1 //===-- ExpandPostRAPseudos.cpp - Pseudo instruction expansion 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 file defines a pass that expands COPY and SUBREG_TO_REG pseudo
11 // instructions after register allocation.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/CodeGen/MachineFunctionPass.h"
16 #include "llvm/CodeGen/MachineInstr.h"
17 #include "llvm/CodeGen/MachineInstrBuilder.h"
18 #include "llvm/CodeGen/MachineRegisterInfo.h"
19 #include "llvm/CodeGen/Passes.h"
20 #include "llvm/CodeGen/TargetInstrInfo.h"
21 #include "llvm/CodeGen/TargetRegisterInfo.h"
22 #include "llvm/CodeGen/TargetSubtargetInfo.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "postrapseudos"
29 
30 namespace {
31 struct ExpandPostRA : public MachineFunctionPass {
32 private:
33   const TargetRegisterInfo *TRI;
34   const TargetInstrInfo *TII;
35 
36 public:
37   static char ID; // Pass identification, replacement for typeid
38   ExpandPostRA() : MachineFunctionPass(ID) {}
39 
40   void getAnalysisUsage(AnalysisUsage &AU) const override {
41     AU.setPreservesCFG();
42     AU.addPreservedID(MachineLoopInfoID);
43     AU.addPreservedID(MachineDominatorsID);
44     MachineFunctionPass::getAnalysisUsage(AU);
45   }
46 
47   /// runOnMachineFunction - pass entry point
48   bool runOnMachineFunction(MachineFunction&) override;
49 
50 private:
51   bool LowerSubregToReg(MachineInstr *MI);
52   bool LowerCopy(MachineInstr *MI);
53 
54   void TransferImplicitOperands(MachineInstr *MI);
55 };
56 } // end anonymous namespace
57 
58 char ExpandPostRA::ID = 0;
59 char &llvm::ExpandPostRAPseudosID = ExpandPostRA::ID;
60 
61 INITIALIZE_PASS(ExpandPostRA, DEBUG_TYPE,
62                 "Post-RA pseudo instruction expansion pass", false, false)
63 
64 /// TransferImplicitOperands - MI is a pseudo-instruction, and the lowered
65 /// replacement instructions immediately precede it.  Copy any implicit
66 /// operands from MI to the replacement instruction.
67 void ExpandPostRA::TransferImplicitOperands(MachineInstr *MI) {
68   MachineBasicBlock::iterator CopyMI = MI;
69   --CopyMI;
70 
71   for (const MachineOperand &MO : MI->implicit_operands())
72     if (MO.isReg())
73       CopyMI->addOperand(MO);
74 }
75 
76 bool ExpandPostRA::LowerSubregToReg(MachineInstr *MI) {
77   MachineBasicBlock *MBB = MI->getParent();
78   assert((MI->getOperand(0).isReg() && MI->getOperand(0).isDef()) &&
79          MI->getOperand(1).isImm() &&
80          (MI->getOperand(2).isReg() && MI->getOperand(2).isUse()) &&
81           MI->getOperand(3).isImm() && "Invalid subreg_to_reg");
82 
83   unsigned DstReg  = MI->getOperand(0).getReg();
84   unsigned InsReg  = MI->getOperand(2).getReg();
85   assert(!MI->getOperand(2).getSubReg() && "SubIdx on physreg?");
86   unsigned SubIdx  = MI->getOperand(3).getImm();
87 
88   assert(SubIdx != 0 && "Invalid index for insert_subreg");
89   unsigned DstSubReg = TRI->getSubReg(DstReg, SubIdx);
90 
91   assert(TargetRegisterInfo::isPhysicalRegister(DstReg) &&
92          "Insert destination must be in a physical register");
93   assert(TargetRegisterInfo::isPhysicalRegister(InsReg) &&
94          "Inserted value must be in a physical register");
95 
96   LLVM_DEBUG(dbgs() << "subreg: CONVERTING: " << *MI);
97 
98   if (MI->allDefsAreDead()) {
99     MI->setDesc(TII->get(TargetOpcode::KILL));
100     LLVM_DEBUG(dbgs() << "subreg: replaced by: " << *MI);
101     return true;
102   }
103 
104   if (DstSubReg == InsReg) {
105     // No need to insert an identity copy instruction.
106     // Watch out for case like this:
107     // %rax = SUBREG_TO_REG 0, killed %eax, 3
108     // We must leave %rax live.
109     if (DstReg != InsReg) {
110       MI->setDesc(TII->get(TargetOpcode::KILL));
111       MI->RemoveOperand(3);     // SubIdx
112       MI->RemoveOperand(1);     // Imm
113       LLVM_DEBUG(dbgs() << "subreg: replace by: " << *MI);
114       return true;
115     }
116     LLVM_DEBUG(dbgs() << "subreg: eliminated!");
117   } else {
118     TII->copyPhysReg(*MBB, MI, MI->getDebugLoc(), DstSubReg, InsReg,
119                      MI->getOperand(2).isKill());
120 
121     // Implicitly define DstReg for subsequent uses.
122     MachineBasicBlock::iterator CopyMI = MI;
123     --CopyMI;
124     CopyMI->addRegisterDefined(DstReg);
125     LLVM_DEBUG(dbgs() << "subreg: " << *CopyMI);
126   }
127 
128   LLVM_DEBUG(dbgs() << '\n');
129   MBB->erase(MI);
130   return true;
131 }
132 
133 bool ExpandPostRA::LowerCopy(MachineInstr *MI) {
134 
135   if (MI->allDefsAreDead()) {
136     LLVM_DEBUG(dbgs() << "dead copy: " << *MI);
137     MI->setDesc(TII->get(TargetOpcode::KILL));
138     LLVM_DEBUG(dbgs() << "replaced by: " << *MI);
139     return true;
140   }
141 
142   MachineOperand &DstMO = MI->getOperand(0);
143   MachineOperand &SrcMO = MI->getOperand(1);
144 
145   bool IdentityCopy = (SrcMO.getReg() == DstMO.getReg());
146   if (IdentityCopy || SrcMO.isUndef()) {
147     LLVM_DEBUG(dbgs() << (IdentityCopy ? "identity copy: " : "undef copy:    ")
148                       << *MI);
149     // No need to insert an identity copy instruction, but replace with a KILL
150     // if liveness is changed.
151     if (SrcMO.isUndef() || MI->getNumOperands() > 2) {
152       // We must make sure the super-register gets killed. Replace the
153       // instruction with KILL.
154       MI->setDesc(TII->get(TargetOpcode::KILL));
155       LLVM_DEBUG(dbgs() << "replaced by:   " << *MI);
156       return true;
157     }
158     // Vanilla identity copy.
159     MI->eraseFromParent();
160     return true;
161   }
162 
163   LLVM_DEBUG(dbgs() << "real copy:   " << *MI);
164   TII->copyPhysReg(*MI->getParent(), MI, MI->getDebugLoc(),
165                    DstMO.getReg(), SrcMO.getReg(), SrcMO.isKill());
166 
167   if (MI->getNumOperands() > 2)
168     TransferImplicitOperands(MI);
169   LLVM_DEBUG({
170     MachineBasicBlock::iterator dMI = MI;
171     dbgs() << "replaced by: " << *(--dMI);
172   });
173   MI->eraseFromParent();
174   return true;
175 }
176 
177 /// runOnMachineFunction - Reduce subregister inserts and extracts to register
178 /// copies.
179 ///
180 bool ExpandPostRA::runOnMachineFunction(MachineFunction &MF) {
181   LLVM_DEBUG(dbgs() << "Machine Function\n"
182                     << "********** EXPANDING POST-RA PSEUDO INSTRS **********\n"
183                     << "********** Function: " << MF.getName() << '\n');
184   TRI = MF.getSubtarget().getRegisterInfo();
185   TII = MF.getSubtarget().getInstrInfo();
186 
187   bool MadeChange = false;
188 
189   for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
190        mbbi != mbbe; ++mbbi) {
191     for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
192          mi != me;) {
193       MachineInstr &MI = *mi;
194       // Advance iterator here because MI may be erased.
195       ++mi;
196 
197       // Only expand pseudos.
198       if (!MI.isPseudo())
199         continue;
200 
201       // Give targets a chance to expand even standard pseudos.
202       if (TII->expandPostRAPseudo(MI)) {
203         MadeChange = true;
204         continue;
205       }
206 
207       // Expand standard pseudos.
208       switch (MI.getOpcode()) {
209       case TargetOpcode::SUBREG_TO_REG:
210         MadeChange |= LowerSubregToReg(&MI);
211         break;
212       case TargetOpcode::COPY:
213         MadeChange |= LowerCopy(&MI);
214         break;
215       case TargetOpcode::DBG_VALUE:
216         continue;
217       case TargetOpcode::INSERT_SUBREG:
218       case TargetOpcode::EXTRACT_SUBREG:
219         llvm_unreachable("Sub-register pseudos should have been eliminated.");
220       }
221     }
222   }
223 
224   return MadeChange;
225 }
226