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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/raw_ostream.h"
25 #include "llvm/Target/TargetInstrInfo.h"
26 #include "llvm/Target/TargetRegisterInfo.h"
27 #include "llvm/Target/TargetSubtargetInfo.h"
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "codegen-cp"
31 
32 STATISTIC(NumDeletes, "Number of dead copies deleted");
33 
34 namespace {
35   typedef SmallVector<unsigned, 4> RegList;
36   typedef DenseMap<unsigned, RegList> SourceMap;
37   typedef DenseMap<unsigned, MachineInstr*> Reg2MIMap;
38 
39   class MachineCopyPropagation : public MachineFunctionPass {
40     const TargetRegisterInfo *TRI;
41     const TargetInstrInfo *TII;
42     const MachineRegisterInfo *MRI;
43 
44   public:
45     static char ID; // Pass identification, replacement for typeid
MachineCopyPropagation()46     MachineCopyPropagation() : MachineFunctionPass(ID) {
47       initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
48     }
49 
getAnalysisUsage(AnalysisUsage & AU) const50     void getAnalysisUsage(AnalysisUsage &AU) const override {
51       AU.setPreservesCFG();
52       MachineFunctionPass::getAnalysisUsage(AU);
53     }
54 
55     bool runOnMachineFunction(MachineFunction &MF) override;
56 
getRequiredProperties() const57     MachineFunctionProperties getRequiredProperties() const override {
58       return MachineFunctionProperties().set(
59           MachineFunctionProperties::Property::AllVRegsAllocated);
60     }
61 
62   private:
63     void ClobberRegister(unsigned Reg);
64     void CopyPropagateBlock(MachineBasicBlock &MBB);
65     bool eraseIfRedundant(MachineInstr &Copy, unsigned Src, unsigned Def);
66 
67     /// Candidates for deletion.
68     SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;
69     /// Def -> available copies map.
70     Reg2MIMap AvailCopyMap;
71     /// Def -> copies map.
72     Reg2MIMap CopyMap;
73     /// Src -> Def map
74     SourceMap SrcMap;
75     bool Changed;
76   };
77 }
78 char MachineCopyPropagation::ID = 0;
79 char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
80 
81 INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
82                 "Machine Copy Propagation Pass", false, false)
83 
84 /// Remove any entry in \p Map where the register is a subregister or equal to
85 /// a register contained in \p Regs.
removeRegsFromMap(Reg2MIMap & Map,const RegList & Regs,const TargetRegisterInfo & TRI)86 static void removeRegsFromMap(Reg2MIMap &Map, const RegList &Regs,
87                               const TargetRegisterInfo &TRI) {
88   for (unsigned Reg : Regs) {
89     // Source of copy is no longer available for propagation.
90     for (MCSubRegIterator SR(Reg, &TRI, true); SR.isValid(); ++SR)
91       Map.erase(*SR);
92   }
93 }
94 
95 /// Remove any entry in \p Map that is marked clobbered in \p RegMask.
96 /// The map will typically have a lot fewer entries than the regmask clobbers,
97 /// so this is more efficient than iterating the clobbered registers and calling
98 /// ClobberRegister() on them.
removeClobberedRegsFromMap(Reg2MIMap & Map,const MachineOperand & RegMask)99 static void removeClobberedRegsFromMap(Reg2MIMap &Map,
100                                        const MachineOperand &RegMask) {
101   for (Reg2MIMap::iterator I = Map.begin(), E = Map.end(), Next; I != E;
102        I = Next) {
103     Next = std::next(I);
104     unsigned Reg = I->first;
105     if (RegMask.clobbersPhysReg(Reg))
106       Map.erase(I);
107   }
108 }
109 
ClobberRegister(unsigned Reg)110 void MachineCopyPropagation::ClobberRegister(unsigned Reg) {
111   for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
112     CopyMap.erase(*AI);
113     AvailCopyMap.erase(*AI);
114 
115     SourceMap::iterator SI = SrcMap.find(*AI);
116     if (SI != SrcMap.end()) {
117       removeRegsFromMap(AvailCopyMap, SI->second, *TRI);
118       SrcMap.erase(SI);
119     }
120   }
121 }
122 
123 /// Return true if \p PreviousCopy did copy register \p Src to register \p Def.
124 /// This fact may have been obscured by sub register usage or may not be true at
125 /// all even though Src and Def are subregisters of the registers used in
126 /// PreviousCopy. e.g.
127 /// isNopCopy("ecx = COPY eax", AX, CX) == true
128 /// isNopCopy("ecx = COPY eax", AH, CL) == false
isNopCopy(const MachineInstr & PreviousCopy,unsigned Src,unsigned Def,const TargetRegisterInfo * TRI)129 static bool isNopCopy(const MachineInstr &PreviousCopy, unsigned Src,
130                       unsigned Def, const TargetRegisterInfo *TRI) {
131   unsigned PreviousSrc = PreviousCopy.getOperand(1).getReg();
132   unsigned PreviousDef = PreviousCopy.getOperand(0).getReg();
133   if (Src == PreviousSrc) {
134     assert(Def == PreviousDef);
135     return true;
136   }
137   if (!TRI->isSubRegister(PreviousSrc, Src))
138     return false;
139   unsigned SubIdx = TRI->getSubRegIndex(PreviousSrc, Src);
140   return SubIdx == TRI->getSubRegIndex(PreviousDef, Def);
141 }
142 
143 /// Remove instruction \p Copy if there exists a previous copy that copies the
144 /// register \p Src to the register \p Def; This may happen indirectly by
145 /// copying the super registers.
eraseIfRedundant(MachineInstr & Copy,unsigned Src,unsigned Def)146 bool MachineCopyPropagation::eraseIfRedundant(MachineInstr &Copy, unsigned Src,
147                                               unsigned Def) {
148   // Avoid eliminating a copy from/to a reserved registers as we cannot predict
149   // the value (Example: The sparc zero register is writable but stays zero).
150   if (MRI->isReserved(Src) || MRI->isReserved(Def))
151     return false;
152 
153   // Search for an existing copy.
154   Reg2MIMap::iterator CI = AvailCopyMap.find(Def);
155   if (CI == AvailCopyMap.end())
156     return false;
157 
158   // Check that the existing copy uses the correct sub registers.
159   MachineInstr &PrevCopy = *CI->second;
160   if (!isNopCopy(PrevCopy, Src, Def, TRI))
161     return false;
162 
163   DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; Copy.dump());
164 
165   // Copy was redundantly redefining either Src or Def. Remove earlier kill
166   // flags between Copy and PrevCopy because the value will be reused now.
167   assert(Copy.isCopy());
168   unsigned CopyDef = Copy.getOperand(0).getReg();
169   assert(CopyDef == Src || CopyDef == Def);
170   for (MachineInstr &MI :
171        make_range(PrevCopy.getIterator(), Copy.getIterator()))
172     MI.clearRegisterKills(CopyDef, TRI);
173 
174   Copy.eraseFromParent();
175   Changed = true;
176   ++NumDeletes;
177   return true;
178 }
179 
CopyPropagateBlock(MachineBasicBlock & MBB)180 void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
181   DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
182 
183   for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
184     MachineInstr *MI = &*I;
185     ++I;
186 
187     if (MI->isCopy()) {
188       unsigned Def = MI->getOperand(0).getReg();
189       unsigned Src = MI->getOperand(1).getReg();
190 
191       assert(!TargetRegisterInfo::isVirtualRegister(Def) &&
192              !TargetRegisterInfo::isVirtualRegister(Src) &&
193              "MachineCopyPropagation should be run after register allocation!");
194 
195       // The two copies cancel out and the source of the first copy
196       // hasn't been overridden, eliminate the second one. e.g.
197       //  %ECX<def> = COPY %EAX
198       //  ... nothing clobbered EAX.
199       //  %EAX<def> = COPY %ECX
200       // =>
201       //  %ECX<def> = COPY %EAX
202       //
203       // or
204       //
205       //  %ECX<def> = COPY %EAX
206       //  ... nothing clobbered EAX.
207       //  %ECX<def> = COPY %EAX
208       // =>
209       //  %ECX<def> = COPY %EAX
210       if (eraseIfRedundant(*MI, Def, Src) || eraseIfRedundant(*MI, Src, Def))
211         continue;
212 
213       // If Src is defined by a previous copy, the previous copy cannot be
214       // eliminated.
215       for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
216         Reg2MIMap::iterator CI = CopyMap.find(*AI);
217         if (CI != CopyMap.end()) {
218           DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
219           MaybeDeadCopies.remove(CI->second);
220         }
221       }
222 
223       DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
224 
225       // Copy is now a candidate for deletion.
226       if (!MRI->isReserved(Def))
227         MaybeDeadCopies.insert(MI);
228 
229       // If 'Def' is previously source of another copy, then this earlier copy's
230       // source is no longer available. e.g.
231       // %xmm9<def> = copy %xmm2
232       // ...
233       // %xmm2<def> = copy %xmm0
234       // ...
235       // %xmm2<def> = copy %xmm9
236       ClobberRegister(Def);
237 
238       // Remember Def is defined by the copy.
239       for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
240            ++SR) {
241         CopyMap[*SR] = MI;
242         AvailCopyMap[*SR] = MI;
243       }
244 
245       // Remember source that's copied to Def. Once it's clobbered, then
246       // it's no longer available for copy propagation.
247       RegList &DestList = SrcMap[Src];
248       if (std::find(DestList.begin(), DestList.end(), Def) == DestList.end())
249         DestList.push_back(Def);
250 
251       continue;
252     }
253 
254     // Not a copy.
255     SmallVector<unsigned, 2> Defs;
256     const MachineOperand *RegMask = nullptr;
257     for (const MachineOperand &MO : MI->operands()) {
258       if (MO.isRegMask())
259         RegMask = &MO;
260       if (!MO.isReg())
261         continue;
262       unsigned Reg = MO.getReg();
263       if (!Reg)
264         continue;
265 
266       assert(!TargetRegisterInfo::isVirtualRegister(Reg) &&
267              "MachineCopyPropagation should be run after register allocation!");
268 
269       if (MO.isDef()) {
270         Defs.push_back(Reg);
271         continue;
272       }
273 
274       // If 'Reg' is defined by a copy, the copy is no longer a candidate
275       // for elimination.
276       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
277         Reg2MIMap::iterator CI = CopyMap.find(*AI);
278         if (CI != CopyMap.end()) {
279           DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
280           MaybeDeadCopies.remove(CI->second);
281         }
282       }
283       // Treat undef use like defs for copy propagation but not for
284       // dead copy. We would need to do a liveness check to be sure the copy
285       // is dead for undef uses.
286       // The backends are allowed to do whatever they want with undef value
287       // and we cannot be sure this register will not be rewritten to break
288       // some false dependencies for the hardware for instance.
289       if (MO.isUndef())
290         Defs.push_back(Reg);
291     }
292 
293     // The instruction has a register mask operand which means that it clobbers
294     // a large set of registers.  Treat clobbered registers the same way as
295     // defined registers.
296     if (RegMask) {
297       // Erase any MaybeDeadCopies whose destination register is clobbered.
298       for (SmallSetVector<MachineInstr *, 8>::iterator DI =
299                MaybeDeadCopies.begin();
300            DI != MaybeDeadCopies.end();) {
301         MachineInstr *MaybeDead = *DI;
302         unsigned Reg = MaybeDead->getOperand(0).getReg();
303         assert(!MRI->isReserved(Reg));
304 
305         if (!RegMask->clobbersPhysReg(Reg)) {
306           ++DI;
307           continue;
308         }
309 
310         DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
311               MaybeDead->dump());
312 
313         // erase() will return the next valid iterator pointing to the next
314         // element after the erased one.
315         DI = MaybeDeadCopies.erase(DI);
316         MaybeDead->eraseFromParent();
317         Changed = true;
318         ++NumDeletes;
319       }
320 
321       removeClobberedRegsFromMap(AvailCopyMap, *RegMask);
322       removeClobberedRegsFromMap(CopyMap, *RegMask);
323       for (SourceMap::iterator I = SrcMap.begin(), E = SrcMap.end(), Next;
324            I != E; I = Next) {
325         Next = std::next(I);
326         if (RegMask->clobbersPhysReg(I->first)) {
327           removeRegsFromMap(AvailCopyMap, I->second, *TRI);
328           SrcMap.erase(I);
329         }
330       }
331     }
332 
333     // Any previous copy definition or reading the Defs is no longer available.
334     for (unsigned Reg : Defs)
335       ClobberRegister(Reg);
336   }
337 
338   // If MBB doesn't have successors, delete the copies whose defs are not used.
339   // If MBB does have successors, then conservative assume the defs are live-out
340   // since we don't want to trust live-in lists.
341   if (MBB.succ_empty()) {
342     for (MachineInstr *MaybeDead : MaybeDeadCopies) {
343       assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg()));
344       MaybeDead->eraseFromParent();
345       Changed = true;
346       ++NumDeletes;
347     }
348   }
349 
350   MaybeDeadCopies.clear();
351   AvailCopyMap.clear();
352   CopyMap.clear();
353   SrcMap.clear();
354 }
355 
runOnMachineFunction(MachineFunction & MF)356 bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
357   if (skipFunction(*MF.getFunction()))
358     return false;
359 
360   Changed = false;
361 
362   TRI = MF.getSubtarget().getRegisterInfo();
363   TII = MF.getSubtarget().getInstrInfo();
364   MRI = &MF.getRegInfo();
365 
366   for (MachineBasicBlock &MBB : MF)
367     CopyPropagateBlock(MBB);
368 
369   return Changed;
370 }
371 
372