MachineCopyPropagation.cpp revision 9ebfbf8b9fd5f982e0db9293808bd32168615ba9 
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/Target/TargetRegisterInfo.h"
20#include "llvm/Support/Debug.h"
21#include "llvm/Support/ErrorHandling.h"
22#include "llvm/Support/raw_ostream.h"
23#include "llvm/ADT/BitVector.h"
24#include "llvm/ADT/DenseMap.h"
25#include "llvm/ADT/SetVector.h"
26#include "llvm/ADT/SmallVector.h"
27#include "llvm/ADT/Statistic.h"
28using namespace llvm;
29
30STATISTIC(NumDeletes, "Number of dead copies deleted");
31
32namespace {
33  class MachineCopyPropagation : public MachineFunctionPass {
34    const TargetRegisterInfo *TRI;
35    BitVector ReservedRegs;
36
37  public:
38    static char ID; // Pass identification, replacement for typeid
39    MachineCopyPropagation() : MachineFunctionPass(ID) {
40     initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
41    }
42
43    virtual bool runOnMachineFunction(MachineFunction &MF);
44
45  private:
46    void SourceNoLongerAvailable(unsigned Reg,
47                               DenseMap<unsigned, unsigned> &SrcMap,
48                               DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
49    bool CopyPropagateBlock(MachineBasicBlock &MBB);
50  };
51}
52char MachineCopyPropagation::ID = 0;
53char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
54
55INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
56                "Machine Copy Propagation Pass", false, false)
57
58void
59MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
60                              DenseMap<unsigned, unsigned> &SrcMap,
61                              DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
62  DenseMap<unsigned, unsigned>::iterator SI = SrcMap.find(Reg);
63  if (SI != SrcMap.end()) {
64    unsigned MappedDef = SI->second;
65    // Source of copy is no longer available for propagation.
66    if (AvailCopyMap.erase(MappedDef)) {
67      for (const uint16_t *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
68        AvailCopyMap.erase(*SR);
69    }
70  }
71  for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
72    SI = SrcMap.find(*AS);
73    if (SI != SrcMap.end()) {
74      unsigned MappedDef = SI->second;
75      if (AvailCopyMap.erase(MappedDef)) {
76        for (const uint16_t *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
77          AvailCopyMap.erase(*SR);
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  DenseMap<unsigned, unsigned> 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 (!ReservedRegs.test(Def) &&
151            (!ReservedRegs.test(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      CI = CopyMap.find(Src);
181      if (CI != CopyMap.end())
182        MaybeDeadCopies.remove(CI->second);
183      for (const uint16_t *AS = TRI->getAliasSet(Src); *AS; ++AS) {
184        CI = CopyMap.find(*AS);
185        if (CI != CopyMap.end())
186          MaybeDeadCopies.remove(CI->second);
187      }
188
189      // Copy is now a candidate for deletion.
190      MaybeDeadCopies.insert(MI);
191
192      // If 'Src' is previously source of another copy, then this earlier copy's
193      // source is no longer available. e.g.
194      // %xmm9<def> = copy %xmm2
195      // ...
196      // %xmm2<def> = copy %xmm0
197      // ...
198      // %xmm2<def> = copy %xmm9
199      SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
200
201      // Remember Def is defined by the copy.
202      // ... Make sure to clear the def maps of aliases first.
203      for (const uint16_t *AS = TRI->getAliasSet(Def); *AS; ++AS) {
204        CopyMap.erase(*AS);
205        AvailCopyMap.erase(*AS);
206      }
207      CopyMap[Def] = MI;
208      AvailCopyMap[Def] = MI;
209      for (const uint16_t *SR = TRI->getSubRegisters(Def); *SR; ++SR) {
210        CopyMap[*SR] = MI;
211        AvailCopyMap[*SR] = MI;
212      }
213
214      // Remember source that's copied to Def. Once it's clobbered, then
215      // it's no longer available for copy propagation.
216      SrcMap[Src] = Def;
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      DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(Reg);
246      if (CI != CopyMap.end())
247        MaybeDeadCopies.remove(CI->second);
248      for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
249        CI = CopyMap.find(*AS);
250        if (CI != CopyMap.end())
251          MaybeDeadCopies.remove(CI->second);
252      }
253    }
254
255    // The instruction has a register mask operand which means that it clobbers
256    // a large set of registers.  It is possible to use the register mask to
257    // prune the available copies, but treat it like a basic block boundary for
258    // now.
259    if (RegMaskOpNum >= 0) {
260      // Erase any MaybeDeadCopies whose destination register is clobbered.
261      const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
262      for (SmallSetVector<MachineInstr*, 8>::iterator
263           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
264           DI != DE; ++DI) {
265        unsigned Reg = (*DI)->getOperand(0).getReg();
266        if (ReservedRegs.test(Reg) || !MaskMO.clobbersPhysReg(Reg))
267          continue;
268        (*DI)->eraseFromParent();
269        Changed = true;
270        ++NumDeletes;
271      }
272
273      // Clear all data structures as if we were beginning a new basic block.
274      MaybeDeadCopies.clear();
275      AvailCopyMap.clear();
276      CopyMap.clear();
277      SrcMap.clear();
278      continue;
279    }
280
281    for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
282      unsigned Reg = Defs[i];
283
284      // No longer defined by a copy.
285      CopyMap.erase(Reg);
286      AvailCopyMap.erase(Reg);
287      for (const uint16_t *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
288        CopyMap.erase(*AS);
289        AvailCopyMap.erase(*AS);
290      }
291
292      // If 'Reg' is previously source of a copy, it is no longer available for
293      // copy propagation.
294      SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
295    }
296  }
297
298  // If MBB doesn't have successors, delete the copies whose defs are not used.
299  // If MBB does have successors, then conservative assume the defs are live-out
300  // since we don't want to trust live-in lists.
301  if (MBB.succ_empty()) {
302    for (SmallSetVector<MachineInstr*, 8>::iterator
303           DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
304         DI != DE; ++DI) {
305      if (!ReservedRegs.test((*DI)->getOperand(0).getReg())) {
306        (*DI)->eraseFromParent();
307        Changed = true;
308        ++NumDeletes;
309      }
310    }
311  }
312
313  return Changed;
314}
315
316bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
317  bool Changed = false;
318
319  TRI = MF.getTarget().getRegisterInfo();
320  ReservedRegs = TRI->getReservedRegs(MF);
321
322  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
323    Changed |= CopyPropagateBlock(*I);
324
325  return Changed;
326}
327