
247      unsigned Reg = MO.getReg();
248      if (!Reg)
249        continue;
250
251      if (TargetRegisterInfo::isVirtualRegister(Reg))
252        report_fatal_error("MachineCopyPropagation should be run after"
253                           " register allocation!");
254
255      if (MO.isDef()) {
256        Defs.push_back(Reg);
257        continue;
258      }
259
260      // If 'Reg' is defined by a copy, the copy is no longer a candidate
261      // for elimination.
262      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
263        DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
264        if (CI != CopyMap.end()) {
265          DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
266          MaybeDeadCopies.remove(CI->second);
267        }
268      }
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        removeCopy(*DI);
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        removeCopy(*DI);
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.getTarget().getRegisterInfo();
339  TII = MF.getTarget().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