MipsDelaySlotFiller.cpp revision 6522a9e04bcfa447299f4fd10ee9afffd5834a47 
1//===-- DelaySlotFiller.cpp - Mips Delay Slot Filler ----------------------===//
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// Simple pass to fills delay slots with useful instructions.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "delay-slot-filler"
15
16#include "Mips.h"
17#include "MipsTargetMachine.h"
18#include "llvm/CodeGen/MachineFunctionPass.h"
19#include "llvm/CodeGen/MachineInstrBuilder.h"
20#include "llvm/Support/CommandLine.h"
21#include "llvm/Target/TargetMachine.h"
22#include "llvm/Target/TargetInstrInfo.h"
23#include "llvm/Target/TargetRegisterInfo.h"
24#include "llvm/ADT/SmallSet.h"
25#include "llvm/ADT/Statistic.h"
26
27using namespace llvm;
28
29STATISTIC(FilledSlots, "Number of delay slots filled");
30STATISTIC(UsefulSlots, "Number of delay slots filled with instructions that"
31                       " are not NOP.");
32
33static cl::opt<bool> DisableDelaySlotFiller(
34  "disable-mips-delay-filler",
35  cl::init(false),
36  cl::desc("Disable the delay slot filler, which attempts to fill the Mips"
37           "delay slots with useful instructions."),
38  cl::Hidden);
39
40// This option can be used to silence complaints by machine verifier passes.
41static cl::opt<bool> SkipDelaySlotFiller(
42  "skip-mips-delay-filler",
43  cl::init(false),
44  cl::desc("Skip MIPS' delay slot filling pass."),
45  cl::Hidden);
46
47namespace {
48  struct Filler : public MachineFunctionPass {
49    typedef MachineBasicBlock::instr_iterator InstrIter;
50    typedef MachineBasicBlock::reverse_instr_iterator ReverseInstrIter;
51
52    TargetMachine &TM;
53    const TargetInstrInfo *TII;
54    InstrIter LastFiller;
55
56    static char ID;
57    Filler(TargetMachine &tm)
58      : MachineFunctionPass(ID), TM(tm), TII(tm.getInstrInfo()) { }
59
60    virtual const char *getPassName() const {
61      return "Mips Delay Slot Filler";
62    }
63
64    bool runOnMachineBasicBlock(MachineBasicBlock &MBB);
65    bool runOnMachineFunction(MachineFunction &F) {
66      if (SkipDelaySlotFiller)
67        return false;
68
69      bool Changed = false;
70      for (MachineFunction::iterator FI = F.begin(), FE = F.end();
71           FI != FE; ++FI)
72        Changed |= runOnMachineBasicBlock(*FI);
73      return Changed;
74    }
75
76    bool isDelayFiller(MachineBasicBlock &MBB,
77                       InstrIter candidate);
78
79    void insertCallUses(InstrIter MI,
80                        SmallSet<unsigned, 32> &RegDefs,
81                        SmallSet<unsigned, 32> &RegUses);
82
83    void insertDefsUses(InstrIter MI,
84                        SmallSet<unsigned, 32> &RegDefs,
85                        SmallSet<unsigned, 32> &RegUses);
86
87    bool IsRegInSet(SmallSet<unsigned, 32> &RegSet,
88                    unsigned Reg);
89
90    bool delayHasHazard(InstrIter candidate,
91                        bool &sawLoad, bool &sawStore,
92                        SmallSet<unsigned, 32> &RegDefs,
93                        SmallSet<unsigned, 32> &RegUses);
94
95    bool
96    findDelayInstr(MachineBasicBlock &MBB, InstrIter slot,
97                   InstrIter &Filler);
98
99
100  };
101  char Filler::ID = 0;
102} // end of anonymous namespace
103
104/// runOnMachineBasicBlock - Fill in delay slots for the given basic block.
105/// We assume there is only one delay slot per delayed instruction.
106bool Filler::
107runOnMachineBasicBlock(MachineBasicBlock &MBB) {
108  bool Changed = false;
109  LastFiller = MBB.instr_end();
110
111  for (InstrIter I = MBB.instr_begin(); I != MBB.instr_end(); ++I)
112    if (I->hasDelaySlot()) {
113      ++FilledSlots;
114      Changed = true;
115
116      InstrIter D;
117
118      if (!DisableDelaySlotFiller && findDelayInstr(MBB, I, D)) {
119        MBB.splice(llvm::next(I), &MBB, D);
120        ++UsefulSlots;
121      } else
122        BuildMI(MBB, llvm::next(I), I->getDebugLoc(), TII->get(Mips::NOP));
123
124      // Record the filler instruction that filled the delay slot.
125      // The instruction after it will be visited in the next iteration.
126      LastFiller = ++I;
127
128      // Set InsideBundle bit so that the machine verifier doesn't expect this
129      // instruction to be a terminator.
130      LastFiller->setIsInsideBundle();
131     }
132  return Changed;
133
134}
135
136/// createMipsDelaySlotFillerPass - Returns a pass that fills in delay
137/// slots in Mips MachineFunctions
138FunctionPass *llvm::createMipsDelaySlotFillerPass(MipsTargetMachine &tm) {
139  return new Filler(tm);
140}
141
142bool Filler::findDelayInstr(MachineBasicBlock &MBB,
143                            InstrIter slot,
144                            InstrIter &Filler) {
145  SmallSet<unsigned, 32> RegDefs;
146  SmallSet<unsigned, 32> RegUses;
147
148  insertDefsUses(slot, RegDefs, RegUses);
149
150  bool sawLoad = false;
151  bool sawStore = false;
152
153  for (ReverseInstrIter I(slot); I != MBB.instr_rend(); ++I) {
154    // skip debug value
155    if (I->isDebugValue())
156      continue;
157
158    // Convert to forward iterator.
159    InstrIter FI(llvm::next(I).base());
160
161    if (I->hasUnmodeledSideEffects()
162        || I->isInlineAsm()
163        || I->isLabel()
164        || FI == LastFiller
165        || I->isPseudo()
166        //
167        // Should not allow:
168        // ERET, DERET or WAIT, PAUSE. Need to add these to instruction
169        // list. TBD.
170        )
171      break;
172
173    if (delayHasHazard(FI, sawLoad, sawStore, RegDefs, RegUses)) {
174      insertDefsUses(FI, RegDefs, RegUses);
175      continue;
176    }
177
178    Filler = FI;
179    return true;
180  }
181
182  return false;
183}
184
185bool Filler::delayHasHazard(InstrIter candidate,
186                            bool &sawLoad, bool &sawStore,
187                            SmallSet<unsigned, 32> &RegDefs,
188                            SmallSet<unsigned, 32> &RegUses) {
189  if (candidate->isImplicitDef() || candidate->isKill())
190    return true;
191
192  // Loads or stores cannot be moved past a store to the delay slot
193  // and stores cannot be moved past a load.
194  if (candidate->mayLoad()) {
195    if (sawStore)
196      return true;
197    sawLoad = true;
198  }
199
200  if (candidate->mayStore()) {
201    if (sawStore)
202      return true;
203    sawStore = true;
204    if (sawLoad)
205      return true;
206  }
207
208  assert((!candidate->isCall() && !candidate->isReturn()) &&
209         "Cannot put calls or returns in delay slot.");
210
211  for (unsigned i = 0, e = candidate->getNumOperands(); i!= e; ++i) {
212    const MachineOperand &MO = candidate->getOperand(i);
213    unsigned Reg;
214
215    if (!MO.isReg() || !(Reg = MO.getReg()))
216      continue; // skip
217
218    if (MO.isDef()) {
219      // check whether Reg is defined or used before delay slot.
220      if (IsRegInSet(RegDefs, Reg) || IsRegInSet(RegUses, Reg))
221        return true;
222    }
223    if (MO.isUse()) {
224      // check whether Reg is defined before delay slot.
225      if (IsRegInSet(RegDefs, Reg))
226        return true;
227    }
228  }
229  return false;
230}
231
232// Insert Defs and Uses of MI into the sets RegDefs and RegUses.
233void Filler::insertDefsUses(InstrIter MI,
234                            SmallSet<unsigned, 32> &RegDefs,
235                            SmallSet<unsigned, 32> &RegUses) {
236  // If MI is a call or return, just examine the explicit non-variadic operands.
237  MCInstrDesc MCID = MI->getDesc();
238  unsigned e = MI->isCall() || MI->isReturn() ? MCID.getNumOperands() :
239                                                MI->getNumOperands();
240
241  // Add RA to RegDefs to prevent users of RA from going into delay slot.
242  if (MI->isCall())
243    RegDefs.insert(Mips::RA);
244
245  for (unsigned i = 0; i != e; ++i) {
246    const MachineOperand &MO = MI->getOperand(i);
247    unsigned Reg;
248
249    if (!MO.isReg() || !(Reg = MO.getReg()))
250      continue;
251
252    if (MO.isDef())
253      RegDefs.insert(Reg);
254    else if (MO.isUse())
255      RegUses.insert(Reg);
256  }
257}
258
259//returns true if the Reg or its alias is in the RegSet.
260bool Filler::IsRegInSet(SmallSet<unsigned, 32> &RegSet, unsigned Reg) {
261  // Check Reg and all aliased Registers.
262  for (MCRegAliasIterator AI(Reg, TM.getRegisterInfo(), true);
263       AI.isValid(); ++AI)
264    if (RegSet.count(*AI))
265      return true;
266  return false;
267}
268