xref: /llvm-project/llvm/lib/CodeGen/MachineCopyPropagation.cpp (revision 1fa5bcbe2ae447344d6cbba3f19daa9a5bed5de8) 
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"
28 using namespace llvm;
29 
30 STATISTIC(NumDeletes, "Number of dead copies deleted");
31 
32 namespace {
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 }
52 char MachineCopyPropagation::ID = 0;
53 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
54 
55 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
56                 "Machine Copy Propagation Pass", false, false)
57 
58 void
59 MachineCopyPropagation::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 unsigned *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
68         AvailCopyMap.erase(*SR);
69     }
70   }
71   for (const unsigned *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 unsigned *SR = TRI->getSubRegisters(MappedDef); *SR; ++SR)
77           AvailCopyMap.erase(*SR);
78       }
79     }
80   }
81 }
82 
83 static 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 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
103   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; // Candidates for deletion
104   DenseMap<unsigned, MachineInstr*> AvailCopyMap;   // Def -> available copies map
105   DenseMap<unsigned, MachineInstr*> CopyMap;        // Def -> copies map
106   DenseMap<unsigned, unsigned> SrcMap;              // Src -> Def map
107 
108   bool Changed = false;
109   for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
110     MachineInstr *MI = &*I;
111     ++I;
112 
113     if (MI->isCopy()) {
114       unsigned Def = MI->getOperand(0).getReg();
115       unsigned Src = MI->getOperand(1).getReg();
116 
117       if (TargetRegisterInfo::isVirtualRegister(Def) ||
118           TargetRegisterInfo::isVirtualRegister(Src))
119         report_fatal_error("MachineCopyPropagation should be run after"
120                            " register allocation!");
121 
122       DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
123       if (CI != AvailCopyMap.end()) {
124         MachineInstr *CopyMI = CI->second;
125         unsigned SrcSrc = CopyMI->getOperand(1).getReg();
126         if (!ReservedRegs.test(Def) &&
127             (!ReservedRegs.test(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
128             (SrcSrc == Def || TRI->isSubRegister(SrcSrc, Def))) {
129           // The two copies cancel out and the source of the first copy
130           // hasn't been overridden, eliminate the second one. e.g.
131           //  %ECX<def> = COPY %EAX<kill>
132           //  ... nothing clobbered EAX.
133           //  %EAX<def> = COPY %ECX
134           // =>
135           //  %ECX<def> = COPY %EAX
136           //
137           // Also avoid eliminating a copy from reserved registers unless the
138           // definition is proven not clobbered. e.g.
139           // %RSP<def> = COPY %RAX
140           // CALL
141           // %RAX<def> = COPY %RSP
142 
143           // Clear any kills of Def between CopyMI and MI. This extends the
144           // live range.
145           for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
146             I->clearRegisterKills(Def, TRI);
147 
148           MI->eraseFromParent();
149           Changed = true;
150           ++NumDeletes;
151           continue;
152         }
153       }
154 
155       // If Src is defined by a previous copy, it cannot be eliminated.
156       CI = CopyMap.find(Src);
157       if (CI != CopyMap.end())
158         MaybeDeadCopies.remove(CI->second);
159       for (const unsigned *AS = TRI->getAliasSet(Src); *AS; ++AS) {
160         CI = CopyMap.find(*AS);
161         if (CI != CopyMap.end())
162           MaybeDeadCopies.remove(CI->second);
163       }
164 
165       // Copy is now a candidate for deletion.
166       MaybeDeadCopies.insert(MI);
167 
168       // If 'Src' is previously source of another copy, then this earlier copy's
169       // source is no longer available. e.g.
170       // %xmm9<def> = copy %xmm2
171       // ...
172       // %xmm2<def> = copy %xmm0
173       // ...
174       // %xmm2<def> = copy %xmm9
175       SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
176 
177       // Remember Def is defined by the copy.
178       CopyMap[Def] = MI;
179       AvailCopyMap[Def] = MI;
180       for (const unsigned *SR = TRI->getSubRegisters(Def); *SR; ++SR) {
181         CopyMap[*SR] = MI;
182         AvailCopyMap[*SR] = MI;
183       }
184 
185       // Remember source that's copied to Def. Once it's clobbered, then
186       // it's no longer available for copy propagation.
187       SrcMap[Src] = Def;
188 
189       continue;
190     }
191 
192     // Not a copy.
193     SmallVector<unsigned, 2> Defs;
194     for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
195       MachineOperand &MO = MI->getOperand(i);
196       if (!MO.isReg())
197         continue;
198       unsigned Reg = MO.getReg();
199       if (!Reg)
200         continue;
201 
202       if (TargetRegisterInfo::isVirtualRegister(Reg))
203         report_fatal_error("MachineCopyPropagation should be run after"
204                            " register allocation!");
205 
206       if (MO.isDef()) {
207         Defs.push_back(Reg);
208         continue;
209       }
210 
211       // If 'Reg' is defined by a copy, the copy is no longer a candidate
212       // for elimination.
213       DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(Reg);
214       if (CI != CopyMap.end())
215         MaybeDeadCopies.remove(CI->second);
216       for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
217         CI = CopyMap.find(*AS);
218         if (CI != CopyMap.end())
219           MaybeDeadCopies.remove(CI->second);
220       }
221     }
222 
223     for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
224       unsigned Reg = Defs[i];
225 
226       // No longer defined by a copy.
227       CopyMap.erase(Reg);
228       AvailCopyMap.erase(Reg);
229       for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
230         CopyMap.erase(*AS);
231         AvailCopyMap.erase(*AS);
232       }
233 
234       // If 'Reg' is previously source of a copy, it is no longer available for
235       // copy propagation.
236       SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
237     }
238   }
239 
240   // If MBB doesn't have successors, delete the copies whose defs are not used.
241   // If MBB does have successors, then conservative assume the defs are live-out
242   // since we don't want to trust live-in lists.
243   if (MBB.succ_empty()) {
244     for (SmallSetVector<MachineInstr*, 8>::iterator
245            DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
246          DI != DE; ++DI) {
247       if (!ReservedRegs.test((*DI)->getOperand(0).getReg())) {
248         (*DI)->eraseFromParent();
249         Changed = true;
250         ++NumDeletes;
251       }
252     }
253   }
254 
255   return Changed;
256 }
257 
258 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
259   bool Changed = false;
260 
261   TRI = MF.getTarget().getRegisterInfo();
262   ReservedRegs = TRI->getReservedRegs(MF);
263 
264   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
265     Changed |= CopyPropagateBlock(*I);
266 
267   return Changed;
268 }
269