MipsDelaySlotFiller.cpp revision 864f66085cd9543070ef01b9f7371c110ecd7898 
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> EnableDelaySlotFiller(
34  "enable-mips-delay-filler",
35  cl::init(false),
36  cl::desc("Fill the Mips delay slots useful instructions."),
37  cl::Hidden);
38
39// This option can be used to silence complaints by machine verifier passes.
40static cl::opt<bool> SkipDelaySlotFiller(
41  "skip-mips-delay-filler",
42  cl::init(false),
43  cl::desc("Skip MIPS' delay slot filling pass."),
44  cl::Hidden);
45
46namespace {
47  struct Filler : public MachineFunctionPass {
48    typedef MachineBasicBlock::instr_iterator InstrIter;
49    typedef MachineBasicBlock::reverse_instr_iterator ReverseInstrIter;
50
51    TargetMachine &TM;
52    const TargetInstrInfo *TII;
53    InstrIter LastFiller;
54
55    static char ID;
56    Filler(TargetMachine &tm)
57      : MachineFunctionPass(ID), TM(tm), TII(tm.getInstrInfo()) { }
58
59    virtual const char *getPassName() const {
60      return "Mips Delay Slot Filler";
61    }
62
63    bool runOnMachineBasicBlock(MachineBasicBlock &MBB);
64    bool runOnMachineFunction(MachineFunction &F) {
65      if (SkipDelaySlotFiller)
66        return false;
67
68      bool Changed = false;
69      for (MachineFunction::iterator FI = F.begin(), FE = F.end();
70           FI != FE; ++FI)
71        Changed |= runOnMachineBasicBlock(*FI);
72      return Changed;
73    }
74
75    bool isDelayFiller(MachineBasicBlock &MBB,
76                       InstrIter candidate);
77
78    void insertCallUses(InstrIter MI,
79                        SmallSet<unsigned, 32> &RegDefs,
80                        SmallSet<unsigned, 32> &RegUses);
81
82    void insertDefsUses(InstrIter MI,
83                        SmallSet<unsigned, 32> &RegDefs,
84                        SmallSet<unsigned, 32> &RegUses);
85
86    bool IsRegInSet(SmallSet<unsigned, 32> &RegSet,
87                    unsigned Reg);
88
89    bool delayHasHazard(InstrIter candidate,
90                        bool &sawLoad, bool &sawStore,
91                        SmallSet<unsigned, 32> &RegDefs,
92                        SmallSet<unsigned, 32> &RegUses);
93
94    bool
95    findDelayInstr(MachineBasicBlock &MBB, InstrIter slot,
96                   InstrIter &Filler);
97
98
99  };
100  char Filler::ID = 0;
101} // end of anonymous namespace
102
103/// runOnMachineBasicBlock - Fill in delay slots for the given basic block.
104/// We assume there is only one delay slot per delayed instruction.
105bool Filler::
106runOnMachineBasicBlock(MachineBasicBlock &MBB) {
107  bool Changed = false;
108  LastFiller = MBB.instr_end();
109
110  for (InstrIter I = MBB.instr_begin(); I != MBB.instr_end(); ++I)
111    if (I->hasDelaySlot()) {
112      ++FilledSlots;
113      Changed = true;
114
115      InstrIter D;
116
117      if (EnableDelaySlotFiller && findDelayInstr(MBB, I, D)) {
118        MBB.splice(llvm::next(I), &MBB, D);
119        ++UsefulSlots;
120      } else
121        BuildMI(MBB, llvm::next(I), I->getDebugLoc(), TII->get(Mips::NOP));
122
123      // Record the filler instruction that filled the delay slot.
124      // The instruction after it will be visited in the next iteration.
125      LastFiller = ++I;
126
127      // Set InsideBundle bit so that the machine verifier doesn't expect this
128      // instruction to be a terminator.
129      LastFiller->setIsInsideBundle();
130     }
131  return Changed;
132
133}
134
135/// createMipsDelaySlotFillerPass - Returns a pass that fills in delay
136/// slots in Mips MachineFunctions
137FunctionPass *llvm::createMipsDelaySlotFillerPass(MipsTargetMachine &tm) {
138  return new Filler(tm);
139}
140
141bool Filler::findDelayInstr(MachineBasicBlock &MBB,
142                            InstrIter slot,
143                            InstrIter &Filler) {
144  SmallSet<unsigned, 32> RegDefs;
145  SmallSet<unsigned, 32> RegUses;
146
147  insertDefsUses(slot, RegDefs, RegUses);
148
149  bool sawLoad = false;
150  bool sawStore = false;
151
152  for (ReverseInstrIter I(slot); I != MBB.instr_rend(); ++I) {
153    // skip debug value
154    if (I->isDebugValue())
155      continue;
156
157    // Convert to forward iterator.
158    InstrIter FI(llvm::next(I).base());
159
160    if (I->hasUnmodeledSideEffects()
161        || I->isInlineAsm()
162        || I->isLabel()
163        || FI == LastFiller
164        || I->isPseudo()
165        //
166        // Should not allow:
167        // ERET, DERET or WAIT, PAUSE. Need to add these to instruction
168        // list. TBD.
169        )
170      break;
171
172    if (delayHasHazard(FI, sawLoad, sawStore, RegDefs, RegUses)) {
173      insertDefsUses(FI, RegDefs, RegUses);
174      continue;
175    }
176
177    Filler = FI;
178    return true;
179  }
180
181  return false;
182}
183
184bool Filler::delayHasHazard(InstrIter candidate,
185                            bool &sawLoad, bool &sawStore,
186                            SmallSet<unsigned, 32> &RegDefs,
187                            SmallSet<unsigned, 32> &RegUses) {
188  if (candidate->isImplicitDef() || candidate->isKill())
189    return true;
190
191  // Loads or stores cannot be moved past a store to the delay slot
192  // and stores cannot be moved past a load.
193  if (candidate->mayLoad()) {
194    if (sawStore)
195      return true;
196    sawLoad = true;
197  }
198
199  if (candidate->mayStore()) {
200    if (sawStore)
201      return true;
202    sawStore = true;
203    if (sawLoad)
204      return true;
205  }
206
207  assert((!candidate->isCall() && !candidate->isReturn()) &&
208         "Cannot put calls or returns in delay slot.");
209
210  for (unsigned i = 0, e = candidate->getNumOperands(); i!= e; ++i) {
211    const MachineOperand &MO = candidate->getOperand(i);
212    unsigned Reg;
213
214    if (!MO.isReg() || !(Reg = MO.getReg()))
215      continue; // skip
216
217    if (MO.isDef()) {
218      // check whether Reg is defined or used before delay slot.
219      if (IsRegInSet(RegDefs, Reg) || IsRegInSet(RegUses, Reg))
220        return true;
221    }
222    if (MO.isUse()) {
223      // check whether Reg is defined before delay slot.
224      if (IsRegInSet(RegDefs, Reg))
225        return true;
226    }
227  }
228  return false;
229}
230
231// Insert Defs and Uses of MI into the sets RegDefs and RegUses.
232void Filler::insertDefsUses(InstrIter MI,
233                            SmallSet<unsigned, 32> &RegDefs,
234                            SmallSet<unsigned, 32> &RegUses) {
235  // If MI is a call or return, just examine the explicit non-variadic operands.
236  MCInstrDesc MCID = MI->getDesc();
237  unsigned e = MI->isCall() || MI->isReturn() ? MCID.getNumOperands() :
238                                                MI->getNumOperands();
239
240  // Add RA to RegDefs to prevent users of RA from going into delay slot.
241  if (MI->isCall())
242    RegDefs.insert(Mips::RA);
243
244  for (unsigned i = 0; i != e; ++i) {
245    const MachineOperand &MO = MI->getOperand(i);
246    unsigned Reg;
247
248    if (!MO.isReg() || !(Reg = MO.getReg()))
249      continue;
250
251    if (MO.isDef())
252      RegDefs.insert(Reg);
253    else if (MO.isUse())
254      RegUses.insert(Reg);
255  }
256}
257
258//returns true if the Reg or its alias is in the RegSet.
259bool Filler::IsRegInSet(SmallSet<unsigned, 32> &RegSet, unsigned Reg) {
260  // Check Reg and all aliased Registers.
261  for (MCRegAliasIterator AI(Reg, TM.getRegisterInfo(), true);
262       AI.isValid(); ++AI)
263    if (RegSet.count(*AI))
264      return true;
265  return false;
266}
267