MachineSink.cpp revision b6f5417edb8ad11e06d3a6527e452945e5349a97 
1//===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
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 pass
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "machine-sink"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/CodeGen/MachineRegisterInfo.h"
17#include "llvm/CodeGen/MachineDominators.h"
18#include "llvm/Target/TargetRegisterInfo.h"
19#include "llvm/Target/TargetInstrInfo.h"
20#include "llvm/Target/TargetMachine.h"
21#include "llvm/ADT/Statistic.h"
22#include "llvm/Support/Compiler.h"
23#include "llvm/Support/Debug.h"
24using namespace llvm;
25
26STATISTIC(NumSunk, "Number of machine instructions sunk");
27
28namespace {
29  class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
30    const TargetMachine   *TM;
31    const TargetInstrInfo *TII;
32    MachineFunction       *CurMF; // Current MachineFunction
33    MachineRegisterInfo  *RegInfo; // Machine register information
34    MachineDominatorTree *DT;   // Machine dominator tree for the current Loop
35
36  public:
37    static char ID; // Pass identification
38    MachineSinking() : MachineFunctionPass(&ID) {}
39
40    virtual bool runOnMachineFunction(MachineFunction &MF);
41
42    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43      MachineFunctionPass::getAnalysisUsage(AU);
44      AU.addRequired<MachineDominatorTree>();
45      AU.addPreserved<MachineDominatorTree>();
46    }
47  private:
48    bool ProcessBlock(MachineBasicBlock &MBB);
49    bool SinkInstruction(MachineInstr *MI, bool &SawStore);
50    bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
51  };
52} // end anonymous namespace
53
54char MachineSinking::ID = 0;
55static RegisterPass<MachineSinking>
56X("machine-sink", "Machine code sinking");
57
58FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
59
60/// AllUsesDominatedByBlock - Return true if all uses of the specified register
61/// occur in blocks dominated by the specified block.
62bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
63                                             MachineBasicBlock *MBB) const {
64  assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
65         "Only makes sense for vregs");
66  for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
67       E = RegInfo->reg_end(); I != E; ++I) {
68    if (I.getOperand().isDef()) continue;  // ignore def.
69
70    // Determine the block of the use.
71    MachineInstr *UseInst = &*I;
72    MachineBasicBlock *UseBlock = UseInst->getParent();
73    if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
74      // PHI nodes use the operand in the predecessor block, not the block with
75      // the PHI.
76      UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
77    }
78    // Check that it dominates.
79    if (!DT->dominates(MBB, UseBlock))
80      return false;
81  }
82  return true;
83}
84
85
86
87bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
88  DOUT << "******** Machine Sinking ********\n";
89
90  CurMF = &MF;
91  TM = &CurMF->getTarget();
92  TII = TM->getInstrInfo();
93  RegInfo = &CurMF->getRegInfo();
94  DT = &getAnalysis<MachineDominatorTree>();
95
96  bool EverMadeChange = false;
97
98  while (1) {
99    bool MadeChange = false;
100
101    // Process all basic blocks.
102    for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end();
103         I != E; ++I)
104      MadeChange |= ProcessBlock(*I);
105
106    // If this iteration over the code changed anything, keep iterating.
107    if (!MadeChange) break;
108    EverMadeChange = true;
109  }
110  return EverMadeChange;
111}
112
113bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
114  bool MadeChange = false;
115
116  // Can't sink anything out of a block that has less than two successors.
117  if (MBB.succ_size() <= 1) return false;
118
119  // Walk the basic block bottom-up.  Remember if we saw a store.
120  bool SawStore = false;
121  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
122    MachineBasicBlock::iterator LastIt = I;
123    if (SinkInstruction(--I, SawStore)) {
124      I = LastIt;
125      ++NumSunk;
126    }
127  }
128
129  return MadeChange;
130}
131
132/// SinkInstruction - Determine whether it is safe to sink the specified machine
133/// instruction out of its current block into a successor.
134bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
135  // Check if it's safe to move the instruction.
136  if (!MI->isSafeToMove(TII, SawStore))
137    return false;
138
139  // FIXME: This should include support for sinking instructions within the
140  // block they are currently in to shorten the live ranges.  We often get
141  // instructions sunk into the top of a large block, but it would be better to
142  // also sink them down before their first use in the block.  This xform has to
143  // be careful not to *increase* register pressure though, e.g. sinking
144  // "x = y + z" down if it kills y and z would increase the live ranges of y
145  // and z only the shrink the live range of x.
146
147  // Loop over all the operands of the specified instruction.  If there is
148  // anything we can't handle, bail out.
149  MachineBasicBlock *ParentBlock = MI->getParent();
150
151  // SuccToSinkTo - This is the successor to sink this instruction to, once we
152  // decide.
153  MachineBasicBlock *SuccToSinkTo = 0;
154
155  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
156    const MachineOperand &MO = MI->getOperand(i);
157    if (!MO.isReg()) continue;  // Ignore non-register operands.
158
159    unsigned Reg = MO.getReg();
160    if (Reg == 0) continue;
161
162    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
163      // If this is a physical register use, we can't move it.  If it is a def,
164      // we can move it, but only if the def is dead.
165      if (MO.isUse() || !MO.isDead())
166        return false;
167    } else {
168      // Virtual register uses are always safe to sink.
169      if (MO.isUse()) continue;
170
171      // If it's not safe to move defs of the register class, then abort.
172      if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
173        return false;
174
175      // FIXME: This picks a successor to sink into based on having one
176      // successor that dominates all the uses.  However, there are cases where
177      // sinking can happen but where the sink point isn't a successor.  For
178      // example:
179      //   x = computation
180      //   if () {} else {}
181      //   use x
182      // the instruction could be sunk over the whole diamond for the
183      // if/then/else (or loop, etc), allowing it to be sunk into other blocks
184      // after that.
185
186      // Virtual register defs can only be sunk if all their uses are in blocks
187      // dominated by one of the successors.
188      if (SuccToSinkTo) {
189        // If a previous operand picked a block to sink to, then this operand
190        // must be sinkable to the same block.
191        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
192          return false;
193        continue;
194      }
195
196      // Otherwise, we should look at all the successors and decide which one
197      // we should sink to.
198      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
199           E = ParentBlock->succ_end(); SI != E; ++SI) {
200        if (AllUsesDominatedByBlock(Reg, *SI)) {
201          SuccToSinkTo = *SI;
202          break;
203        }
204      }
205
206      // If we couldn't find a block to sink to, ignore this instruction.
207      if (SuccToSinkTo == 0)
208        return false;
209    }
210  }
211
212  // If there are no outputs, it must have side-effects.
213  if (SuccToSinkTo == 0)
214    return false;
215
216  DEBUG(cerr << "Sink instr " << *MI);
217  DEBUG(cerr << "to block " << *SuccToSinkTo);
218
219  // If the block has multiple predecessors, this would introduce computation on
220  // a path that it doesn't already exist.  We could split the critical edge,
221  // but for now we just punt.
222  // FIXME: Split critical edges if not backedges.
223  if (SuccToSinkTo->pred_size() > 1) {
224    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
225    return false;
226  }
227
228  // Determine where to insert into.  Skip phi nodes.
229  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
230  while (InsertPos != SuccToSinkTo->end() &&
231         InsertPos->getOpcode() == TargetInstrInfo::PHI)
232    ++InsertPos;
233
234  // Move the instruction.
235  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
236                       ++MachineBasicBlock::iterator(MI));
237  return true;
238}
239