TwoAddressInstructionPass.cpp revision d10fd9791c20fd8368fa0ce94b626b769c6c8ba0 
1//===-- TwoAddressInstructionPass.cpp - Two-Address instruction 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 implements the TwoAddress instruction pass which is used
11// by most register allocators. Two-Address instructions are rewritten
12// from:
13//
14//     A = B op C
15//
16// to:
17//
18//     A = B
19//     A op= C
20//
21// Note that if a register allocator chooses to use this pass, that it
22// has to be capable of handling the non-SSA nature of these rewritten
23// virtual registers.
24//
25// It is also worth noting that the duplicate operand of the two
26// address instruction is removed.
27//
28//===----------------------------------------------------------------------===//
29
30#define DEBUG_TYPE "twoaddrinstr"
31#include "llvm/CodeGen/Passes.h"
32#include "llvm/Function.h"
33#include "llvm/CodeGen/LiveVariables.h"
34#include "llvm/CodeGen/MachineFunctionPass.h"
35#include "llvm/CodeGen/MachineInstr.h"
36#include "llvm/CodeGen/MachineRegisterInfo.h"
37#include "llvm/Target/MRegisterInfo.h"
38#include "llvm/Target/TargetInstrInfo.h"
39#include "llvm/Target/TargetMachine.h"
40#include "llvm/Support/Debug.h"
41#include "llvm/Support/Compiler.h"
42#include "llvm/ADT/Statistic.h"
43#include "llvm/ADT/STLExtras.h"
44using namespace llvm;
45
46STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions");
47STATISTIC(NumCommuted        , "Number of instructions commuted to coalesce");
48STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address");
49
50namespace {
51  struct VISIBILITY_HIDDEN TwoAddressInstructionPass
52   : public MachineFunctionPass {
53    static char ID; // Pass identification, replacement for typeid
54    TwoAddressInstructionPass() : MachineFunctionPass((intptr_t)&ID) {}
55
56    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
57
58    /// runOnMachineFunction - pass entry point
59    bool runOnMachineFunction(MachineFunction&);
60  };
61
62  char TwoAddressInstructionPass::ID = 0;
63  RegisterPass<TwoAddressInstructionPass>
64  X("twoaddressinstruction", "Two-Address instruction pass");
65}
66
67const PassInfo *llvm::TwoAddressInstructionPassID = X.getPassInfo();
68
69void TwoAddressInstructionPass::getAnalysisUsage(AnalysisUsage &AU) const {
70  AU.addRequired<LiveVariables>();
71  AU.addPreserved<LiveVariables>();
72  AU.addPreservedID(PHIEliminationID);
73  MachineFunctionPass::getAnalysisUsage(AU);
74}
75
76/// runOnMachineFunction - Reduce two-address instructions to two
77/// operands.
78///
79bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
80  DOUT << "Machine Function\n";
81  const TargetMachine &TM = MF.getTarget();
82  const TargetInstrInfo &TII = *TM.getInstrInfo();
83  LiveVariables &LV = getAnalysis<LiveVariables>();
84
85  bool MadeChange = false;
86
87  DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n";
88  DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
89
90  for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
91       mbbi != mbbe; ++mbbi) {
92    for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
93         mi != me; ++mi) {
94      const TargetInstrDescriptor *TID = mi->getInstrDescriptor();
95
96      bool FirstTied = true;
97      for (unsigned si = 1, e = TID->numOperands; si < e; ++si) {
98        int ti = TID->getOperandConstraint(si, TOI::TIED_TO);
99        if (ti == -1)
100          continue;
101
102        if (FirstTied) {
103          ++NumTwoAddressInstrs;
104          DOUT << '\t'; DEBUG(mi->print(*cerr.stream(), &TM));
105        }
106        FirstTied = false;
107
108        assert(mi->getOperand(si).isRegister() && mi->getOperand(si).getReg() &&
109               mi->getOperand(si).isUse() && "two address instruction invalid");
110
111        // if the two operands are the same we just remove the use
112        // and mark the def as def&use, otherwise we have to insert a copy.
113        if (mi->getOperand(ti).getReg() != mi->getOperand(si).getReg()) {
114          // rewrite:
115          //     a = b op c
116          // to:
117          //     a = b
118          //     a = a op c
119          unsigned regA = mi->getOperand(ti).getReg();
120          unsigned regB = mi->getOperand(si).getReg();
121
122          assert(MRegisterInfo::isVirtualRegister(regA) &&
123                 MRegisterInfo::isVirtualRegister(regB) &&
124                 "cannot update physical register live information");
125
126#ifndef NDEBUG
127          // First, verify that we don't have a use of a in the instruction (a =
128          // b + a for example) because our transformation will not work. This
129          // should never occur because we are in SSA form.
130          for (unsigned i = 0; i != mi->getNumOperands(); ++i)
131            assert((int)i == ti ||
132                   !mi->getOperand(i).isRegister() ||
133                   mi->getOperand(i).getReg() != regA);
134#endif
135
136          // If this instruction is not the killing user of B, see if we can
137          // rearrange the code to make it so.  Making it the killing user will
138          // allow us to coalesce A and B together, eliminating the copy we are
139          // about to insert.
140          if (!LV.KillsRegister(mi, regB)) {
141            // If this instruction is commutative, check to see if C dies.  If
142            // so, swap the B and C operands.  This makes the live ranges of A
143            // and C joinable.
144            // FIXME: This code also works for A := B op C instructions.
145            if ((TID->Flags & M_COMMUTABLE) && mi->getNumOperands() >= 3) {
146              assert(mi->getOperand(3-si).isRegister() &&
147                     "Not a proper commutative instruction!");
148              unsigned regC = mi->getOperand(3-si).getReg();
149              if (LV.KillsRegister(mi, regC)) {
150                DOUT << "2addr: COMMUTING  : " << *mi;
151                MachineInstr *NewMI = TII.commuteInstruction(mi);
152                if (NewMI == 0) {
153                  DOUT << "2addr: COMMUTING FAILED!\n";
154                } else {
155                  DOUT << "2addr: COMMUTED TO: " << *NewMI;
156                  // If the instruction changed to commute it, update livevar.
157                  if (NewMI != mi) {
158                    LV.instructionChanged(mi, NewMI);  // Update live variables
159                    mbbi->insert(mi, NewMI);           // Insert the new inst
160                    mbbi->erase(mi);                   // Nuke the old inst.
161                    mi = NewMI;
162                  }
163
164                  ++NumCommuted;
165                  regB = regC;
166                  goto InstructionRearranged;
167                }
168              }
169            }
170
171            // If this instruction is potentially convertible to a true
172            // three-address instruction,
173            if (TID->Flags & M_CONVERTIBLE_TO_3_ADDR) {
174              // FIXME: This assumes there are no more operands which are tied
175              // to another register.
176#ifndef NDEBUG
177              for (unsigned i = si+1, e = TID->numOperands; i < e; ++i)
178                assert(TID->getOperandConstraint(i, TOI::TIED_TO) == -1);
179#endif
180
181              if (MachineInstr *New = TII.convertToThreeAddress(mbbi, mi, LV)) {
182                DOUT << "2addr: CONVERTING 2-ADDR: " << *mi;
183                DOUT << "2addr:         TO 3-ADDR: " << *New;
184                mbbi->erase(mi);                 // Nuke the old inst.
185                mi = New;
186                ++NumConvertedTo3Addr;
187                // Done with this instruction.
188                break;
189              }
190            }
191          }
192
193        InstructionRearranged:
194          const TargetRegisterClass* rc = MF.getRegInfo().getRegClass(regA);
195          TII.copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
196
197          MachineBasicBlock::iterator prevMi = prior(mi);
198          DOUT << "\t\tprepend:\t"; DEBUG(prevMi->print(*cerr.stream(), &TM));
199
200          // Update live variables for regA
201          LiveVariables::VarInfo& varInfo = LV.getVarInfo(regA);
202          varInfo.DefInst = prevMi;
203
204          // update live variables for regB
205          LiveVariables::VarInfo& varInfoB = LV.getVarInfo(regB);
206          // regB is used in this BB.
207          varInfoB.UsedBlocks[mbbi->getNumber()] = true;
208          if (LV.removeVirtualRegisterKilled(regB, mbbi, mi))
209            LV.addVirtualRegisterKilled(regB, prevMi);
210
211          if (LV.removeVirtualRegisterDead(regB, mbbi, mi))
212            LV.addVirtualRegisterDead(regB, prevMi);
213
214          // replace all occurences of regB with regA
215          for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
216            if (mi->getOperand(i).isRegister() &&
217                mi->getOperand(i).getReg() == regB)
218              mi->getOperand(i).setReg(regA);
219          }
220        }
221
222        assert(mi->getOperand(ti).isDef() && mi->getOperand(si).isUse());
223        mi->getOperand(ti).setReg(mi->getOperand(si).getReg());
224        MadeChange = true;
225
226        DOUT << "\t\trewrite to:\t"; DEBUG(mi->print(*cerr.stream(), &TM));
227      }
228    }
229  }
230
231  return MadeChange;
232}
233